Re: [Haskell-cafe] New slogan for haskell.org
On 26 Nov 2007, at 15:50, Henning Thielemann wrote: On Mon, 26 Nov 2007, Thomas Davie wrote: On 26 Nov 2007, at 15:15, Henning Thielemann wrote: On Thu, 4 Oct 2007, Don Stewart wrote: The Haskell website has the rather strange motivational text: Haskell is a general purpose, purely functional programming language featuring static typing, higher order functions, polymorphism, type classes, and monadic effects. Haskell compilers are freely available for almost any computer. To continue an old thread: What about turning the strange words like 'monadic effects' into links to glossary articles? Btw. where is 'lazy' ? I believe the point of this discussion was that anyone reading the Haskell webpage will currently get about as far as featuring static typing, and go this is all very nice, but what exactly does this language do for me? Why should I use it?. Take for example what the python website says: I didn't want to repeat the discussion. I think the discussion ended with: Anything more helpful would be too long for the title line at haskell.org, and a more detailed explanation (but not a generic advertisement like that from Python) should be reachable easily. Now my idea was, that making links to glossary articles leaves the slogan as short as it is, and allows people to find out quickly about the words they still don't know. An explanation why Haskell's features are useful for programmers is still required. But the point is that this section of the site is the bit that's meant to be an advertisement -- we're trying to encourage people to read more, and quite frankly, making it a fist full of links would make at least me think Well bugger this if I have to read 10 pages before I even have a clue what it is. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] St. Petersburg Game
main = do let b = 0 let c = randomRIO (1,2) until (c == 1) increment b return b This is intended to print the number of consecutive heads (i.e., 2) before the first tail, but I get the following error: ERROR StPetersburg.hs:8 - Type error in application *** Expression : until (c == 1) increment b *** Term : c == 1 *** Type : Bool *** Does not match : Int - Bool I don't really see what's going on, so any help will be more than welcome. I hope this is a suitable question for the Haskell Café list. I'm not familiar with the problem, so I won't comment on how I would implement it. However what you appear to be doing is trying to write something in an imperative style. If you want to generate random coin tosses and count how many are heads, I suggest you write a function that returns an infinite list of coin toss results. Something like tosses :: IO ([Int]) tosses = do ts - tosses return (randomRIO (1,2):ts) Then your main function merely needs to count them: main = do ts - tosses return $ countHeads ts countHeads = if (head fg == 1) then 0 else length fg where fg = head $ group ts Your immediate error is caused by a misunderstanding of how until works. Until essentially is a restricted while loop implemented with recursion. It takes three things: 1) A condition for stopping looping 2) A thing to do in the loop 3) A value to start with. Because there's no mutable state, a while loop can't alter the program state, so we must do something else instead. What we do is we have a function for computing whether we're done looping or not, and we pass a value into it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] New slogan for haskell.org
On 27 Nov 2007, at 14:44, David Menendez wrote: On Nov 26, 2007 1:44 PM, Thomas Davie [EMAIL PROTECTED] wrote: But the point is that this section of the site is the bit that's meant to be an advertisement -- we're trying to encourage people to read more, Are we? I thought Haskell.org was intended to describe what Haskell *is*. There are plenty of articles and blog posts and wiki pages out there that advocate Haskell. I don't see why the main web page needs to be polluted with marketing. Because someone's first contact with Haskell is likely to be someone saying I use this really cool language called Haskell, or a lecturer teaching it to them. In either case, if a tiny amount of interest is sparked, their likely second contact is likely to be haskell.org (through guessing or googling). Quite frankly, there's nothing going to put me off a language more than a paragraph full of unknown buzz words that I have to look up on the front page. There's plenty of places on Haskell.org where we can describe what haskell *is*, but the front page should be used for grabbing peoples attention and telling them why it's useful. Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] New slogan for haskell.org
On 28 Nov 2007, at 13:41, Ian Lynagh wrote: On Wed, Nov 28, 2007 at 09:27:39AM +0100, Thomas Schilling wrote: Sorry, but are you talking of *one* homepage? This can all go into own wiki pages that are aimed at certain audiences, but this really can't all fit on the front page. I'm reminded of http://www.shiregames.com/shiregames/ We could do something similar, with a column for When you hear programming, if you immediately think of C or perl, then please read the following: and maybe ML or lisp for the other column. I don't know if it's a good idea or not, just something to think about. That's an excellent idea as far as I'm concerned. We get the advertising pitch to the uninitiated, and the old hands keep their navigation routes to the important documentation. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What is the role of $!?
On 29 Nov 2007, at 06:32, PR Stanley wrote: Hi Thanks for the response. JCC: In most languages, if you have some expression E, and when the computer attempts to evaluate E it goes in to an infinite loop, then when the computer attempts to evaluate the expression f(E), it also goes into an infinite loop, regardless of what f is. That's the definition of a strict language. PRS: Does that mean that a strict language is also imperative? Nope, not at all. Just a strict language has slightly fewer programs it can evaluate correctly, as more will loop infinitely. Either e or f(e) could result in an infinite loop. JCC: In Haskell, this isn't the case ---we can write functions f such that the computation f(E) terminates, even when E does not. (:) is one such function, as are some functions built from it, such as (++); xn ++ ys terminates whenever xn does, even if ys is an infinite loop. This is what makes it easy and convenient to build infinite loops in Haskell; in most strict languages, if you said let fibs = 0 : 1 : zipWith (+) fibs (tail fibs) the language would insist on evaluating fibs before it actually assigned anything to the memory cell for fibs, giving rise to an infinite loop. (For this reason, most strict languages make such definitions compile-time errors). Unfortunately, non-strictness turns out to be a pain in the ass to implement, since it means when the code generator sees an expression, it can't just generate code to evaluate it --- it has to hide the code somewhere else, and then substitute a pointer to that code for the value of the expression. PRS: Is there a kind of strictness applied when the compiler/ interpreter sorts the various sub-expressions into little memory compartments indexed with pointers for later evaluation? To put it another way, does lazy evaluation begin with the outer-most expression, the most abstract, and determine what sshould go where in relation to the subsequent inner expressions? For example: takeWhile (20) [0..9] ++ [10..] The compiler determiens at the outset that the result of takeWhile is a list followed by the calculation of the length of that list based on the predicate (20), and then calls ++ which is for all intents and purposes on its own an infinite loop. Is this what happens? Not really. For lazy evaluation the compiler doesn't decide the order statically -- it merely gives the program rules to follow for what the next expression to be evaluated should be. Lets look at a slightly simpler example: takeWhile ( 2) (map (+1) [0..]) We will always attempt to evaluate the outermost left most expression. We do this by matching against the rules given in the program, to make this clearer, here are the rules for takeWhile and map: takeWhile _ []= [] takeWhile p (x:xs) | p x = x : takeWhile p xs | otherwise = [] map _ [] = [] map f (x:xs) = f x : map f xs takeWhile ( 2) (map (+1) [0..]) -- We start by evaluating the leftmost outermost expression. We attempt to match on the first rule of takeWhile, and discover that we can't because we don't know whether the result of (map (+1) [0..]) is the empty list or not. Therefore we demand the evaluation of (map +1) [0..]) - takeWhile ( 2) ((+1) 0 : map (+1) [1..]) -- We now know that we don't have the empty list, so we must use the second rule of takeWhile. We must evaluate the guard first though: - (2) ((+1) 0) | -- To do this, we must evaluate ((+1) 0) - (2) 1 | -- This evaluates to True, so we may insert the right hand side -- note that x remains evaluated - True | 1 : takeWhile (2) (map (+1) [1..]) -- We can drop the guard now, but lets carry on. We have already evaluated the outermost expression, so lets evaluate the next in. Again pattern matching on takeWhile demands the evaluation of map: - 1 : takeWhile (2) ((+1) 1 : map (+1) [2..]) -- We again, can pattern match on takeWhile, and must evaluate the guard again: - 1 : ((2) ((+1) 1) |) -- Again, we must evaluate the result of the addition - 1 : ((2) 2 |) -- This time we get False, so we must evaluate the next guard - 1 : (otherwise |) -- otherwise is a synonym for True, so we use this right hand side. - 1 : (True | []) -- and we can get rid of the guard, and prettify the result, giving us: - [1] Note that we followed a set of rules that gave us non-strict semantics. The set of rules is called lazy evaluation. We may come up with several other sets of rules that give us different evaluation orders, but still non-strict semantics (e.g. Optimistic Evaluation). This is a very simple example, that's to say, I am aware that the compiler may be faced with a much more complex job of applying lazy evaluation. Nevertheless, I wonder if there are a set of fundamental rules to which the compiler must always adhere in lazy
Re: [Haskell-cafe] Re: Trees
One could alway store a node's depth at each node -- then you must search for u and v, creating a list of what nodes you found at each depth, and finally, simply compare the lists -- O(n) in the depth of u and v. Bob On 3 Dec 2007, at 08:40, apfelmus wrote: Adrian Neumann wrote: data Tree a = Leaf a | Node a [Tree a] But now the assignments require more than a simple top-down traversal. For example: given a tree t and two nodes u,v, find the first common ancestor. Well, this problem doesn't make much sense in Haskell. How do you specify the subtrees u and v in the first place? Regards, apfelmus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] OOP'er with (hopefully) trivial questions.....
On 17 Dec 2007, at 10:46, Nicholls, Mark wrote: I can obviously at a later date add a new class Triangle, and not have to touch any of the above code…. Yes, and you can indeed do a similar thing in Haskell. The natural thing to do here would be to define a type Shape... data Shape = Circle Int | Rectangle Int Int | Square Int area :: Shape - Int -- Note, this is an interesting type if you want the area of circles area (Circle r) = pi * r^2 area (Rectangle h w) = h * w area (Square l) = area (Rectangle l l) If however, you *really* want to keep your shapes as being seperate types, then you'll want to invoke the class system (note, not the same as OO classes). class Shape a where area :: a - Int newtype Circle = C Int instance Shape Circle where area (C r) = pi * r^2 newtype Rectangle = R Int Int instance Shape Rectangle where area (R h w) = h * w newtype Square = Sq Int instance Shape Square where area (Sq l) = l * l -- Now we can do something with our shapes doubleArea :: Shape a = a - Int doubleArea s = (area s) * 2 Hope that helps Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] OOP'er with (hopefully) trivial questions.....
On 17 Dec 2007, at 11:14, Nicholls, Mark wrote: OK I'll have to digest this and mess about a bitbut can I make an observation at this point If I define Shape like data Shape = Circle Int | Rectangle Int Int | Square Int Isn't this now closed...i.e. the statement is effectively defining that shape is this and only ever thisi.e. can I in another module add new types of Shape? (sorry about all the quotation marks, but it's a minefield of potential confusions over types, classes etc). That's correct, another module could not add constructors to this type. The idea here is that you tell it all of the possible ways to construct Shape, and can then write functions to deal with it elsewhere. My other observation is...are the things on the right hand side of the the ='s sign not types? Correct, they're constructors. So you could never for example write a function that accepts only Rectangles (unless you start getting into odd type extensions) The lower version makes more sense to me...I'll have to give it a go Both versions make sense. They differ only in how heavy weight they are. Defining a type allows you to do pattern matching on the constructors, and is a much better way of defining anything you know the structure of in the first place. Using the class system on the other hand, gives you more flexibility, but at the cost of a lot of readability. The class system is designed to be able to describe things that aren't explicitly the same type, but exhibit similar properties. For example the Eq class describes all things that are equatable, it defines the (==) and (/=) operators. Your Shape class describes all types in which it's sane to compute an area. A P.S. would be...I tend to write code rather than mess about in the GHCi shell.is there a way in code to output the type of a value..i.e. the :t operation? Take a look at the Typable class. Although, pretty much any code that you can compile can be loaded into ghci without modification, and that's by far the easier way of finding the types of things. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] OOP'er with (hopefully) trivial questions.....
On 17 Dec 2007, at 12:22, Nicholls, Mark wrote: Ok... Thanks I need to revisit data and newtype to work out what the difference is I think. Beware in doing so -- type, and newtype are not the same either. type creates a type synonim. That is, if I were to declare type Jam = Int then Jam and Int from that point on become completely interchangable, the only thing this does is make things readable. For example, a parser might be described as a function that takes a list of tokens, and outputs a parse tree, and a list of unparsed tokens: type Parser = [Token] - (ParseTree, [Token]) if I write some parser combinators, I can now give them clear types like | :: Parser - Parser - Parser I could however still write this, and it would have *exactly* the same meaning. | :: ([Token] - (ParseTree, [Token])) - ([Token] - (ParseTree, [Token])) - [Token] - (ParseTree, [Token]) newtype on the other hand introduces a new type to the type system. Because of this, the type system has to be able to tell when you're using your new type, so a tag gets attached. newtype Ham = Ham Int This creates a type that contains only an integer, but is different from Int (and Jam) in the type system's eyes. Thus, I cannot for example write (Ham 5) + (Ham 6) Because Ham is not Int and thus (+) does not work (or actually, more specifically, Ham is not a member of the class Num, the numeric types, and therefore (+) doesn't work). This can of course be fixed thus: newtype Ham = Ham Int deriving Num Hope that helps Tom Davie p.s. Sorry for the slip with the newtype Rectangle. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On 12 Jan 2008, at 23:16, Hugh Perkins wrote: On Jan 12, 2008 10:54 PM, Henning Thielemann [EMAIL PROTECTED] wrote: On Sat, 12 Jan 2008, Hugh Perkins wrote: I guess that Haskell's referential transparence means the answers to the isPerfectSquare will be cached, ie automatically memoized? (not sure if is correct term?) http://www.haskell.org/haskellwiki/Memoization Interesting... but I dont understand... I thought that referential transparence meant that once the answer to a function has been calculated once, it will always be the same, and that the interpreter can, and will, cache this answer? So, if I call f( 20 ) once, for some, arbitrary, f, it will have to go away and calculate f(20), but if I call it multiple times, it will just return the value it already calculated? No, Memorisation has it's costs too... Suppose you wanted to computer map f [1..100]? Each time f was called, your program would look up a table of on average 50 results for f. That doesn't sound very efficient if f is a simple function. Now suppose you're running a program for several hours -- imagine how large your table would become, and how slow your lookup would be. What you can do however, is introduced constants. Constants are evaluated once and only once, so using them, you can tell the compiler exactly what should be memorized. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackape package lackage
I'd love to see that map normalised by the population of the country – would be interesting to see where Haskell is popular. Bob On 22 Mar 2010, at 16:22, Don Stewart wrote: We're watching *massive* traffic right now due to HP release. It's not down, just very very busy. For fun, here's a map of who's downloading Haskell: http://imgur.com/flwPF.png 74 countries in 12 hours, and counting. - Don dougal: Hackage seems to be down again. $ cabal update Downloading package list from server 'http://hackage.haskell.org/packages/archive' ^Ccabal: interrupted $ ping -c3 hackage.haskell.org PING abbot.galois.com (69.30.63.204) 56(84) bytes of data. --- abbot.galois.com ping statistics --- 3 packets transmitted, 0 received, 100% packet loss, time 2012ms ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [OT?] Haskell-inspired functions for BASH
Unfortunately though, h4sh seems to be broken, for one, there's no fps package (apparently required), and hsplugins won't build with 6.12.1. Bob On 1 Apr 2010, at 15:41, Jeremy Shaw wrote: How about: http://www.cse.unsw.edu.au/~dons/h4sh.html It brings a lot of familiar Haskell functions to the command-line. And *is* actually written in Haskell ;) - jeremy On Wed, Mar 31, 2010 at 6:05 PM, Patrick LeBoutillier patrick.leboutill...@gmail.com wrote: Hi all, I've been studying Haskell for about a year now, and I've really come to like it. In my daily work I write a lot of BASH shell scripts and I thought I'd try add some of the haskell features and constructs to BASH to make my scripting life a bit easier. So I've been working on a small BASH function library that implements some basic functional programming building blocks. Note: There is no actual Haskell code involved here. I put up the full manpage here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=24564 Source is here: http://svn.solucorp.qc.ca/repos/solucorp/bashkell/trunk/trunk/ All this is very prototypical, but here is an example of some of the stuff I've got so far (map, filter, foldr): $ ls data 1.txt 2.txt # basic map, argument goes on the command line $ ls -d data/* | map basename 1.txt 2.txt # map with lambda expression $ ls -d data/* | map '\f - basename $f .txt' 1 2 # simple filter, also works with lambda $ ls -d data/* | map basename | filter 'test 1.txt =' 1.txt # sum $ ls -d data/* | map '\f - basename $f .txt' | foldr '\x acc - echo $(($x + $acc))' 0 3 Basically I'm looking for a bit of feedback/info: - Does anyone know if there are already similar projets out there? - Does anyone find this interesting? - Any other comment/suggestion/feedback - Where's a good place to promote such a project? Thanks a lot, Patrick LeBoutillier -- = Patrick LeBoutillier Rosemère, Québec, Canada ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskellers hate GUIs!!
On 2 Apr 2010, at 21:01, Brandon S. Allbery KF8NH wrote: On Apr 2, 2010, at 15:21 , Thomas Schilling wrote: On 2 April 2010 20:15, Brandon S. Allbery KF8NH allb...@ece.cmu.edu wrote: On Apr 2, 2010, at 10:41 , David Leimbach wrote: Having said that, are there any plans to make it really easy to get gtk2hs working on Mac OS X? It's in MacPorts. But that's the variant using X11, no? There now is a Gtk+ framework, BTW, native Cocoa support is now part of the standard Gtk+ distribution. Unfortunately, it still doesn't behave anything like a Cocoa application.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell.org re-design
On 7 Apr 2010, at 02:53, Ben Millwood wrote: On Wed, Apr 7, 2010 at 2:22 AM, Thomas Schilling nomin...@googlemail.com wrote: I have set a maximum width on purpose so that it doesn't degrade too badly on big screens. I've never really trusted this argument - it's not required that the browser window occupy the entire screen, so why not let the user choose how wide they want their text? Unfortunately, because the majority operating system has such bad window management that all users do make their windows take up the entire screen. :( Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: instance Eq (a - b)
Your instances of Finite are not quite right: bottom :: a bottom = doSomethingToLoopInfinitely. instance Finite () where allValues = [(), bottom] instance Finite Nothing where allValues = [bottom] Though at a guess an allValuesExculdingBottom function is also useful, perhaps the class should be class Finite a where allValuesExcludingBottom :: [a] allValues :: Finite a = [a] allValues = (bottom:) . allValuesExcludingBottom Bob On 14 Apr 2010, at 08:01, Ashley Yakeley wrote: Joe Fredette wrote: this is bounded, enumerable, but infinite. The question is whether there are types like this. If so, we would need a new class: class Finite a where allValues :: [a] instance (Finite a,Eq b) = Eq (a - b) where p == q = fmap p allValues == fmap q allValues instance (Finite a,Eq a) = Traversable (a - b) where sequenceA afb = fmap lookup (sequenceA (fmap (\a - fmap (b - (a,b)) (afb a)) allValues)) where lookup :: [(a,b)] - a - b lookup (a,b):_ a' | a == a' = b lookup _:r a' = lookup r a' lookup [] _ = undefined instance Finite () where allValues = [()] data Nothing instance Finite Nothing where allValues = [] -- Ashley Yakeley ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 08:29, Ashley Yakeley wrote: On Wed, 2010-04-14 at 08:13 +0100, Thomas Davie wrote: Your instances of Finite are not quite right: bottom :: a bottom = doSomethingToLoopInfinitely. instance Finite () where allValues = [(), bottom] Bottom is not a value, it's failure to evaluate to a value. But if one did start considering bottom to be a value, one would have to distinguish different ones. For instance, (error ABC) vs. (error PQR). Obviously this is not finite. Certainly bottom is a value, and it's a value in *all* Haskell types. Of note, bottom is very important to this question – two functions are not equal unless their behaviour when handed bottom is equal. We also don't need to distinguish different bottoms, there is only one bottom value, the runtime has different side effects when it occurs at different times though. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:01, Jonas Almström Duregård wrote: But if one did start considering bottom to be a value, one would have to distinguish different ones. For instance, (error ABC) vs. (error PQR). Obviously this is not finite. Nor is it computable, since it must distinguish terminating programs from nonterminating ones (i.e. the halting problem). On a side note, since instance (Finite a, Finite b) = Finite (a - b) should be possible, one can even compare some higher order functions with this approach ;). f,g :: Bool - Int f x = 6 g x = 6 We can in Haskell compute that these two functions are equal, without solving the halting problem. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:08, Jonas Almström Duregård wrote: f,g :: Bool - Int f x = 6 g x = 6 We can in Haskell compute that these two functions are equal, without solving the halting problem. Of course, this is the nature of generally undecidable problems. They are decidable in some cases, but not in general. Well yes, but we already knew that this was true of function equality – we can't tell in general. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:12, Jonas Almström Duregård wrote: f,g :: Bool - Int f x = 6 g x = 6 We can in Haskell compute that these two functions are equal, without solving the halting problem. what about these? f,g :: Bool - Int f x = 6 g x = x `seq` 6 As pointed out on #haskell by roconnor, we apparently don't care, this is a shame... We only care that x == y = f x == g y, and x == y can't tell if _|_ == _|_. It's a shame that we can't use this to tell if two functions are equally lazy (something I would consider part of the semantics of the function). Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:17, Ashley Yakeley wrote: Thomas Davie wrote: Certainly bottom is a value, and it's a value in *all* Haskell types. This is a matter of interpretation. If you consider bottom to be a value, then all the laws fail. For instance, (==) is supposed to be reflexive, but undefined == undefined is not True for almost any type. For this reason I recommend fast and loose reasoning: http://www.cs.nott.ac.uk/~nad/publications/danielsson-et-al-popl2006.html It might be nice to have a definition of whether we consider bottom to be a value in Haskell then, because the definition of second and fmap on tuples are different because of this consideration: fmap f (x,y) = (x,f y) second f ~(x,y) = (x,f y) Because we consider that the Functor laws must hold for all values in the type (including bottom). Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:25, Ashley Yakeley wrote: Thomas Davie wrote: Because we consider that the Functor laws must hold for all values in the type (including bottom). This is not so for IO, which is an instance of Functor. fmap id undefined is not bottom. It isn't? fPrelude fmap id (undefined :: IO ()) *** Exception: Prelude.undefined Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:31, Ashley Yakeley wrote: On Wed, 2010-04-14 at 09:29 +0100, Thomas Davie wrote: It isn't? fPrelude fmap id (undefined :: IO ()) *** Exception: Prelude.undefined ghci is helpfully running the IO action for you. Try this: seq (fmap id (undefined :: IO ())) not bottom Ah, rubbish... I guess this further reinforces my point though – we have a mixture of places where we consider _|_ when considering laws, and places where we don't consider _|_. This surely needs better defined somewhere. For reference, the fmap on tuples which ignores the bottom case for the sake of the laws is useful :(. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:35, Jonas Almström Duregård wrote: what about these? f,g :: Bool - Int f x = 6 g x = x `seq` 6 As pointed out on #haskell by roconnor, we apparently don't care, this is a shame... We only care that x == y = f x == g y, and x == y can't tell if _|_ == _|_. So the facts that (1) f == g (2) f undefined = 6 (3) g undefined = undefined is not a problem? Yeh :( Shame, isn't it. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: instance Eq (a - b)
On 14 Apr 2010, at 09:39, Ashley Yakeley wrote: Thomas Davie wrote: I guess this further reinforces my point though – we have a mixture of places where we consider _|_ when considering laws, and places where we don't consider _|_. This surely needs better defined somewhere. It's easy: don't consider bottom as a value, and the laws work fine. If it were this easy, then why is our instance of Functor on tuples gimped? Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] help with Haskell programming
I'm not certain exactly what you mean, but I *think* you mean: func :: (a - Bool) - (a - Bool) func = (not .) Bob On 18 Apr 2010, at 16:35, Mujtaba Boori wrote: Hello I am kinda newbie in Haskell you can help help me with some programming I am trying to make function like for example func :: (a - Bool) - (a - Bool) this function make calculation and return bool . I want to be able to make bool True when It is False and False when it is True while returning the a. Thank you -- Mujtaba Ali Alboori ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: help with Haskell programming
To do this, you need not just fmap (composition), but also ap, or the combined form, liftA2: func = liftA2 (||) Bob On 18 Apr 2010, at 18:21, Keith Sheppard wrote: Using composition can be tricky with more than one arg. I just want to be sure you're not really looking for something like: func :: (a - Bool) - (b - Bool) - (a - b - Bool) keeping with your given type I think you're looking for something like: func f1 f2 x = (f1 x) || (f2 x) I'm sure there is a nice way to do this with function composition but I still find composition less intuitive than explicit args in cases like this. On Sun, Apr 18, 2010 at 1:00 PM, Mujtaba Boori mujtaba.bo...@gmail.com wrote: Thanks for helping me but I have another problem (sorry for asking) . I tried to figure it out . how about if I want to compare two kind with () (||) for func :: (a - Bool) - (a - Bool) - (a - Bool) I tried some thing like func = ((||) .) This is the annoying part about Haskell . I can not understand composition . On Sun, Apr 18, 2010 at 4:35 PM, Mujtaba Boori mujtaba.bo...@gmail.com wrote: Hello I am kinda newbie in Haskell you can help help me with some programming I am trying to make function like for example func :: (a - Bool) - (a - Bool) this function make calculation and return bool . I want to be able to make bool True when It is False and False when it is True while returning the a. Thank you -- Mujtaba Ali Alboori -- Mujtaba Ali Alboori ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- keithsheppard.name ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why Either = Left | Right instead of something like Result = Success | Failure
On 27 May 2010, at 15:25, Ionut G. Stan wrote: Hi, I was just wondering if there's any particular reason for which the two constructors of the Either data type are named Left and Right. I'm thinking that something like Success | Failure or Right | Wrong would have been a little better. The reason I guess is that Success/Failure and Right/Wrong are a lot less general than Left/Right. One can use Either for types with two possible valid types contained within, it doesn't only have to be used for types where one is for correct results and the other for erroneous. Of course, there's nothing stopping you implementing your own type :) Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: Sum type branches as extended types (as Type!Constructor)
On 3 Jun 2010, at 16:14, Gabriel Riba wrote:
Extending sum types with data constructors would spare runtime errors or
exception control,
when applying functions to inappropriate branches, as in the example ...
data List a = Nil | Cons a (List a) -- List!Nil and List!Cons
-- as extended types
* Actual system, with runtime errors (as in GHC Data.List head) or
exception throwing
hd :: List a - a
hd (Cons x _) - x
hd Nil - error error: hd: empty list -- error or exception throwing
* Proposed system extending types with constructors as Type!Constructor:
User must do pattern matching before applying the constructor-specific
type function.
In ''var @ (Constructor _ _)'' the compiler should append the constructor
to the type as a pair (Type, Constructor) as an extended type for ''var''
No need for runtime errors or exception control
hd :: List!Cons a - a
hd (Cons x _) = x
using it:
headOf :: List a - Maybe a
headOf list = case list of
li @ (Cons _ _) - Just hd li -- extTypeOf li == ( 'List', 'Cons')
-- should pass typechecker for List!Cons
li @ Nil - Just hd li -- compiler error !!
-- extTypeOf ('List','Nil') don't match
_ - Just hd list -- compiler error !!
-- extTypeOf ('List',Nothing) don't match
Maybe we could take out importance on the number of _ wildcards (constructor
arity) with a syntax like.
li @ (Cons ...)
li @ (Nil ...)
This looks fairly similar to total functional programming, though putting the
onus on the caller to make sure it meets preconditions, rather than the callee
to make sure it's the right type.
In total functional programming we would say
head :: List a - Maybe a
head (Cons x _) = Just x
head Nil = Nothing
We'd then allow the caller to deal with the maybe any way it likes (commonly
with fmap, or with the maybe function).
Bob___
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Re: [Haskell-cafe] Physical equality
On 28 Jun 2010, at 09:38, José Romildo Malaquias wrote: Is there in Haskell a non monadic function of type a - a - Bool which test for physical equality of two values? It would return True if only if both values are the same object in memory. For instance: value1 = good value2 = good eq value1 value2 = False value1 = good value2 = value1 eq value1 value2 = True This simply isn't possible without manually tagging values yourself (or with a library), it would violate referential transparency. Remember, a function, called twice with semantically identical arguments must always return the same value, that isn't true of eq. Even if this weren't an issue, you're relying heavily on the runtime's behaviour here. There's nothing to stop the runtime, in the first example, observing that the two values are identical, and making them both take up the same memory – they are after all immutable, so that's totally safe. There's similarly nothing to stop the runtime, in the second example, arbitrarily copying the the string (although it's probably not a great idea for efficiency). Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How easy is it to hire Haskell programmers
On 3 Jul 2010, at 03:39, Don Stewart wrote: ivan.miljenovic: Hmm, interesting. Applicative and Traversable are two classes I've never used and don't really understand the purpose of. I have no idea what hsc2hs is. I keep hearing finger trees mentioned, but only in connection to papers that I can't access. So I guess that means that I don't count as a knowledgable Haskell programmer. :-( RWH is free and online, and covers many useful things. There's no excuse :-) Knowing about something /= knowing how to use it. I own and have read RWH, but I've never had to use hsc2hs, or Applicative, etc. Writing libraries that bind to C is a great way to have to use a lot of hsc2hs (or c2hs), so clearly you need to contribute more libraries :-) Alternatively, they're already busy contributing lots of purely functional libraries, instead of doing (still valuable) work getting C cludges working. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Are you a Haskell expert? [How easy is it to hire Haskell programmers]
On 3 Jul 2010, at 11:04, Brandon S Allbery KF8NH wrote: -BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 7/3/10 05:57 , Andrew Coppin wrote: Agreed. So let me rephrase: Why should _every_ Haskell library involve C? ;-) Who says they do, or should? Dons rather implied it... The suggestion is that someone who hasn't used hsc2hs is an inexperienced Haskeller... I'd bet though that there are many *extremely* experienced haskellers who have never once in their life written a C binding. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] lists of arbitrary depth
On 13 Jul 2010, at 10:11, Shlomi Vaknin wrote: Thank you all for replying! I am really beginning my baby steps in this fascinating language, and was just wondering if it was possible to naturally scan lists with arbitrary lists (aka trees :) ). Trees aren't lists, Trees are trees... Here's how you create one* data Tree a = Leaf | Branch a [Tree a] This roughly says, a tree can be made up of two possible things... First, it could be a leaf, and secondly, it could be a branch with an element at it, and a list of subtrees. Some example trees using this data type: 1) Leaf 2) Branch 5 [] 3) Branch 5 [Leaf] 4) Branch 5 [Branch 10 [], Branch 20 [], Leaf, Branch 50 [Branch 10 [], Leaf]] When you have really strong typing, you also have very well specified types. You don't just use a list as if it were a tree, you declare what a tree is. Bob * In this case an arbitrarily branching tree, we could ofc declare different forms of tree here.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Can we come out of a monad?
On 1 Aug 2010, at 11:43, Ertugrul Soeylemez wrote: Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: No, a pure function is one without any side effects. There are no functions with side effects in Haskell, unless you use hacks like unsafePerformIO. Every Haskell function is perfectly referentially transparent, i.e. pure. This is why we badly need a new term, say, io-pure. That means, neither has side effects, nor produces an action that when run by the runtime has side effects. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Exercise in point free-style
Shorter, although perhaps less insightful. Bob On 2 Sep 2006, at 01:36, Lennart Augustsson wrote: An easy way to solve this is to ask lambdabot. Log on to the Haskell IRC channel: lennart: @pl \ f l - l ++ map f l lambdabot: ap (++) . map Notice how it's much shorter than the Hughes' solution. :) -- Lennart On Sep 1, 2006, at 13:11 , John Hughes wrote: From: Julien Oster [EMAIL PROTECTED] Subject: [Haskell-cafe] Exercise in point free-style I was just doing Exercise 7.1 of Hal Daumé's very good Yet Another Haskell Tutorial. It consists of 5 short functions which are to be converted into point-free style (if possible). It's insightful and after some thinking I've been able to come up with solutions that make me understand things better. But I'm having problems with one of the functions: func3 f l = l ++ map f l Looks pretty clear and simple. However, I can't come up with a solution. Is it even possible to remove one of the variables, f or l? If so, how? Thanks, Julien Oh, YES!! Two ways to remove l: func3a f = uncurry ((.map f).(++)) . pair func3b f = uncurry (flip (++).map f) . pair And just to make sure they're right: propab new f l = func3 f l == new f l where types = f :: Int-Int quickCheck (propab func3a) quickCheck (propab func3b) If you don't mind swapping the arguments, then propc f l = func3 f l == func3c l f where types = f :: Int-Int func3c l = (l++) . (`map` l) With the arguments swapped, you can even remove both! propd f l = func3 f l == func3d l f where types = f :: Int - Int func3d = uncurry ((.(flip map)) . (.) . (++)) . pair MUCH clearer! The trick is to observe that l is duplicated, so you need to use a combinator that duplicates something. The only one available here is pair, which you then have to combine with uncurry. It would be nicer to have (f g) x = (f x,g x) available. ( is one of the arrow combinators). Then you could remove l by func3e f = uncurry (++) . (id map f) which is sort of readable, and remove both by func3f = (uncurry (++).) . (id ) . map John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] smallest double eps
On 30 Sep 2006, at 17:19, Brian Hulley wrote: Lennart Augustsson wrote: Hang on, hang on, now I'm getting confused. First you asked for the smallest (positive) x such that 1+x /= x which is around x=4.5e15. 1 + 0 /= 0 0 is smaller than 4.5e15 So I don't understand this at all... But then 0 isn't positive. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Composing functions with runST
It's true that this is the typical way of learning Haskell, but I for one think it's a bad way of learning Haskell. Very few real world programs get by without the impure stuff, so if you give the newbie the impression that it isn't there (by postponing it) there's a chance he'll run into a situation where he needs it before it's been even mentioned (queue newbie going bah, academic language and switching to C++). On the contrary, I think it's an excellent way of learning Haskell. I'm writing a lot of useful Haskell code with only one IO action (interact). I don't think I could reasonably construct an introductory problem that couldn't be solved with it, and I haven't yet found an application for which I've needed more. I think it's destructive to teach people we have a wonderful new paradigm of programming that solves all sorts of problems, but all we're going to use it for is doing what we did with C++ anyway. That's just my 2¢ -- I like Haskell specifically because I don't have to do things in order and I don't have to do things in an imperative style, I would love for more people to be taught about this wonderful thing. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Win32 help please
On 4 Feb 2007, at 17:59, Stefan O'Rear wrote: On Sun, Feb 04, 2007 at 10:42:23PM +1100, John Ky wrote: # hsc2hs mywin32.hsc # ghc -fffi mywin32.hs C:/system/ghc/ghc-6.6/libHSrts.a(Main.o):Main.c:(.text+0x1b): undefined reference to `__stginit_ZCMain' C:/system/ghc/ghc-6.6/libHSrts.a(Main.o):Main.c:(.text+0x3f): undefined reference to `ZCMain_main_closure' collect2: ld returned 1 exit status What am I missing? A function named main. s/function/constant/ In a module called Main. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell and GUI
There's also the HOC (Haskell Objective-C bridge), which lets you use Apple's Cocoa APIs. Bob On 14 Jan 2008, at 22:09, Torsten Otto wrote: Seeing my woes with FranTk - what else is out there that people use if a (simple) GUI is desired for a Haskell app? Just a few textboxes and a button or two would do me. Thanks in advance! Regards, Torsten ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Doubting Haskell
A quick note here. This is a *really* excellent tutorial on a variety
of subjects. It shows how monad operators can be used responsibly (to
clarify code, not obfuscate it), it shows how chosing a good data
structure and a good algorithm can work wonders for your code, and on
a simplistic level, it shows how to build a database in Haskell.
Would it be possible to clean this up and put it in the wiki somewhere?
Thanks
Bob
On 20 Feb 2008, at 09:58, Cale Gibbard wrote:
(I'm copying the list on this, since my reply contains a tutorial
which might be of use to other beginners.)
On 19/02/2008, Alan Carter [EMAIL PROTECTED] wrote:
Hi Cale,
On Feb 19, 2008 3:48 PM, Cale Gibbard [EMAIL PROTECTED] wrote:
Just checking up, since you haven't replied on the list. Was my
information useful? Did I miss any questions you might have had? If
you'd like, I posted some examples of using catch here:
Thanks for your enquiry! My experiment continues. I did put a
progress
report on the list - your examples together with a similar long an
short pair got me over the file opening problem, and taught me some
things about active whitespace :-) I couldn't get withFile working
(says out of scope, maybe 'cos I'm ghc 6.6 on my Mac)
Make sure to put:
import System.IO
at the top of your source file, if you haven't been. This should
import everything documented here:
http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html
but it turned out the line I was looking for (collapsed from the
examples)
was:
text - readFile data.txt `catch` \_ - return
This ensures the program never loses control, crashing or becoming
unpredictable by attempting to use an invalid resource, by yielding
an
empty String if for any reason the file read fails. Then an empty
String makes it very quickly through parsing. I guess that's quite
functiony :-)
Amazing how easy once I knew how. Even stranger that I couldn't
find a
bread and butter example of it.
Then I was going very quickly for a while. My file is dumped from a
WordPress MySql table. Well formed lines have 4 tab separated fields
(I'm using pipes for tabs here):
line id | record id | property | value
Line IDs are unique and don't matter. All lines with the same record
ID give a value to a property in the same record, similar to this:
1|1|name|arthur
2|1|quest|seek holy grail
3|1|colour|blue
4|2|name|robin
5|2|quest|run away
6|2|colour|yellow
Organizing that was a joy. It took minutes:
let cutUp = tail (filter (\fields - (length fields) == 4)
(map (\x - split x '\t')
(lines text)))
This should almost certainly be a function of text:
cutUp text = tail (filter (\fields - (length fields) == 4)
(map (\x - split x '\t') (lines
text)))
I found a split on someone's blog (looking for a library tokenizer),
but I can understand it just fine. I even get to chuck out ill-formed
lines and remove the very first (which contains MySql column names)
on
the way through!
Sadly, there's no general library function for doing this. We have
words and lines (and words would work here, if your fields never have
spaces), but nobody's bothered to put anything more general for simple
splitting into the base libraries (though I'm sure there's plenty on
hackage -- MissingH has a Data.String.Utils module which contains
split and a bunch of others, for example). However, for anything more
complicated, there are also libraries like Parsec, which are generally
really effective, so I highly recommend looking at that at some point.
I then made a record to put things in, and wrote some lines to play
with it (these are the real property names):
data Entry = Entry
{ occupation:: String
, iEnjoyMyJob :: Int
, myJobIsWellDefined:: Int
, myCoworkersAreCooperative :: Int
, myWorkplaceIsStressful:: Int
, myJobIsStressful :: Int
, moraleIsGoodWhereIWork:: Int
, iGetFrustratedAtWork :: Int
}
...
let e = Entry{occupation = , iEnjoyMyJob = 0}
let f = e {occupation = alan}
let g = f {iEnjoyMyJob = 47}
putStrLn ((occupation g) ++ ++ (show (iEnjoyMyJob g)))
Then I ran into another quagmire. I think I have to use Data.Map to
build a collection of records keyed by record id, and fill them in by
working through the list of 4 item lists called cutUp. As with the
file opening problem I can find a few examples that convert a list of
tuples to a Data.Map, one to one. I found a very complex example that
convinced me a map from Int to a record is possible, but gave me no
understanding of how to do it. I spent a while trying to use foldl
before I decided it can't be appropriate (I need to pass more
values).
So I tried a couple of recursive functions, something like:
type Entries = M.Map Int Entry
...
let entries = loadEntries cutUp
...
loadEntries :: [[String]] - Entries
loadEntries [] = M.empty Entries
loadEntries [x : xs] = loadEntry
Re: [Haskell-cafe] Re: ANN: haskell-src-exts 0.3.2
On 17 Mar 2008, at 23:41, Niklas Broberg wrote: Could this be used to add support for refactoring of source files containing language extensions? Because if I'm correct, the current most popular refactoring solution (I forgot the name) for Haskell does not support extensions. I supppose you're talking about HaRe, that Thomas Schilling linked to. I have no idea how that system is built so I can't answer your question. But in principle I don't see why not. :-) I believe that the limitation is that they use Programatica's parser to get an AST to run their refactorings on. I think they've looked several times at using ghc's apis to do this, but hit various problems. I think that the main problem is that no other parser preserves things like code layout and commenting, which is of course pretty critical to refactoring programs in a sane kind of way. Thanks Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] monadic debugging
On 16 Apr 2008, at 00:04, Bulat Ziganshin wrote: Hello Vasili, Wednesday, April 16, 2008, 2:53:32 AM, you wrote: I have an Linux executable of my Haskell library and test case. I see there are several debuggers, e.g. Buddha, Hat, etc. Which debugger is currently preferred for monadic (imperative) code? Thanks. i use print mainly :) btw, there is also built-in ghci debugger, i suspect that it's closest one to the usual debuggers and most useful one for imperative code (but i never tried anything, so don't trust me :) Having worked lots on Hat, and studied all (I hope or I've got a hole in my research) of the debuggers out there, I'd have to say that debugging monadic code is still very much an unsolved problem. Putting print statements in is probably your best option. You may want to try hat-delta, or buddha's functional mapping mode -- both of them should be capable of reducing sequences of monadic operations to a single operation and a function map. Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fw: I have a problem
First, I'd refer you to this list's rules on homework, and what people will or won't answer. Secondly to that though, rather than provide a solution, I'll give you an idea that may lead to you coming up with a solution. First, try and write a function that can test if your first list is at the start of the second list: startlist [2,3] [2,3,4,5] True startlist [2,3] [1,2,3,4] False Then use this function to write your sublist function. Hope that helps Tom Davie On 24 Apr 2008, at 10:29, cetin tozkoparan wrote: I have a problem which i can't solve. Is there any one who has an idea? Two lists is sent as parameter to a function. If second list contains first list, return true, else return false. This comparision must be in order of first list. You can look at examples. function type as follows: sublist:: [a] - [a] - Bool examples: For instance [2,4,5] list is sub list of [3,7,2,4,5,9] list but not of [3,7,4,2,5,9] list. sublist [2,8] [1,5,6,2,4,7,8,2] False sublist [1,2,3] [0,1,2,3,4,5,6] True sublist [5,4] [1,4,5,7,8,3,5,4] True sublist [1,2,4,3,4,5,7,8,9,5] [8,2,3,1,2,4,3,4,5,7,8,9,5,1,6,2] True Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unapplying function definitions?
On 4 May 2008, at 17:33, PR Stanley wrote: Hi What on earth is unapplying function definitions? The following is taken from chapter 13 of the Hutton book: ...when reasoning about programs, function definitions can be both applied from left to right and unapplied from right to left. Well, because of referential transparency, we can say that the left hand side of a function is exactly equal to the right hand side. Thus, we can instead of applying functions, and making progress towards a normal form, unapply them and get further away from a normal form... for example: 5 = head [5,6,7,8,9] = head ([5,6] ++ [7] ++ [8,9]) = head (([] ++ [5] ++ [6]) ++ [7] ++ [8,9]) ... There are of course an infinite number of ways of doing this, so it's usually only interesting, if we have some reason for applying a specific expansion. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO Help
On 8 May 2008, at 16:31, Mark Wallsgrove wrote: Was there? I have been google'ing that problem for ages.. Just one more thing. I have to make a menu system where the user chooses what functionality they want. Because you cannot change a value once it is set I have used recursion so that when something changes it then calls the menu back up. I feel this is way to memory consuming. Is there another way? While this method feels like it should consume lots of memory, it in fact doesn't. Remember that you're dealing with a graph machine, and that no stack is maintained of all the calls you've made. The garbage collector will simply follow you through the menu system clearing up the memory behind you. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] inserting values in a binary tree
On 10 May 2008, at 00:35, PR Stanley wrote: Hi data Ord a = Tree a = Nil | Node (Tree a) a (Tree a) How would one go about inserting a value in a binary search tree of the above description? All you need to do is consider what the trees should look like in the two cases: If I try and insert an item into a completely empty tree, what do I end up with? I'll give you a hint, it has one Node, and 2 Nils. If I have a Node, do I need to insert into the left tree, or the right tree? Take it from there Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type Coercion
On 28 May 2008, at 09:34, PR Stanley wrote: Hi (16 :: Float) is a perfectly legitimate statement although I'm surprised that it's allowed in a type strong language such as Haskell. It's a bit like casting in good old C. What's going on here? It's not a coercion -- it happens at compile time. In a coercion, 16 starts off it's runtime life as an integer, gets a couple of things done to it, and then is coerced into a floating point number. What's happening here is you are telling the compiler I would like the number 16, but a floating point version of it please. That instance of 16 always will have type Float. Slightly more detail: numeric literals like this normally have the type Int, but get pre-processed by the compiler. Whenever you write 16, the compiler writes (fromInteger 16). This has the effect that instead of having the type Int, your literal has the type Num a = a. You adding the type signature constrains it to being a Float again. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] modelling C in Haskell ..
Yes, you must write them seperately as something like
data A = A Int B
data B = B1 Int
| B2 Int Int
one of the many wonders of Haskell -- it encourages you to split up
your code into nice small chunks.
Bob
On 30 May 2008, at 08:46, Galchin, Vasili wrote:
Hello,
I don't want to write kludgy Haskell code!
typedef struct blah
{
int val1;
union {
int val2;
struct {
int val3;
int val4;
}
}
}C_type;
question: in Haskell, can I embed definition of the union inside
of the C typedef, i.e. recursion definition? Or must I have a
separate definition for the union which I instantiate inside the
Haskell typedef, i.e. Haskell data?
Kind regards, Vasili
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Re: [Haskell-cafe] automatically deriving Map and Filter on datatypes etc.
Even deriving an instance of Functor seems rather implausable, what should it do for data Wierd a b = Nil | A a (Wierd a b) | B b (Wierd a b) Should fmap's function argument operate on 'a's, 'b's, or both? Bob On 5 Jun 2008, at 10:28, Miguel Mitrofanov wrote: Well, it's certainly not possible for filter, at least, not without additional hints to the compiler. For example, consider this type: data Weird a = A | B a (Weird a) (Weird a) filter p A = A filter p (B x w1 w2) | p x = B x (filter p w1) (filter p w2) | otherwise = ? On 5 Jun 2008, at 12:03, Cetin Sert wrote: Hi ^_^, Let's say we have the following data type and functions: data Tab a = (:↺:) | a :↓: Tab a | Tab a :↙↘: (Tab a,Tab a) deriving (Eq, Show, Read) map f (:↺:) = (:↺:) map f (a :↓: t) = f a :↓: map f t map f (h :↙↘: (l,r)) = map f h :↙↘: (map f l, map f r) filter p (:↺:) = (:↺:) filter p (a :↓: t) | p a = filter p t | otherwise = a :↓: filter p t filter p (h :↙↘: (l,r)) = filter p h :↙↘: (filter p l, filter p r) is it possible to automatically derive map and filter? data Tab a = (:↺:) | a :↓: Tab a | Tab a :↙↘: (Tab a,Tab a) deriving (Eq, Show, Read, Map, Filter) If not, do you think it might be nice to have something like this in the future? Best Regards, Cetin Sert ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Topkata
On 14 Jun 2008, at 12:45, Christoph Bauer wrote: Hi All, Topkata is a simple OpenGL Game written in Haskell. It's not very advanced. Goal so far is to guide a ball trough an labyrinth to the opposite corner. The web page shows an screenshot. It's only tested under Linux. http://home.arcor.de/chr_bauer/topkata.html FeedbackPatches are welcome. BTW, I had a lot of support in #haskell! Looks awesome, and I completely agree with Niel -- hackage it up! In the mean time -- who knows enough to make ghc target ARM, and get this to link against the iPhone libraries? This would be quite a coup if it could be made to run there! Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Topkata
On 15 Jun 2008, at 07:41, Deborah Goldsmith wrote: On Jun 14, 2008, at 1:06 PM, Don Stewart wrote: tom.davie: In the mean time -- who knows enough to make ghc target ARM, and get this to link against the iPhone libraries? This would be quite a coup if it could be made to run there! I'd be interested. We should start a wiki page for Haskell on the iphone.. It's an interesting idea, but I think it would need to be a cross- compiler. Does ghc support cross-compilation? The most obvious place to start I guess would be using -fvia-C and the C cross compiler that apple supply. I'll certainly be looking into this as soon as I get an iPhone (which unfortunately I need to wait for 3G one in Belgium). Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How to do this in FP way?
One half of all Haskell coders will tell you that mutable state isn't a good starting point to learn Haskell, the other half will tell you the same because they want to be cool kids, too. And the one left over will point out that he asked how to do this the FP way, not the imperative way? If it was me btw, I'd take a stab at the problem being that each time we do something a time gets updated and we want to know how much time has passed since we last did something. I'd approach this by generating a lazy list of times at which we started doing something, and then generating a lazy list of time differences. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How to do this in FP way?
On 16 Jun 2008, at 18:28, Achim Schneider wrote: Thomas Davie [EMAIL PROTECTED] wrote: One half of all Haskell coders will tell you that mutable state isn't a good starting point to learn Haskell, the other half will tell you the same because they want to be cool kids, too. And the one left over will point out that he asked how to do this the FP way, not the imperative way? There's no difference, as you can't do time-accounting non-strict and still expect it to give meaningful results: I'm merely trying to be helpful. None of the other solutions allow for the IO Monad. Firstly, I'd phrase that differently -- the IO Monad doesn't allow for the other solutions -- the other solutions are the truly functional ones. Secondly, I'm curious as to why you think that the two are incompatible, are you saying that for any meaningful kind of computation we need to resort to IORefs? I'd strongly contest that idea. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How to do this in FP way?
On 16 Jun 2008, at 19:24, Achim Schneider wrote: Thomas Davie [EMAIL PROTECTED] wrote: On 16 Jun 2008, at 18:28, Achim Schneider wrote: Thomas Davie [EMAIL PROTECTED] wrote: One half of all Haskell coders will tell you that mutable state isn't a good starting point to learn Haskell, the other half will tell you the same because they want to be cool kids, too. And the one left over will point out that he asked how to do this the FP way, not the imperative way? There's no difference, as you can't do time-accounting non-strict and still expect it to give meaningful results: I'm merely trying to be helpful. None of the other solutions allow for the IO Monad. Firstly, I'd phrase that differently -- the IO Monad doesn't allow for the other solutions -- the other solutions are the truly functional ones. Secondly, I'm curious as to why you think that the two are incompatible, are you saying that for any meaningful kind of computation we need to resort to IORefs? I'd strongly contest that idea. We have to resort to IO actions to get the time, and to IORefs because we need to chain up different calls to getCurrentTime using the IO Monad. The rest of the program can work with whatever you like best. And in what way is this incompatible with either FRP as pierre suggested, or with generating an infinite list of times at which we call the function, and scanning it to find the differences? Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Qualified import syntax badly designed (?)
I think a better design for namespacing might be: import Data.Map as M implicit (Map) import Data.Map as M explicit (lookup) Why 'implicit' and 'explicit'? Do you mean something like 'include' and 'exclude'? To me at least, implicit and explicit make more sense. I don't want to exclude importing lookup, I want to make it so I have to explicitly tag lookup as being M.lookup. Similarly, I don't want to include Map (as opposed to all the other things I'm getting from Data.Map), I just want to make it so that when I say Map, I implicitly mean M.Map. Personally I'd extend this syntax (something Neil may have had in mind), so that import Data.Map as M (lookup, union) implicit (Map) gives me M.lookup, M.union and Map while import Data.Map as M hiding (union) explicit (lookup) gives me everything in Data.Map with no qualification except for union and lookup, plus it gives me M.lookup. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.Derive problems
On 10 Jul 2008, at 21:25, Ron Alford wrote: On Thu, Jul 10, 2008 at 3:18 PM, Neil Mitchell [EMAIL PROTECTED] wrote: Hi Ron, I'm using GHC 6.8.3 with $ cabal --version cabal-install version 0.5.1 using version 1.4.0.1 of the Cabal library I installed Data.Derive from hackage, only to be unable to find the 'derive' binary! Did you do the runhaskell Setup configure runhaskell Setup build runhaskell Setup install? I used 'sudo cabal install derive'. I did find the binary - in my user's .cabal/bin directory! Odd that it should default to that when run as root. I don't have the darcs version working yet, though. I've found that cabal install doesn't actually install anything on my system either, despite going through all the motions. I just download the tarballs of hackage and config/build/install them myself, and I then end up with all the relevant bits installed. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] a newbie's question
On Apr 21, 2005, at 3:47 PM, SCOTT J. wrote: Hi, I'm beginning to study Haskell, For the following a = [1,2,3] b = there do x - a y - b return (x , y) Winhugs cannot run it. Gives Syntax error in input (unexpected backslash ( lambda)) Your problem is that you're using monads to grab the contents of a and b, while a and b are not monadic... You probably if you're only just setting out don't want to pay attention to any of the do notation or monadic code. To get the result it looks like you want, all you need to do is this: (a, b) you can then define this as a new constant: c = (a, b) Hope that helps Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Specify array or list size?
No, it introduces a variable of type array of 50 ints, which can be converted to pointer to int. It matters when you make a pointer of such arrays, an array of such arrays, or sizeof such array. In C++ the size can be matched by template parameter, and you can have separate overloadings for separate array sizes. I'm not familiar with your C++ example (not being familiar with C++), but I think that it's a bit of a stretch of the imagination to say that C introduces a variable of type array of 50 ints, the fact that this is now an array of 50 integers is never checked at any point in the compilation or run, and I'm not sure it can be even if KR had wanted to. If I'm thinking straight then *any* array definition merely gets re-written to a memory allocation of the relevant amount of ram, and beyond this point it is forever of type pointer to array content type. As an example: int bobsArray[5]; bobsArray[6] = 23; is not badly typed it is merely a badly broken program. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Specify array or list size?
On May 7, 2005, at 8:07 PM, Marcin 'Qrczak' Kowalczyk wrote: Thomas Davie [EMAIL PROTECTED] writes: I'm not familiar with your C++ example (not being familiar with C++), but I think that it's a bit of a stretch of the imagination to say that C introduces a variable of type array of 50 ints, the fact that this is now an array of 50 integers is never checked at any point in the compilation or run, and I'm not sure it can be even if KR had wanted to. The size is taken into account when such array type is an element of another array, and by sizeof. int (*p)[50]; /* p may legally point only to arrays of 50 ints each */ ++p; /* p is assumed to point into an array, and is moved by one element, i.e. by 50 ints */ I'm not sure what you're trying to prove by saying that... There is still no type information that says that the contents of p are an array of 50 elements... I can still attempt to access element 51 and get a runtime memory error. The type of p is still int**, not pointer to array of 50 ints As an example: int bobsArray[5]; bobsArray[6] = 23; is not badly typed - it is merely a badly broken program. Because the array size is not taken into account by indexing. But it's a part of the type. These issues are independent, for example in C# both are the opposite. I don't think it is part of the type... Does the compiler ever know any more about the type of bobsArray other than it's a pointer to an integer? I think that the above code can be directly translated to: int *bobsArray; bobsArray = (int *)malloc(5 * sizeof(int)); bobsArray[6] = 23 Which stores exactly the same about on type information. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Compiling with NHC98
Incidentally, if you aren't already familiar with make or some other build system, I strongly recommend looking into one. Even for a project with only two files, having a build system keep track of compilation dependencies makes things a lot less tedious. In random addition to this... hmake will automatically keep track of Haskell dependencies, and will also allow you to easily compile your code to work with the hat (hmake -hat) tracing suite. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type system extension
Hi, I'd just been writing some code and an interesting idea for an extension to Haskell's type system sprang into my head. I have no idea if people have played with it, but it looked vaguely useful to me, so I thought I'd see what everyone else thought. Supposing you have these types: type Export = String data SCode = SModule String [Export] | SUnknown It would be useful to specify a function as so: doSomethingToAModule :: SCode(SModule) - SomeRandomOtherType which would specify, not only that the first argument was of type SCode, but more specifically, that it used the SModule constructor. This would then allow you to safely only write a case for the SModule constructor, and not worry about a runtime error when pattern matching failed on an SUnknown (as this would be checked at compile time). I hope this makes sense What does anyone think of the idea, and is there an obvious flaw in the plan? Thanks Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type system extension
On May 16, 2005, at 12:46 AM, Neil Mitchell wrote: Hi, Yes, sounds like a good idea. I'm not sure the right approach is to make the user give this information though - the code will very likely be something like doSomethingToAModule (SModule a b) = f a b from which you can derive the type SCode(SModule) very easily. As the expressions get more complex, you will want more substantial annotations - i.e. SCode(SModule(_,[])|SUnknown) for something which either exports nothing, or is unknown. At this point getting the programmer to type in essentially the same information twice is likely to become annoying. I'm not certain I agree with you. Where I do agree is that the types are liable to get very complex fairly quickly, and this may well be the flaw in the plan, however, I think Haskell benefits greatly from asking the programmer to provide the same information twice in slightly different forms. The type system after all is essentially a method of providing a sanity check -- does the code actually match up with what the programmer specified as a type. My current work on my PhD is all related to checking that a Haskell program cannot raise a pattern match error, and it is accomplished in a similar sort of method to what you are suggesting, and achieves similar goals. This work is still ongoing, but a first order checker exists for a subset of Haskell already - so progress is being made. I haven't thought about this for more than half a day when the idea popped into my head, so obviously you've dealt with it a bit more, but I wonder if this is only half the problem. By the sounds of it you are doing type inferencing from the program (as you explained above), which allows for a certain level of checks. However, type errors are not only thrown when the type inference system can't generate types to fit the program, but also when the programmer has specified types that are different to that the inference worked out. Thanks for a very interesting reply Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type extensions
Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not... Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. So I would eno up with three data types data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag data MyBwTaggedTree = Branch BwTag Int MyBwTaggedTree MyBwTaggedTree | Node BwTag Int and several functions to move from one to another. (Or define the most complex and not always use all the attrdbutes). What I would prefer is to be able to spocify something like: data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree extends MyTree with BwTag=True data MyBwTaggedTree extends MyBwTree with Int=0 Specifying that myBwTree is the same as MyTree but with an extra argument on each node, and to generate functions to translate between them that fill in True as a default value for the extra tag. Is this possible? Thanks Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type extensions
On 1 Jun 2005, at 15:54, Henning Thielemann wrote: On Wed, 1 Jun 2005, Thomas Davie wrote: Hi, I was wondering if I hat missed something and it was possible to do this within the Haskell type system or not... Essentially I would like some sort of inderritance property for Haskell types, I often find myself wanting to for example extend a tree with black/white colouring, or later extend the tree with some sort of ID, etc. So I would eno up with three data types data MyTree = Branch MyTree MyTree | Leaf type BwTag = Bool data MyBwTree = Branch BwTag MyBwTree MyBwTree | Node BwTag What about data MyTree a = Branch a (MyTree a) (MyTree a) | Node a and the types MyTree () MyTree Bool MyTree (Bool, Int) ? That's exactly what I would normally do, but my data type is in a library and is not parameterised. Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monadic vs pure style (was: pros and cons of sta tic typing and side effects)
On Aug 30, 2005, at 12:13 PM, Bayley, Alistair wrote:From: Duncan Coutts [mailto:[EMAIL PROTECTED]] This is often a misconception, that just because you find you need to'do' something in the middle of your algorithm, that you need to convert it wholly to monadic style. Yes. However, Wadler makes a convincing (at least to me) case that themonadic style is easier to extend. The code changes for the monadic styleappear to be more localised.Something else I noticed about my non-monadic code was the way I wasthreading state through functions. I was tempted to introduce a State monadto make this easier to manage, but then I decided to try mutable arraysinstead, so that experiment was not attempted. So it might well have beenbetter in monadic style anyway, even with immutable arrays.I'm conscious that for most (?) monads, monadic code can be invoked fromnon-monadic code. I'm only aware of the IO monad as a one-way trap. Sochanging code from pure to monadic doesn't necessarily involve program-widechanges, unless the monad you're introducing happens to be IO. In my arrayexample, I introduced STArrays, but the main interface remained pure(non-monadic), which was my goal.I was also wondering what the disadvantages of monadic style are? Are therecompiler optimisations which are not possible with monadic code?Both the advantage and the disadvantage is that you break lazy evaluation. 90% of the time lazyness is your friend and monadifying your code can break some nice features, but there is an occasional 10% of the time when it's useful to break lazyness.On a side note, whenever I find myself tempted to pass state around, I consider whether using CPS is better... It provides some method of ordering code, but doesn't break lazyness.Just 2¢ from a relative newbie.Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functional vs Imperative
On 13 Sep 2005, at 14:45, Dhaemon wrote:
Hello,
I'm quite interested in haskell, but there is something I don't
understand(intuitively). I've been crawling the web for an answer,
but nothing talks to me...
So I was hoping I could find some help here:
How is evaluating an expression different from performing action?
I'm puzzled... Doesn't it amount to the same thing? Maybe I have a
wrong definition of evaluating(determine the value of an
expression)?
Examples would be appreciated.
Also, just for kicks, may I had this: I read the code of some
haskell-made programs and was astonished. Yes! It was clean and
all, but there were dos everywhere... Why use a function language
if you use it as an imperative one?(i.e. most of the apps in http://
haskell.org/practice.html)
The difference is all about referential transparency -- in short, a
function given the same inputs will always give the same result.
This is not the same as in imperative languages, where functions/
methods/actions can have 'side-effects' that change the behavior of
the rest of the program.
Take this example:
C program:
#define square(x) ((x) * (x))
#define inc(x) ((x)++)
int myFunc (int *x)
{
return square(inc(*x));
}
the C preprocessor will re-write the return line to:
return x)++)) * (((x)++)));
this will be performed in sequence, so, x will be incremented
(changing the value of x), and that result will be multiplied by x
incremented again.
so if we run myFunc(y), where y is 5, what we get is 5 incremented
to 6, and them multiplied by 6 incremented to 7. So the result of
the function is 42 (when you might reasonably expect 36), and y is
incremented by 2, when you might reasonably expect it to be
incremented by 1.
Haskell program:
square x = x * x
inc = (+1)
myFunc = square . inc
and we now call myFunc 5, we get this evaluation:
myFunc 5 is reduced to (square . inc) 5
(square . inc) 5 is reduced to square (inc 5)
square (inc 5) is reduced to square ((+1) 5)
square ((+1) 5) is reduced to square 6
square 6 is reduced to 6 * 6
6 * 6 is reduced to 36
If you want to study these reductions on a few more examples, you
might want to download the Hat tracer, and use hat-anim to display
reductions step by step.
Bob
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Re: [Haskell-cafe] Functional vs Imperative
On 13 Sep 2005, at 16:22, David Roundy wrote: On Tue, Sep 13, 2005 at 01:45:52PM +, Dhaemon wrote: Also, just for kicks, may I had this: I read the code of some haskell-made programs and was astonished. Yes! It was clean and all, but there were dos everywhere... Why use a function language if you use it as an imperative one?(i.e. most of the apps in http://haskell.org/practice.html) Monadic code isn't synonymous with imperative code, and do only indicates that you're looking at monadic code. The Maybe monad is an example of a very useful, very non-imperative monad that can be used to cleanly write functional code. On the other hand, IO is always monadic, so perhaps you're looking at IO code. But I'd assert that even monadic IO code isn't quite the same as true imperative code. I'd probably say that the difference has to do with whether you create modifiable variables. When you start doing that, whether you're in the ST monad or the IO monad, I think you're writing imperative-style code. But I think that that sort of usage is actually pretty uncommon. I would tend to argue that even in those monads you aren't really writing imperative style code -- you still can't have side effects. The point of the monad is that it preserves referential transparency while doing something ordered. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbe question
The reason is that you can define =- as on operator so for example, in this (obfuscated) code: (=-) x y = x * y sq y = y =- y Thus, in your code, you had an operator on the LHS of the definition, and the interpreter baulked at it. Bob On 27 Sep 2005, at 10:34, [EMAIL PROTECTED] wrote: Hi i can not load program test1 into hugs, but test2 works. Am i missing some special syntax? greetings, Philip -- test1 -- foo :: Maybe Int - Int foo Nothing =-1 foo (Just a)= a -- test2 -- foo :: Maybe Int - Int foo Nothing = -1 -- ^ -- +-- note an extra space foo (Just a)= a ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbe question
On 27 Sep 2005, at 16:53, [EMAIL PROTECTED] wrote: On 27 Sep, Wolfgang Jeltsch wrote: Hello, obviously, Hugs thinks that =- is a special operator. In Haskell you have the ability to define your own operators, so it would be possible to define an operator =-. I would suggest that you always put spaces around the = in declarations. Best wishes, Wolfgang Hello, thank you for fast reply. Ok, but what is the semantic of '=-' ? If it's an operator, it should have some impact (right term?). The semantics are whatever you define them to be: (=-) x y = doSomeFunkyStuff x y Note that this also introduces problems with comments, a common mistake people make is to not put a space after the -- comment symbol, so they may end up with: --| something The compiler then interprets --| as an operator. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] how to break foldl' ?
On 30 Sep 2005, at 11:33, gary ng wrote: Hi, say if I want to sum a list of numbers but only until it hits a max limit. Currently, I control it through the function and basically do nothing when the max is hit. However, if the list is very long, would this mean the same function would be called for the rest of the list which can be a waste of cycle ? In an imperative language, I just break/return in the middle of the loop. No - lazy evaluation guarantees that if a reduct is never needed, it is never reduced. So as your function never needs the latter values in the list, it is never evaluated. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] how to break foldl' ?
Again, it depends how takeWhile is implemented -- if it's not tail recursive, the compiler will usually manage to run such functions in constant space. Bob On 30 Sep 2005, at 16:02, gary ng wrote: Once again, many thanks to all who taught me about this small little problem. Don't even know there is init/last and thought there is only head/tail. But just for my curiosity, would the takeWhile still store the intermediate result till my result is reached ? If so, and my list is really very long(and I need to go to 1/2 of its length), I would still use a lot more memory than imperative method or even the foldl one(where in both case, I just take one element) ? --- Henning Thielemann [EMAIL PROTECTED] wrote: No problem: last (takeWhile (maxX) (scanl (+) 0 xs)) Convinced? The first sum which exceeds the limit could be computed with head (dropWhile (=maxX) (scanl (+) 0 xs)) __ Yahoo! Mail - PC Magazine Editors' Choice 2005 http://mail.yahoo.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] file i/o
The other thing to mention, is that if you have the ability to change file formats, it may be better to make just a slight adjustment... If you make it look exactly like the haskell data structure you want: [(Foo, [1,2,3,4,5,6,7]) ,(Bar, [7,6,5,4,3,2,1]) ,...] Then your parser becomes even simpler: parseFile :: FilePath - IO [(String,[Int])] parseFile = do src - readFile x return $ read src On Jan 3, 2006, at 11:33 AM, Neil Mitchell wrote: Hi Robert, The first thing to mention is that Haskell uses linked-lists, not arrays as the standard list type structure, so [1,2] is actually a linked list. The next thing to note is that Haskell is *lazy*. It won't do work that it doens't have to. This means that you can return a linked list with all the lines in the file, but they won't actually be read til they are required. i.e. Haskell cleverly worries about all the getting a next line as required stuff, without you even noticing - it will read it line by line. A simple function that does some of what you want is: parseFile :: FilePath - IO [(String, [Int])] parseFile x = do src - readFile x return (map parseLine (lines src)) parseLine :: String - (String, [Int]) parseLine = for you to write :) The other point is that Haskell linked lists have to have every element of the same type, so you can't have [test,1] as a linked list, what you actually want is a tuple, written (test,1) - a tuple is of fixed length and all elements can be of different type. Hope that helps, Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Can I use Haskell for web programming
On Jan 21, 2006, at 8:34 PM, Maurício wrote: They both look cool. Do you think I'll be able to find someone to host professional sites using those libraries? [], Maurício Try http://contextshift.co.uk/vps.html ... or I guess any other virtual server hosts, but they're cheep and good. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lists of Lists
On 8 Mar 2006, at 14:21, zell_ffhut wrote: Thank you, It's working as planed now Trying to do a function now that changes the value of an element of the list. In programming languages i've used in the past, this would be done somthing like - changeValue x i [xs] = [xs] !! i = x where x is the value to change to, i is the index of the value to change, and [xs] is the list. This however, dosen't work in Haskell. How would this be done in Haskell? Put simply it isn't. One of the percepts of a functional language is that variables are bound, not set - once a variable has a value it has that value forever. What you want to do is return a new list, that looks like the old one, but has one value changed changeValue x 0 (y:ys) = (x:ys) changeValue x n (y:ys) = y:(changeValue x (n-1) ys) Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Porting GHC to OSX86?
On Mar 21, 2006, at 8:09 PM, Deling Ren wrote: Hi there, Has anyone made any attempt to port GHC to Mac OS X on x86? Wolfgang Thaller’s binary package runs over Rosetta but slow (not surprising). It can not be used to compile a native version either (I got some errors related to machine registers). I tried to do a bootstrap but can't find the .HC files mentioned in the manual. They don't seem to be on the download page of GHC. Any ideas? Why not use darwin ports to build it? Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Editors for Haskell
On May 25, 2006, at 6:14 PM, Jeremy O'Donoghue wrote: Hi Walt, I'm using Haskell (GHC and Hugs) on several different platforms. Windows, OS X and Linux systems. Assuming that you want your students to be able to use any of the above platforms, the only options I know of which work well on all of the platforms are Emacsen, Vim, hIDE, Eclipse and JEdit. I'd like to have an IDE that works well for medium to large size projects. I know of Eclipse and hIDE. Vim works fine but I'd like more. hiDE seems to be in process. When working on Macs I've found SubEthaEdit to be by far the best Haskell editor, emailing the guy tends to have quite good results in terms of getting it free if you say you're involved in education. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Can we come out of a monad?
On 11 Aug 2010, at 12:39, Ertugrul Soeylemez wrote: Martijn van Steenbergen mart...@van.steenbergen.nl wrote: On 8/2/10 7:09, Ertugrul Soeylemez wrote: Given the definition of a Haskell function, Haskell is a pure language. The notion of a function in other languages is not: int randomNumber(); The result of this function is an integer. You can't replace the function call by its result without changing the meaning of the program. I'm not sure this is fair. It's perfectly okay to replace a call randomNumber() by that method's *body* (1), which is what you argue is okay in Haskell. This is not the same. In Haskell you can replace the function call by its /result/, not its body. You can always do that. But the result of an IO-based random number generator is an IO computation, not a value. It's not source code either, and it's not a function body. It's a computation, something abstract without a particular representation. It's still rather papering over the cracks to call this pure though. The IO based computation itself still has a result that you *can't* replace the IO based computation with. The fact that it's evaluated by the runtime and not strictly in haskell may give us a warm fuzzy feeling inside, but it still means we have to watch out for a lot of things we don't normally have to in a very pure[1] computation. Bob [1] Bob's arbitrary definition 1 – very pure computations are ones which can be replaced with their result without changing the behavior of the program *even* if said result is computed in the runtime and not by the Haskel program.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Can we come out of a monad?
On 11 Aug 2010, at 14:17, Ertugrul Soeylemez wrote: There is a fundamental difference between an IO computation's result and a Haskell function's result. The IO computation is simply a value, not a function. That's a rather odd distinction to make – a function is simply a value in a functional programming language. You're simply wrapping up we're talking about haskell functions when we talk about referential transparency, not about IO actions in a way that maintains the warm fuzzy feeling. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] On to applicative
On 26 Aug 2010, at 08:01, michael rice wrote: Hmm... it was my understanding that the example was showing how to *avoid* having to create a lot of functions that do the same thing but have different numbers of arguments. From the Wiki page: Anytime you feel the need to define different higher order functions to accommodate for function-arguments with a different number of arguments, think about how defining a proper instance of Applicative can make your life easier. Not so? Very much so – instead of defining liftA2, liftA3 etc like this, just use pure to get things into the applicative, and write * instead of ' ' to apply applicatives and you're done. Don't write liftA3 sumsq (Just 3) (Just 4) (Just 5) Write (pure sumsq) * (pure 3) * (pure 4) * (pure 5) or you can get rid of that first pure with a quick fmap: sumsq $ (pure 3) * (pure 4) * (pure 5) Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Do expression definition
On 13 Sep 2010, at 10:28, Gleb Alexeyev wrote: On 09/13/2010 12:23 PM, Michael Lazarev wrote: 2010/9/13 Henning Thielemannlemm...@henning-thielemann.de: It means that variables bound by let, may be instantiated to different types later. Can you give an example, please? testOk = let f = id in (f 42, f True) --testNotOk :: Monad m = m (Int, Bool) --testNotOk = do f - return id -- return (f 42, f True) Try uncommenting the 'testNotOk' definition. There's no later here at all. Two seperate definitions in a Haskell program act as if they have always been defined, are defined, and always will be defined, they are not dealt with in sequence (except for pattern matching but that doesn't apply here). Instead, what's going on here is scoping. The f in testOk is a different f to the one in testNotOkay, distinguished by their scope. Finally, this is not how you use a let in a do expression, here's how you should do it: testOk2 :: Monad m = m (Int, Bool) testOk2 = do let f = id return (f 42, f True) Thanks Tom Davie___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generating arbitrary functions with QuickCheck?
On 15 Sep 2010, at 16:29, Matias Eyzaguirre wrote: Hi, I'v been reading a small paper/lesson on writing parser combinators in Haskell, and it seems more or less straightforward. In this case a parser is defined thusly: type Parser a = String - Maybe (a, String) And then it goes on to list some simple parsers, and then starts going on about combinators. I was wondering how one would write quickcheck properties for the items presented in the paper, and the answer seemed fairly straightforward for the actual parsers. But how on earth would you write a test for a combinator? Presumable one would need to make the type an instance of Arbitrary. I see two problems: Firstly, as far as i can tell, one cannot declare a type synonym to be an instance of a type class, thus how would you make it an instance of Arbitrary? The standard solution here is to create a newtype, and generate them instead. Secondly, (and more importantly, or at least more interesting) I can see how one would make a generator for simple compound data types, but how on earth do you make a generator produce functions? With some difficulty, but it can be done. You could for example, select at random, a combinator or axiomatic parser, on selecting a combinator, generate a pair of parsers to hand to it with a slightly lower chance of generating a combinator and stick them together. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: tls, native TLS/SSL protocolimplementation
While I agree with the potential benefits, I also worry that you will end up making something that is far less well tested in practice. For widely used and fairly low-level libraries like gnutls, openssl and zlib, I'm just skeptical that the benefits outweigh the risks and costs. Anyway, it's just a feeling. Please do prove me wrong :-) This certainly isn't a proof by a long shot, but my feeling on at least low-level libraries is exactly the reverse of this. C libraries are usually designed to be extremely stateful (this certainly includes openssl), and because of that any Haskell wrapper for them ends up being heavily IO based. The result of this is that any code that incorporates it ends up being trapped in an IO mess to do essentially pure (yes, I know the arguments about IO being pure, you know what I mean) things. It's precisely these libraries that we need not just implemented in a native way, but designed in a pure, beautiful, simple way for Haskell. While I can see your point about potentially introducing new security holes, and producing much less trusted code, I feel having tidy, pure libraries that we can all integrate into our Haskell is a benefit that far outweighs this. Especially when we have nice things like the type system, which can be used to alleviate many of the security worries. Tom Davie___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Serialization of (a - b) and IO a
On 11 Nov 2010, at 08:36, Luke Palmer wrote: On Thu, Nov 11, 2010 at 12:53 AM, Jesse Schalken jesseschal...@gmail.com wrote: I have had a look at hs-plugins, but it is unclear how to derive a simple pair of functions `(a - b) - ByteString` and `ByteString - Either ParseError (a - b)`, for example, from the functionality it provides, if it is possible at all. I guess such a thing requires thorough digging into the depths of GHC, (or maybe even LLVM if an architecture independent representation is sought, but I don't know enough to say.). Perhaps this is more a question for those interested and knowledgable in Haskell compilation (and, to some extent, decompilation). If not Haskell, are there any languages which provide a simple serialization and deserialization of functions? As far as I know, GHC has no support for this. There are issues with the idea that will come out pretty fast, such as: (1) Those cannot be pure functions, because it differentiate denotationally equal functions. So it would have to be at least (a - b) - IO ByteString. I don't think I agree, I didn't see a rule f == g = serialise f == serialise g anywhere. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why is Haskell flagging this?
On 17 Dec 2010, at 21:44, Christopher Done wrote: On 17 December 2010 18:04, michael rice nowg...@yahoo.com wrote: === f :: [Int] - IO [Int] f lst = do return lst main = do let lst = f [1,2,3,4,5] fmap (+1) lst The problem is that you are applying fmap to a type IO a. fmap (+1) (return [1,2,3]) But to achieve the behaviour you expect, you need another fmap: fmap (fmap (+1)) (return [1,2,3]) Which can be more neatly written with Conal's semantic editor cominators as (fmap . fmap) (+1) (return [1,2,3]) Of course, I question why the list is put in the IO monad at all here... surely this would be much better return $ fmap (+1) [1,2,3] Finally, that has the wrong type for main... perhaps you meant to print it out? main :: IO () main = print $ fmap (+1) [1,2,3] Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC optimizations and unsafePerformIO - Was: Data.Typeable TypeRep Ord instance.
On 1 Jan 2011, at 12:38, Andreas Baldeau wrote: Thinking about this there might be one problem: Without having looked further into this I think perfomance might not be as expected. Using unsafePerformIO affects ghc's optimzations, doesn't it? So I wonder if it's a good idea (from a performance point of view) to use this. 2010/12/30 Andreas Baldeau andr...@baldeau.net: instance Ord TypeRep where compare t1 t2 = compare (unsafePerformIO (typeRepKey t1)) (unsafePerformIO (typeRepKey t2)) typeRepKey :: TypeRep - IO Int typeRepKey (TypeRep (Key i) _ _) = return i So the question is, if ghc could transform this to simply compare the keys throwing away unsafePerformIO and return. Wouldn't a much better plan simply be to take typeRepKey out of the IO monad? Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 18 Apr 2009, at 22:44, Antoine Latter wrote: On Sat, Apr 18, 2009 at 9:03 AM, a...@cs.uu.nl wrote: Utrecht Haskell Compiler -- first release, version 1.0.0 The UHC team is happy to announce the first public release of the Utrecht Haskell Compiler (UHC). UHC supports almost all Haskell98 features plus many experimental extensions. The compiler runs on MacOSX, Windows (cygwin), and various Unix flavors. Features: * Multiple backends, including a bytecode interpreter backend and a GRIN based, full program analysing backend, both via C. * Experimental language extensions, some of which have not been implemented before. * Implementation via attribute grammars and other high-level tools. * Ease of experimentation with language variants, thanks to an aspect-oriented internal organisation. Getting started Download -- UHC is available for download as source distribution via the UHC home page: http://www.cs.uu.nl/wiki/UHC Here you will also find instructions to get started. Status of the implementation Like any university project UHC is very much work in progress. We feel that it is mature and stable enough to offer to the public, but much work still needs to be done; hence we welcome contributions by others. UHC grew out of our Haskell compiler project (called Essential Haskell Compiler, or EHC) over the past 5 years. UHC internally is organised as a combination of separate aspects, which makes UHC very suitable to experiment with; it is relatively easy to build compilers for sublanguages, or to generate related tools such as documentation generators, all from the same code base. Extensions to the language can be described separately, and be switched on or of as need arises. Warning --- Although we think that the compiler is stable enough to compile subtantial Haskell programs, we do not recommend yet to use it for any serious development work in Haskell. We ourselves use the GHC as a development platform! We think however that it provides a great platform for experimenting with language implementations, language extensions, etc. Mailing lists - For UHC users and developers respectively: http://mail.cs.uu.nl/mailman/listinfo/uhc-users http://mail.cs.uu.nl/mailman/listinfo/uhc-developers Bug reporting - Please report bugs at: http://code.google.com/p/uhc/issues/list The UHC Team -- Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/ ~atze . /--| \ Fax : +31-30-2513971 | Email: a...@cs.uu.nl / | ___\ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe After running ./configure on my Intel Mac (running OS 10.5.6 with GHC 6.10), I try to run make uhc and get the following: $ make uhc src/ruler2/files.mk:34: build/ruler2/files-ag-d-dep.mk: No such file or directory src/ruler2/files.mk:35: build/ruler2/files-ag-s-dep.mk: No such file or directory mkdir -p build/shuffle ; \ --module=CDoc -dr -Psrc/shuffle/ -o build/shuffle/CDoc.hs src/ shuffle/CDoc.ag /bin/sh: --module=CDoc: command not found make: Failed to remake makefile `build/ruler2/files-ag-s-dep.mk'. make: Failed to remake makefile `build/ruler2/files-ag-d-dep.mk'. make EHC_VARIANT=`echo install/101/bin/ehc | sed -n -e 's+install/\([0-9_]*\)/bin/ehc.*+\1+p'` ehc-variant src/ruler2/files.mk:34: build/ruler2/files-ag-d-dep.mk: No such file or directory src/ruler2/files.mk:35: build/ruler2/files-ag-s-dep.mk: No such file or directory mkdir -p build/shuffle ; \ --module=CDoc -dr -Psrc/shuffle/ -o build/shuffle/CDoc.hs src/ shuffle/CDoc.ag /bin/sh: --module=CDoc: command not found make[1]: Failed to remake makefile `build/ruler2/files-ag-s-dep.mk'. make[1]: Failed to remake makefile `build/ruler2/files-ag-d-dep.mk'. make EHC_VARIANT_RULER_SEL=((101=HS)).(expr.base patexpr.base tyexpr.base decl.base).(e.int e.char e.var e.con e.str p.str) \ ehc-variant-dflt src/ruler2/files.mk:34: build/ruler2/files-ag-d-dep.mk: No such file or directory src/ruler2/files.mk:35: build/ruler2/files-ag-s-dep.mk: No such file or directory mkdir -p build/shuffle ; \ --module=CDoc -dr -Psrc/shuffle/ -o build/shuffle/CDoc.hs src/ shuffle/CDoc.ag /bin/sh: --module=CDoc: command not found make[2]: Failed to remake makefile `build/ruler2/files-ag-s-dep.mk'. make[2]: Failed to remake makefile `build/ruler2/files-ag-d-dep.mk'. make[1]: *** [ehc-variant] Error 2 make: *** [install/101/bin/ehc] Error 2 This is fairly bewildering. Am I the only one seeing errors like this? This looks like the same error I got – see bug report 1 in the bug
Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 19 Apr 2009, at 00:31, Antoine Latter wrote: On Sat, Apr 18, 2009 at 4:38 PM, Thomas Davie tom.da...@gmail.com wrote: This looks like the same error I got – see bug report 1 in the bug database – the configure script reports that you have uuagc even if you don't – cabal install it, reconfigure, and you should be on your way. Second thing to watch for – it depends on fgl, but this isn't caught by the configure script. Apparently a user install of uuagc and fgl isn't good enough. Fun to know. I've found user installs don't work at all on OS X, various people in #haskell were rather surprised to discover this, so apparently it's not the default behavior on other platforms. It really rather makes cabal install rather odd – because it doesn't actually install anything you can use without providing extra options! Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 19 Apr 2009, at 09:52, Duncan Coutts wrote: On Sun, 2009-04-19 at 00:41 +0200, Thomas Davie wrote: Apparently a user install of uuagc and fgl isn't good enough. Fun to know. I've found user installs don't work at all on OS X, various people in #haskell were rather surprised to discover this, so apparently it's not the default behavior on other platforms. Currently, user installs are the default on all platforms except Windows. It really rather makes cabal install rather odd – because it doesn't actually install anything you can use without providing extra options! It should work fine, you'll need to give more details. This has been the result, at least every time I've installed ghc: $ cabal install xyz $ runhaskell Setup.hs configure -- where abc depends on xyz Configuring abc-0.0... Setup.lhs: At least the following dependencies are missing: xyz -any $ sudo cabal install --global xyz $ runhaskell Setup.hs configure Configuring abc-0.0... $ runhaskell Setup.hs build ... Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 19 Apr 2009, at 11:10, Duncan Coutts wrote: On Sun, 2009-04-19 at 10:02 +0200, Thomas Davie wrote: It really rather makes cabal install rather odd – because it doesn't actually install anything you can use without providing extra options! It should work fine, you'll need to give more details. This has been the result, at least every time I've installed ghc: $ cabal install xyz So this does a per-user install. $ runhaskell Setup.hs configure -- where abc depends on xyz This does a global install. Global packages cannot depend on user packages. You have two choices: $ cabal configure because the cabal program does --user installs by default or use $ runhaskell Setup.hs configure --user which explicitly does a --user install. The reason for this confusion is because the original runghc Setup interface started with global installs and we can't easily change that default. On the other hand, per-user installs are much more convenient so that's the sensible default for the 'cabal' command line program. I don't understand what makes user installs more convenient. Certainly, my preference would be for global all the time – I expect something that says it's going to install something to install it onto my computer, like any other installation program does. What is it that makes user installs more convenient in this situation? Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
I don't understand what makes user installs more convenient. Certainly, my preference would be for global all the time – I expect something that says it's going to install something to install it onto my computer, like any other installation program does. What is it that makes user installs more convenient in this situation? You don't need 'sudo' access for user installs. This means that 'cabal install' works out of the box on every system, without needing admin/root privs (esp. important for students). But students will be used to needing to configure this – in every other installation system out there they need to tell it to install in their user directory. Personally – I find it more convenient to have the install program do what it says it does! Install it! This would save confusion about old tools that do things globally, and not confuse students, because they're all already used to giving extra flags to make install not install things system wide. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Optimizing unamb by determining the state of a thunk?
On 20 Apr 2009, at 09:41, Peter Verswyvelen wrote: I was wandering if it would be possible to optimize unamb by checking if a value is already evaluated to head normal form. So f `unamb` g would then be extremely fast if either f or g is already evaluated to head normal form. Maybe using some vacuum tricks? This function would need to be in IO since it is of course not referentially transparent. Really? Is it any less referentially transparent than unamb already is - i.e. it's referentially transparent, as long as the two values really are equal. Although threads are lightweight in Haskell, forking/waiting/killing surely must have more overhead than just checking the thunk of an expression? Of course one could also make unamb a primitive :-) That would be a lovely solution for me. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Optimizing unamb by determining the state of a thunk?
On 20 Apr 2009, at 10:57, Peter Verswyvelen wrote: On Mon, Apr 20, 2009 at 10:23 AM, Thomas Davie tom.da...@gmail.com wrote: Really? Is it any less referentially transparent than unamb already is - i.e. it's referentially transparent, as long as the two values really are equal. I think it is. Suppose we call the function hnf :: a - Bool. hnf might return a different result for the same argument, since the evaluation of the argument might be happening on a different thread, so the result of hnf depends on the time when it is evaluated. Or am I missing something here? Sure, so hnf would give us a non-determined result, but I don't think that makes unamb any less referentially transparent – the same value is always returned, and always reduced at least to hnf. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Cabal's default install location
There seems to be an assumption amongst the community that a user's home directory is the most useful place for cabal to install to by default. A few people have challenged that. I wanted to find out which one most people do actually prefer, so please go and vote on this poll. http://noordering.wordpress.com/2009/04/21/cabals-default-install-location/ It's no more than a straw poll, I don't know how protected it is against ballot stuffing, but I'm pretty confident that Haskellers are trustworthy enough not to play with it that way. I'll tell you all the result next week. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Being impure within a 'pure' function
On 22 Apr 2009, at 10:38, Daniel K. wrote: Hello, imagine the following situation: You want to implement e.g. Dijkstra's algorithm to find a shortest path between nodes u and v in a graph. This algorithm relies heavily on mutating arrays, so the type signature would look something like getDistance :: Graph - Node - Node - IO Int Not mutating the underlying arrays would probably result in poor performance. BUT: For a constant graph, the distance between two nodes stays the same all the time, so in fact getDistance should be a pure function! So here is my question: Is there a way to write functions in Haskell that do some IO internally, but that are guaranteed to be side- effect free? Of course one would have to make sure that the array that is mutated inside getDistance must not be accessible from outside the function. Is that possible? If not, wouldn't that be desirable? If not, why not? Either, as Eugene suggested, use the ST monad, as is possible in this case (and much better than the solution I'm proposing), or if you *really* can't get out of using IO, use unsafePerformIO. You will though have to provide several guarantees yourself that the type system would normally provide for you (hence the unsafe part - it should really be verifyItsSafeYourselfPerformIO). Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Cabal's default install location
[Moved from the UHC thread – lets stop treading on those guys toes, they did something very very shiny] On 23 Apr 2009, at 07:02, Richard O'Keefe wrote: It's irrelevant, because I _do_ have root access to my machine, How nice to be you. Since the argument is entirely about people who _don't_, your point it? His point is that that kind of person is not the only kind of person, so to base an argument on what they want is as weak as basing an argument on what he wants. It is clear that the only sensible default is no default. That sounds pretty sensible to me too – much like darcs asks what your email address is the first time you work on a repository, cabal should probably ask the first time you run it do you prefer global or user installs? I think the right question is how many people prefer user installs over system installs, wrt. their hackage packages?. No, because the costs are asymmetric. I think this is a case of not seeing the costs to the other users because you're firmly entrenched in your camp. I would have said originally that the costs are asymmetric too – but that it's a much greater cost for the people who expect all installers to do global installs. So I think that the question asked there is a very valid one. However, I do like the solution of not giving any default. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
The results in the poll seem to have stabilised now, so I'll tell you what happened... For user installs: 103 For global installs: 52 Others: 9 Interesting Ideas: • Claus made the suggestion that there be no default, instead that cabal asks you which you prefer the first time you run, or directs you to the configuration file. • Many users made the suggestion that cabal should install globally when running as root, and for a user when running as a user, possibly also displaying a prominent message about what's going on. Conclusions: -- Most people do indeed want user installs, but a very much not insignificant population want global installs. Perhaps one of these other suggestions is the way forward? Bob p.s. I personally very much like Claus' idea.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
On 23 Apr 2009, at 10:02, Matthijs Kooijman wrote: Some material I've read on typography -- can't find the reference now -- suggests ~65 is the best number of characters per line. The advice was, if your page is larger than that, you should make columns. That fits my observations. In particular, I noticed that your emails were particularly comfortable to read, which might also be partly be caused by the extra indent at the start of your lines, which also seems comfortable. Not sure how applicabable all this is to code, though :-) I think the non-applicable to code observation is very likely true – we'd like to be able to write nice descriptive variable names. In doing this, we probably want them to be more than the 1 or 2 characters that Haskellers traditionally use, maybe of the order of 5-10. Given this, it would seem a shame to only be able to fit 6-13 litterals on a line, that sounds like we'll quickly be having to wrap lines with deffinititions of any significance on them. My personal preference with Haskell is to ignore the 78 character limit, but only when layout otherwise becomes horrible otherwise. Haskell is a very horizontal language, and to limit our horizontal space seems pretty weird. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
On 25 Apr 2009, at 10:51, Daniel Fischer wrote: Am Samstag 25 April 2009 08:48:16 schrieb Thomas Davie: On 24 Apr 2009, at 14:37, Loup Vaillant wrote: 2009/4/23 Miguel Mitrofanov miguelim...@yandex.ru: On 23 Apr 2009, at 12:17, Thomas Davie wrote: Haskell is a very horizontal language, and to limit our horizontal space seems pretty weird. +1. I sometimes use lines up to 200 characters long, when I feel they would be more readable. 200 sounds awfully long. Do you have any example? Sure... arrow :: forall (~) b c d e. ( Arrow (~), Show (d ~ e), Show (c ~ d), Show (b ~ c), Show b, Show c, Show d, Show e, Arbitrary (d ~ e), Arbitrary (c ~ d), Arbitrary (b ~ c), Arbitrary b, Arbitrary c, Arbitrary d, Arbitrary e, EqProp (b ~ e), EqProp (b ~ d), EqProp ((b,d) ~ c), EqProp ((b,d) ~ (c,d)), EqProp ((b,e) ~ (d,e)), EqProp ((b,d) ~ (c,e)), EqProp b, EqProp c, EqProp d, EqProp e) = b ~ (c,d,e) - TestBatch . In all seriousness though, that one got broken, but I do find that I occasionally have lines around 100 characters that just look silly if I break them, this is a good example: filterNonRoots (GCase e bs) = filter ((/= e) ^()^ (not . (`elem` bs))) Not that I'd deny that it can sometimes be more readable to have longer lines*, but in this example, would filterNonRoots (GCase e bs) = filter ((/= e) ^()^ (not . (`elem` bs))) be any less readable in your opinion? Yes – this particular line is mixed in with several other pattern matches, each of which has a similar form, laying it out on one line lets you see the similarities and differences, laying it out on two lines creates visual noise. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
On 25 Apr 2009, at 21:09, Jason Dusek wrote: There will always be some people who prefer longer lines. The real issue is, how do we deal with the fundamental disagreement here? It's not like we can have both. Also those people who like long lines -- will they all agree to a long line length? Is there a fundamental disagreement here? There are those who are driven by an archaic standard from the width of the terminal screen, and there are those who are driven by the aesthetics of their code. As always, opinions on aesthetics differ slightly, but overall, everyone seems to mostly agree that we should try to keep lines short where possible, but lengthen then when needed. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] applicative challenge
On 4 May 2009, at 23:15, Thomas Hartman wrote:
{-# LANGUAGE NoMonomorphismRestriction #-}
import Data.List
import Control.Monad
import Control.Applicative
-- Can the function below be tweaked to quit on blank input,
provisioned in the applicative style?
-- which function(s) needs to be rewritten to make it so?
-- Can you tell/guess which function(s) is the problem just by looking
at the code below?
-- If so, can you explain what the strategy for doing so is?
notQuiteRight = takeWhile (not . blank) $ ( sequence . repeat $
echo )
echo = do
l - getLine
putStrLn l
return l
-- this seems to work... is there a way to make it work Applicatively,
with lifted takeWhile?
seemsToWork = sequenceWhile_ (not . blank) (repeat echo)
sequenceWhile_ p [] = return ()
sequenceWhile_ p (mx:mxs) = do
x - mx
if p x
then do sequenceWhile_ p mxs
else return ()
Conor's already give you a comprehensive explanation of why
Applicative can't be used to do this, but that doesn't mean you can't
use applicative style!
How about...
echo = unlines . takeWhile (not . blank) . lines
seemsToWork = interact echo
Bob
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Re: [Haskell-cafe] How to understand the fmap here ?
On 5 May 2009, at 11:27, z_axis wrote: The following code snippets is from xmonad: -- Given a window, find the screen it is located on, and compute -- the geometry of that window wrt. that screen. floatLocation :: Window - X (ScreenId, W.RationalRect) --... rr - snd `fmap` floatLocation w Prelude :i fmap class Functor f where fmap :: (a - b) - f a - f b It seems it is different from the definition of fmap ? sincerely! As the type signature of fmap explains, it transforms a function. Specifically, it starts with a function (a - b), and it transforms it to accept an 'a' inside a functor instead of just an a, and return a 'b' inside the same functor instead of just a b. In other words, fmap applies functions inside containers. We can see from floatLocation that it returns a pair inside a container - specifically, an X container. Fmap takes snd, and transforms it to work on values inside the X. So, snd has type (a,b) - b, thus fmap snd has type f (a,b) - f b. In this case, the type it's being applied to is X (ScreenId, W.RationalRect), so f unifies with X, a with ScreenID and b with W.RationalRect. Making snd `fmap` floatLocation w hav the type X W.RationalRect. Finally, the bind into rr there takes it out of the X monad all together, getting you a W.RationalRect. You may want to read this article which explains some of Haskell's abstraciton mechanisms: http://noordering.wordpress.com/2009/03/31/how-you-shouldnt-use-monad/ Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
