Re: [Haskell-cafe] Proposal: Non-recursive let
For what it's worth, I think a non-recursive in the language would just bring more confusion, in forums, IRC and whereever. The benefits don't seem important at all, and the same effect can be achieved through other means. On Wed, Jul 17, 2013 at 2:20 AM, Richard A. O'Keefe o...@cs.otago.ac.nz wrote: Brian Marick sent me a couple of his stickers. The one I have on my door reads to be less wrong than yesterday. The other one I keep free to bring out and wave around: An example would be handy about now. All of the arguing to and fro -- including mine! -- about non-recursive let has been just so much hot air. I could go on about how the distinction between 'val' and 'val rec' in ML was one of the things I came to dislike intensely, and how Haskell's single coherent approach is one of the things that attracted me to Haskell. But why should anyone else care? When presented with a difficulty, it is very common for some functional language users to propose adding just one more feature from some other language, commonly an imperative one (which ML, Caml, and F# arguably are). Typically this is something that _would_ solve the immediate problem but would create worse problems elsewhere, and there is some other solution, either one already available in the language, or a better one that would solve additional problems or cause fewer ones. The best help for any discussion is A CONCRETE EXAMPLE OF REAL CODE. Not little sketches hacked up for the purpose of discussion, but ACTUAL CODE. The person who initially proposes a problem may think some details are not relevant, whereas someone else may see them as the key to the solution. For example, looking at some code in another mostly- functional language, which had been presented as reason why we needed a new construct, I rewrote it in less than half the number of lines using existing constructors, using only existing features. Without seeing THE ACTUAL CODE that prompted this thread, it is impossible to tell whether that might be the case here. In this specific case, we are seeing state being threaded through a bunch of updates, and IN THE ABSENCE OF THE ACTUAL CODE, it seems to me that monad notation is the most intention-revealing notation available for the purpose in Haskell, and if Haskell did have non-recursive let it would STILL be best to write such code using a state monad so that human beings reading the Haskell code would have some idea of what was happening, because that's how state changes are supposed to be expressed in Haskell, and anything else counts as obfuscation. But THE ACTUAL CODE might show that this case was different in some important way. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] default instance for IsString
Hi Greg
You *can* have what you want -- enable extended defaulting and set
Text as default by
{-# LANGUAGE ExtendedDefaultRules #-}
default (Integer, Double, T.Text) -- keep the default defaults
At the moment you only by chance have one instance of NoDefault in
scope, which is why it shouldn't work at the moment.
(ExtendedDefaultRules is required because the standard defaulting
rules apply only when defaulting numeric literals.)
On Sun, Apr 22, 2012 at 7:55 AM, Greg Weber g...@gregweber.info wrote:
This is a better demonstration of the issue. I am going to open a GHC
bug report, as I can't see how this behavior is desirable.
{-# LANGUAGE OverloadedStrings #-}
import Data.Text as T
class NoDefault a where noDefault :: a - Text
instance NoDefault T.Text where noDefault = id
main = print (noDefault Hello!)
default.hs:7:15:
Ambiguous type variable `a0' in the constraints:
(NoDefault a0) arising from a use of `noDefault'
at default.hs:7:15-23
(Data.String.IsString a0) arising from the literal `Hello!'
at default.hs:7:25-32
Probable fix: add a type signature that fixes these type variable(s)
In the first argument of `print', namely `(noDefault Hello!)'
In the expression: print (noDefault Hello!)
In an equation for `main': main = print (noDefault Hello!)
On Sat, Apr 21, 2012 at 7:51 PM, Greg Weber g...@gregweber.info wrote:
my actual use case looks more like this:
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
import Data.Text as T
class ShowT a where
showT :: a - String
instance ShowT T.Text where
showT = show
instance ShowT String where
showT = show
main = print (showT Hello!)
Ambiguous type variable `a0' in the constraints:
(ShowT a0) arising from a use of `showT' at default.hs:16:15-19
(Data.String.IsString a0) arising from the literal `Hello!'
So I actually want to define a default instance for a typeclass I
define that uses isString instances.
On Sat, Apr 21, 2012 at 6:24 PM, Daniel Peebles pumpkin...@gmail.com wrote:
I think it'll be hard to do that without putting Text in base, which I'm not
sure anyone wants to do.
Dan
On Sat, Apr 21, 2012 at 8:20 PM, Greg Weber g...@gregweber.info wrote:
I would like to default IsString to use the Text instance to avoid
ambiguous type errors.
I see defaulting capability is available for Num. Is there any way to
do this for IsString?
Thanks,
Greg Weber
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Re: [Haskell-cafe] default instance for IsString
The core of it is in the GHC docs' overloaded strings section [1]. It could be clearer though -- reading about defaulting in the reports, in the type defaulting section of GHC docs and in [1] can be a bit confusing. [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#overloaded-strings On Sun, Apr 22, 2012 at 4:54 PM, Greg Weber g...@gregweber.info wrote: Thanks Markus, I think you have saved the day! Even after googling for this extension and searching in the manual I am still coming up pretty blank. Is there somewhere I missed where this is documented or somewhere I can contribute documentation? On Sun, Apr 22, 2012 at 4:47 AM, Markus Läll markus.l...@gmail.com wrote: ExtendedDefaultRules -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] default instance for IsString
Isn't this a special case already? I.e you already can default a string literal to anything you want, the only thing needed is a language pragma (and more obvious docs). The only problem is, that it's not part of the Haskell2010 report, and thus not supported in other compilers (or is it?). On Sun, Apr 22, 2012 at 8:51 PM, Johan Tibell johan.tib...@gmail.com wrote: On Sun, Apr 22, 2012 at 10:37 AM, Brent Yorgey byor...@seas.upenn.edu wrote: I do not think this is a bug. Since type classes are open, GHC does not do any reasoning of the form X is the only instance in scope, so I will pick that one. Other instances could be added at any time (perhaps in other modules). In this particular instance, GHC has no reason to choose the Text instance other than the fact that it is the only instance in scope -- that is, type inference is not enough to determine that the Text instance should be chosen. However, I do agree that it would be nice to have a mechanism for specifying default instances for arbitrary (user-defined) type classes. Couldn't we make a special case for IsString, like we do for Num, given it's special syntactic association with OverloadedStrings? -- Johan ___ Glasgow-haskell-users mailing list glasgow-haskell-us...@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [haskell-cafe] Some reflections on Haskell
What about hoogle/hayoo and hackage? On Tue, Feb 14, 2012 at 5:13 PM, Doug McIlroy d...@cs.dartmouth.edu wrote: Kevin Jardine notices the full Haskell ecosystem ... is huge, and laments the absence of a sophisticated IDE to help manage it. Being a small-code type, I don't personally enjoy IDE's, which are undeniably useful in big projects, at the cost of a whole lot more to learn about programmering in addition to programming. Nevertheless, I share Jardine's concern about the central problem. It is hard to find one's way in this ecosystem. It needn't be, as Java illustrates. To my mind Java's great contribution to the world is its library index--light years ahead of typical documentation one finds at haskell.org, which lacks the guiding hand of a flesh-and-blood librarian. In this matter, it seems, industrial curation can achieve clarity more easily than open source. (To avoid entanglement with social media, this comment is going to Haskell Cafe rather than Google+ where other comments reside.) Doug McIlroy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type class for sequences with dependent types
Hi
This has been discussed here[1] and here[2], and the answer is that
its not possible because of making type inference generally
impossible. It would be a useful feature though, and could be
implemented by requiring the user give additional type signatures to
help the inference algorithm out. But that is AFAIK not implemented
yet.
[1] http://www.haskell.org/pipermail/haskell-cafe/2011-July/093790.html
[2] http://comments.gmane.org/gmane.comp.lang.haskell.glasgow.user/21017
On Thu, Jan 5, 2012 at 3:12 PM, Robbert Krebbers
mailingli...@robbertkrebbers.nl wrote:
Hello,
in a Haskell development I have to represent paths that have elements of
alternating types. GADTs allow me to do this easily:
data AltList :: * - * - * where
Nil :: AltList a a
ICons :: Int - AltList Int b - AltList () b
UCons :: () - AltList () b - AltList Int b
Since I have various kinds of these data structures I defined a type class
for common operations on such data structures.
class DepSequence l where
nil :: l a a
app :: l a b - l b c - l a c
The instance for AltList is as follows:
instance DepSequence AltList where
nil = Nil
Nil `app` k = k
ICons i l `app` k = ICons i (l `app` k)
UCons i l `app` k = UCons i (l `app` k)
This all works nicely, however, I also want ordinary lists to be an instance
of this class too. I tried the following:
type IntListAlt a b = [Int]
instance DepSequence IntListAlt where
nil = []
app = (++)
But GHC does not like this, it yields:
Type synonym `IntListAlt' should have 2 arguments, but has been
given none In the instance declaration for `DepList IntListAlt'
The following alternative works, but it is quite ugly
newtype IntList a b = IntList { getList :: [Int] }
instance DepSequence IntList where
nil = IntList []
app l k = IntList (getList l ++ getList k)
and also does not give me a nil of type [Int] and an app of type [Int] -
[Int] - [Int].
Does anyone know whether Haskell allows me to do this in a better way?
Best,
Robbert
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Re: [Haskell-cafe] How to split this string.
String is really for small strings. Text is more efficent and also has more functionality, including most, if not all, of the functions defined for String. On Mon, Jan 2, 2012 at 3:12 PM, Anupam Jain ajn...@gmail.com wrote: On Mon, Jan 2, 2012 at 5:52 PM, Felipe Almeida Lessa felipe.le...@gmail.com wrote: On Mon, Jan 2, 2012 at 10:12 AM, max m...@mtw.ru wrote: This is the simplest solution of the proposed, in my opinion. Thank you very much. Better yet, don't use String and use Text. Then you just need T.splitOn \r\n [1]. That is actually the opposite of what the OP wants, however it's interesting that Text has a function like that and not the String functions in the standard library. -- Anupam ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Tupling functions
Using fundeps I came up with this: class T funs arg res | funs - arg res, arg res - funs where tuple :: funs - arg - res instance (a ~ a0, a0 ~ a1) = T (a0 - b, a1 - c) a (b, c) where tuple (f, g) a = (f a, g a) instance (a ~ a0, a0 ~ a1, a1 ~ a2) = T (a0 - b, a1 - c, a2 - d) a (b, c, d) where tuple (f, g, h) a = (f a, g a, h a) Intuitively it seems to be exactly the same as the type families' aproach, and looks quite clear too. On 9/15/11, Richard O'Keefe o...@cs.otago.ac.nz wrote: On 14/09/2011, at 2:45 PM, Casey McCann wrote: class Tuple t where type Arg t :: * type Result t :: * tuple :: t - Arg t - Result t instance (x1 ~ x2) = Tuple (x1 - a, x2 - b) where type Arg (x1 - a, x2 - b) = x1 type Result (x1 - a, x2 - b) = (a, b) tuple (f, g) x = (f x, g x) That's it, that's what I was after. Thanks. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: fix-imports-0.1.2
I have used it a little and it worked quite nicely. One thing that to look out for though is that it first checks for modules from the current directory down -- so running it in somewhere you happen to have ghc sources too, will have all that come with it imported from there (and lines like import path.to.ghc.DataMaybe (fromJust) added). But this probably happens only when testing the program in the root of your home directory... (The reason I'm not using it regularly is that I haven't figured out a good way to use it, when developing with Notepad++ over sftp in another environment) -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Arrow instance of function type [a] - [b]
Hi! Is it possible to define an Arrow instance of list to list functions? Something like import Control.Arrow import Control.Category type X a b = [a] - [b] instance Category X where id = map Prelude.id g . f = g Prelude.. f instance Arrow X where arr f = map f first f = unzip first f uncurry zip The problem is that it's not allowed to use partially applied type synonyms. It is however possible to define a type synonym which value is a partially applied type, but I haven't been able to figure out if I could somehow use that fact in defining the X... Is it at all possible, or is a newtype the only way to do it? -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The Lisp Curse
What's stopping it from being put on the official hackage? I use it quite a lot to find well established packages and/or example code, and am quite fond of it. But it is only visible when you know that this exists. On Fri, May 20, 2011 at 10:28 AM, Roel van Dijk vandijk.r...@gmail.comwrote: On 19 May 2011 20:50, Serguey Zefirov sergu...@gmail.com wrote: The solution... I think that some ratings, like used directly by ### packages/projects and indirectly by ### would be nice, but not much. Maybe my reverse dependencies mirror of hackage could be useful here: http://bifunctor.homelinux.net/~roel/hackage/packages/hackage.html The mirror was intended to show the workings of a patch for the old (current) hackage. It is updated daily. The algorithm needs some attention so don't trust it blindly, but in general it is quite accurate. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Reverse Show instance
If you have nested type, then it usually makes sense to have Show defined for the inside types, too, but it's not a requirement. Technically, only when you call 'show' for something in the data type you are defining Show for, *then* you need a Show instance defined for that inside-type. On Fri, May 20, 2011 at 12:15 AM, Andrew Coppin andrewcop...@btinternet.com wrote: On 19/05/2011 10:11 PM, Artyom Kazak wrote: And I can declare an instance for (x, y) which does NOT implies (Show x): instance Show (x, y) where show _ = I'm tuple! Hooray! Ah. So it's a feature. Fortunately I refactored the program where this came up, so it's no longer an issue. I just wanted to see whether or not it was a bug. PS. Wouldn't such an instance require FlexibleContexts or something? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Local copy of hackageDB
This could be a nice feature and not hard to implement, maybe someone could take it up? 2011/4/7 José Pedro Magalhães j...@cs.uu.nl Yes, I have that tarball. I just don't know how to tell cabal-install to use it. Going to each package, individually unpacking and installing it is what I've been doing so far, but I was hoping that could be automated. Cheers, Pedro On Thu, Apr 7, 2011 at 14:18, Magnus Therning mag...@therning.org wrote: On Thu, Apr 7, 2011 at 11:21, Marc Weber marco-owe...@gmx.de wrote: Local copy ? You know that hackage is hosting several thausands of source archives - also old versions you don't want? Do you want to mirror everything locally? Fetching latest versions only to generate hashes takes many hours. (Experience from hack-nix). There's a tar-ball served from http://hackage.haskell.org/packages/hackage.html (http://hackage.haskell.org/cgi-bin/hackage-scripts/archive.tar) which contains the latest version of all packages. I think that's what the OP is using. Downloading a 150MB tar-ball shouldn't take many hours, if you're on a decent connection. /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: mag...@therning.org jabber: mag...@therning.org twitter: magthe http://therning.org/magnus ___ 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] [Haskell-beginners] how to modify show for a list of certain types
Check out unlines! On Sat, Apr 2, 2011 at 4:32 PM, Manfred Lotz manfred.l...@arcor.de wrote: Hi there, I have a list of say type Record which I write to a file and read back from the file if required. The list's content is a single line in the file it doesn't matter how many records the list contains. I would like to modify show for type Record in a way that show does work like the normal show but additionally adds a newline at the end of the string. I'm a bit lost how to do that. Any idea? -- Manfred ___ Beginners mailing list beginn...@haskell.org http://www.haskell.org/mailman/listinfo/beginners ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Learn You a Haskell for Great Good - a few doubts
Sorry, I didn't mean to answer you in particular. I meant to say that for tuples you could (I think) have an enumeration over them without requiring any component be bounded. An example of type (Integer, Integer) you would have: [(0,0) ..] = [(0,0) (0,1) (1,0) (0,2) (1,1) (2,0) ... ] where the order can be visualized by taking diagonals of a table starting from the upper left: 0 1 2 .. 0 (0,0) (0,1) (0,2) 1 (1,0) (1,1) (1,2) 2 (2,0) (2,1) (2,2) .. Would this also have an uncomputable order type? At least for comparing tuples you'd just: lt :: (Integer,Integer) - (Integer,Integer) - Bool (a,b) `lt` (c,d) = let sum1 = (a + b) sum2 = (c + d) in if sum1 == sum2 then a c else sum1 sum2 Implementing fromEnum looks like a bit harder problem.. -- Markus Läll On Fri, Mar 4, 2011 at 5:12 AM, Daniel Fischer daniel.is.fisc...@googlemail.com wrote: On Friday 04 March 2011 03:24:34, Markus wrote: What about having the order by diagonals, like: 0 1 3 2 4 5 and have none of the pair be bounded? I tacitly assumed product order (lexicographic order). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell IDE
I have been using Notepad++ -- it has proper (I think) syntaks highlighting and in the latest version now has line wrapping a la kate: broken lines start at the indent level of the first one. -- Markus Läll On Fri, Mar 4, 2011 at 1:14 AM, Daniel Fischer daniel.is.fisc...@googlemail.com wrote: On Thursday 03 March 2011 22:14:34, Michael Xavier wrote: I use vim (CLI not gvim). I use kate. ___ 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] Fwd: ANN: vector-buffer package 0.1
Hi, Alexander
Here's my take on why the code isnt right:
In the let the v1 and v2 get lazyly bound to code that calls
unsafePerformIO. unsafePerformIO does side-effectful things, but works
outside the IO monad and thus outside the main order of IO actions.
If you create a buffer, push something in and make two lists of it -- v1
and v2 -- then they don't get evaluated to concrete lists (weak head
normal form?) right away, but when they are actually needed. So if you print
v1, buf is read and converted into a list (and printed). v2 stays
unevaluated, but bound to the value of buf. So if you change buf, then
v2 is bound to be different too.
This breaks referential transperency because v1 and v2 by theire
definitions should be exactly the same. And the breakage can only happen
because of using unsafePerformIO, which allowes you to go out in the IO
world and get stuff, but bypass the order of evaluation that IO monad forces
you to otherwise have.
Someone correct me if I'm wrong :)
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On Tue, Feb 15, 2011 at 8:08 AM, Alexander McPhail
haskell.vivian.mcph...@gmail.com wrote:
Hi list,
Could someone explain why the error pointed out by Luke occurred?
From: Luke Palmer lrpal...@gmail.com
I think this would be a very good question for the list. Don't worry,
they're nice helpful folks.
On Mon, Feb 14, 2011 at 10:10 PM, Alexander McPhail
haskell.vivian.mcph...@gmail.com wrote:
Hi Roman,
Can you explain why the following code doesn't work? 'unsafePerformIO'
is
used all the time in hmatrix. For example, adding two vectors, we create
a
new Vector then use that as the target for an FFI C call.
Is the difference because I'm using mutable Storable Vectors? I would
have
thought that unsafeFreeze (at the end) would make sure the problem
reported
by Luke wouldn't occur.
Is there a problem with laziness in the let binding of Luke's example?
I note that in Data.Vector.Storable there is a pure 'convert' function,
which is essentially what I am trying to emulate.
-- | convert to a vector
toVector :: Storable a = Buffer a - (V.Vector a)
toVector (B o n v) = unsafePerformIO $ do
w - M.new n
i - readIORef o
M.unsafeWith v $ \p -
M.unsafeWith w $ \q - do
let n' = n-i
copyArray q (p `advancePtr` i) n'
if i /= 0
then copyArray (q `advancePtr` n') p i
else return ()
V.unsafeFreeze w
{-# INLINE toVector #-}
Vivian
From: Luke Palmer lrpal...@gmail.com
This interface is an outlaw.
main = do
buf - newBuffer 10 :: IO (Buffer Int)
pushNextElement buf 1
let v1 = V.toList (toVector buf)
v2 = V.toList (toVector buf)
print v1
pushNextElement buf 2
print v2
Despite v1 and v2 being defined to equal the exact same thing, this
program prints two distinct lines.
toVector depends on the current state of the buffer. If this is to be
a law-abiding interface, toVector must return a value in IO:
toVector :: (Storable a) = Buffer a - IO (Vector a)
Luke
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Re: [Haskell-cafe] Proving correctness
I think one thing it means, is that, with the typesystem, you just can't do random things where-ever you want. Like, in the pure world if you want to transform values from one type to another, you always need to have implementations of functions available to do that. (You can't just take a pure value, get its mem location and interpret it as something else, without being explicid about it.) So when lining up your code (composing functions), you can be sure, that at least as far as types are concerned, everything is correct -- that such a program, that you wrote, can actually exist == that all the apropriate functions exist. And it is correct only that far -- the value-level coding is still up to you, so no mind-reading... -- Markus Läll On Fri, Feb 11, 2011 at 1:16 PM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: On 11 February 2011 22:06, C K Kashyap ckkash...@gmail.com wrote: Hi Folks, I've come across this a few times - In Haskell, once can prove the correctness of the code - Is this true? I'm not quite sure where you got that... But since Haskell is pure, we can also do equational reasoning, etc. to help prove correctness. Admittedly, I don't know how many people actually do so... I know that static typing and strong typing of Haskell eliminate a whole class of problems - is that related to the proving correctness? Is it about Quickcheck - if so, how is it different from having test sutites in projects using mainstream languages? QuickCheck doesn't prove correctness: I had a bug that survived several releases tested regularly during development with a QC-based testsuite before it arose (as it required a specific condition to be satisfied for the bug to happen). As far as I know, a testsuite - no matter what language or what tools/methodologies are used - for a non-trivial piece of work just provides reasonable degree of assurance of correctness; after all, there could be a bug/problem you hadn't thought of! -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.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] How about Haskell Golf just like vimgolf.com
On Mon, Jan 3, 2011 at 6:19 PM, Ertugrul Soeylemez e...@ertes.de wrote: Alex Kropivny alex.kropi...@gmail.com wrote: Could something like code abstraction be done instead? Haskell lends itself to solving problems in really generic, high level ways that reveal a LOT about the underlying problem structure. Through some combination of descriptive data types, generic type classes, and generic helper functions... You get an extremely clear problem description. Example: https://github.com/amtal/snippets/blob/master/Key.hs (Haskell) versus http://siyobik.info/index.php?module=pastebinid=543 (C++) Clarity is a lot harder to score for, so you'd probably need to score things via votes. (Unless there's a way to measure how generic/high-level code is?) Such a site would fill a very nice role, that the programming language shootout definitely does not fill. Currently the only way to figure out what good Haskell code looks like is to browse lots of blogs, and dig through hackage until you find beautifully written packages. I really like this idea. New concepts in Haskell come up from time to time. Now if there was a competition for code quality and good ideas, they may become more frequent. This could also go for problems, where you have to use some specific feature or extension, like scoped type variables, type families/functional dependencies, or even just typeclasses... (Remember, there was a deep list concatenation problem thread some days ago.) -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] record types and unique names
Yes, they are in the global scope, and from what I gather: they are just
regular functions, created by special syntax.
There are a few obvious solutions (some of which you might have thought
yourself :-):
- rename the accessor or the other function, or
- put the data declaration or the other function in another module and
import qualified, or
- write a typeclass with a 'name' function and fit the non-accessor
function 'name' somehow into that...
I think the best approach is the modular one, but this really depends on
what you are doing.
--
Markus Läll
On Thu, Dec 30, 2010 at 7:01 PM, Aaron Gray aaronngray.li...@gmail.comwrote:
Given a Haskell record type :-
data Test
= Test {
name :: String,
value :: Int
}
test = Test {
name = test,
value = 1
}
main :: IO ()
main = do
putStrLn (name test)
Are name and value in the global name space, as the following gives an
error Multiple declarations of `name' :-
name :: String - String
name s = s
Is there any way round this ?
Many thanks in advance,
Aaron
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Re: [Haskell-cafe] Offer to mirror Hackage
The reason for mirror was avilability, yes, and when the signatures were only on the central sever, then the user could choose not to install packages from mirrors, when they were not available. But now if the signatures were generated by the uploader, then the morrors would be just as secure as the central? I mean -- if we don't trust DNS, then the main hackage has no special security advantages? -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Offer to mirror Hackage
My take on the issue is that we should make it possible to easily mirror hackage (what the OP asked for), so that people could use it when they wanted to, and have a list of the mirrors on the wiki. This way those who are interested can use them. Like when the mirror is faster/closer to them or to help out when hackage is temporarily down. Those who need the security can choose not to use mirrors, or make their own (private), or develop a secure scheme, when it doesn't exist yet. It's perfectly understandable, that people doing work/serious stuff need the guarantees, but I bet a many of us are just playing around and developing things for themselves. -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Offer to mirror Hackage
On Thu, Dec 9, 2010 at 11:04 PM, Richard O'Keefe o...@cs.otago.ac.nz wrote: On 10/12/2010, at 12:18 AM, Markus Läll wrote: My take on the issue is that we should make it possible to easily mirror hackage (what the OP asked for), so that people could use it when they wanted to, and have a list of the mirrors on the wiki. This way those who are interested can use them. Like when the mirror is faster/closer to them or to help out when hackage is temporarily down. Those who need the security can choose not to use mirrors, or make their own (private), or develop a secure scheme, when it doesn't exist yet. Have I misunderstood something? I thought X is a mirror of Y meant X would be a read-only replica of Y, with some sort of protocol between X and Y to keep X up to date. As long as the material from Y replicated at X is *supposed* to be publicly available, I don't see a security problem here. Only Y accepts updates from outside, and it continues to do whatever authentication it would do without a mirror. The mirror X would *not* accept updates. Yes, that's what I think of mirrors too. I don't know if that was what you meant, but yes those mirrors would be just passive copies of the real hackage server (no updates from a user), and serve as a place to download packages from until the original hackage comes back. But for the security issue, ofcourse any host of a mirror could abuse that. But I think for non-critical stuff I wouldn't mind using the mirror if it has shown to be trustworthy. And for people using Haskell a lot, if the making of your own mirror is as simple as installing some package on your webserver and running it, then this would be a great remedy against those hours when something has happened to hackage.. -- Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type level program
Not exactly answering your question, but just in case you hadn't checked, there are a few packages for type level natural numbers on hackage, which do the same things as the program below. The packages are: - type-level-natural-number and co by Gregory Crosswhite, and - type-level-numers by Alexey Khudyakov (which does type-level binary encoding(!) for natural numbers) As for the terminology, I can't tell.. Out of intrest, is this kind of type-level stuff to prove things about your container types and operations on them? -- Markus Läll On Mon, Oct 25, 2010 at 12:52 PM, Patrick Browne patrick.bro...@dit.ie wrote: Hi, I hypothesize that at type level Haskell offers a form of equational logic. At type level the following program[1] could be interpreted as a first order program in equational logic where; 1)Data types represent declarations of constructors (both constants and functions) 2)Type synonyms represent equations assigning constructors to variables 3)Each class contains a non-constructor operation signature with dependencies 4) instances contain equations that define the operations (similar the term rewriting systems (TRS) of Maude/CafeOBJ). 5):t acts as a reduction or evaluation at type level Is the a reasonable description of this program? Regards, Pat [1] From Fritz Ruehr http://www.willamette.edu/~fruehr/haskell/evolution.html -- static Peano constructors and numerals data Zero data Succ n type One = Succ Zero type Two = Succ One type Three = Succ Two type Four = Succ Three -- dynamic representatives for static Peanos zero = undefined :: Zero one = undefined :: One two = undefined :: Two three = undefined :: Three four = undefined :: Four -- addition, a la Prolog class Add a b c | a b - c where add :: a - b - c instance Add Zero b b instance Add a b c = Add (Succ a) b (Succ c) -- multiplication, a la Prolog class Mul a b c | a b - c where mul :: a - b - c instance Mul Zero b Zero instance (Mul a b c, Add b c d) = Mul (Succ a) b d -- factorial, a la Prolog class Fac a b | a - b where fac :: a - b instance Fac Zero One instance (Fac n k, Mul (Succ n) k m) = Fac (Succ n) m -- try, for instance (sorry): -- -- :t fac four This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie ___ 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] What is -
Hi Michael, although I never used it myself, lists seem strange in the way that when combining list monads, then all the values go through the chain one by one -- x will be 1 first, then 2, then 3 and so on.. Try it out, to see. (I think the result is then also a list of all combinations of results.) On Sun, Aug 8, 2010 at 5:26 PM, Christopher Done chrisd...@googlemail.comwrote: On 8 August 2010 16:21, michael rice nowg...@yahoo.com wrote: getLine = \x - -- x is a string at this point [1..] = \x --- x is WHAT at this point? Num n = n A number from the list. ___ 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] Removing alternate items from a list
Yep, the test is done by a rookie. If I get more time, I'll try to look into testing a little more, and redo the timing (if anyone doesn't do it firs) -- using optimizations, more runs per function and the criterion package. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Removing alternate items from a list
So out of curiosity i took the definitions given in this thread, and tried to run timing-tests. Here's what I ran: ghc -prof -auto-all -o Test Test.h Test +RTS -p and then looked in the Test.prof file. All tests I ran from 3 to 10 times (depending on how sure I wanted to be), so the results are not entirely exact. (I copied the average result to the source-file as comments above every function.) As the function doing (x:_:rest) pattern-matching was the fastest I extended the idea from that to (x1:_:x2: ... x10:_:rest), but skipping from 5 to 10, where all steps showed a small increase in performance. So a question: when increasing the pattern matched, is it somekind of way of inlining the matchings, and if so, is there some way of just saying that to the compiler how many recursions you want to inline together to increase speed? Any comments? (besides -O2 ;-) -- I remembered it too late and didn't want to restart... At least for the last two functions it showed a similar difference in seconds as with no -O2) Markus Läll ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Removing alternate items from a list
Forgot the file -- here it is:
module Main where
import Data.Either (rights)
import Data.Function (fix)
test f = putStr $ show $ last $ f $ replicate 1000 (1 :: Int)
main = test matchPattern4
-- 1. zipNums
-- 2. matchPattern
-- 3. zipBoolCycle
-- 4. iterDrop
-- 5. zipBoolCycle2
-- 6. consume
-- 7. eitherr
-- 8. golf
-- 9. matchPattern2
-- 10. matchPattern3
-- 11. matchPattern4
-- 12. matchPattern5
-- 13. matchPattern10
-- 1. total time = 13.72 secs (686 ticks @ 20 ms)
--total alloc = 1,840,007,000 bytes (excludes profiling overheads)
zipNums = map snd . filter (odd . fst) . zip [1,2..]
-- 2. total time =1.82 secs (91 ticks @ 20 ms)
--total alloc = 400,006,752 bytes (excludes profiling overheads)
matchPattern (x:_:zs) = x : matchPattern zs
matchPattern x = x
-- 3. total time =4.46 secs (223 ticks @ 20 ms)
--total alloc = 1,040,006,904 bytes (excludes profiling overhea
zipBoolCycle xs = map fst . filter snd $ zip xs (cycle [False, True])
-- 4 total time =5.20 secs (260 ticks @ 20 ms)
-- total alloc = 940,006,916 bytes (excludes profiling overheads)
iterDrop = map head . takeWhile (not . null) . iterate (drop 2) . drop 1
-- 5 total time =3.68 secs (184 ticks @ 20 ms)
-- total alloc = 820,006,872 bytes (excludes profiling overheads)
zipBoolCycle2 x = [y | (True, y) - zip (cycle [False, True]) x]
-- 6. total time =2.46 secs (123 ticks @ 20 ms)
--total alloc = 420,006,860 bytes (excludes profiling overheads)
data Consume = Take | Skip
consumeBy :: [Consume] - [a] - [a]
consumeBy [] _ = []
consumeBy _ [] = []
consumeBy (tOrS:takesAndSkips) (x:xs) =
case tOrS of Take - x : consumeBy takesAndSkips xs
Skip - consumeBy takesAndSkips xs
consume = consumeBy $ cycle [Take, Skip]
-- 7. total time =4.10 secs (205 ticks @ 20 ms)
--total alloc = 1,000,006,884 bytes (excludes profiling overheads)
eitherr = rights . zipWith ($) (cycle [Left,Right])
-- 8. total time =2.08 secs (104 ticks @ 20 ms)
--total alloc = 420,006,784 bytes (excludes profiling overheads)
golf = (fix $ \f xs - case xs of { (x:_: xs) - x : f xs; _ - [] })
-- 9. total time =1.68 secs (84 ticks @ 20 ms)
--total alloc = 370,006,752 bytes (excludes profiling overheads)
matchPattern2 (a:_:c:_:rest) = a : c : matchPattern2 rest
matchPattern2 (a:_:rest) = a : rest
matchPattern2 (rest) = rest
-- 10. total time =1.58 secs (79 ticks @ 20 ms)
-- total alloc = 360,006,744 bytes (excludes profiling overheads)
matchPattern3 (a:_:c:_:e:_: rest) = a : c : e : matchPattern3 rest
matchPattern3 (a:_:c:_:rest) = a : c : rest
matchPattern3 (a:_:rest) = a : rest
matchPattern3 (rest) = rest
-- 11. total time =1.56 secs (78 ticks @ 20 ms)
-- total alloc = 355,006,752 bytes (excludes profiling overheads)
matchPattern4 (a:_:c:_:e:_:g:_:rest) = a : c : e : g : matchPattern4 rest
matchPattern4 (a:_:c:_:e:_: rest)= a : c : e : rest
matchPattern4 (a:_:c:_:rest) = a : c : rest
matchPattern4 (a:_:rest) = a : rest
matchPattern4 (rest) = rest
-- 12. total time =1.52 secs (76 ticks @ 20 ms)
-- total alloc = 352,006,752 bytes (excludes profiling overheads)
matchPattern5 (a:_:c:_:e:_:g:_:i:_:rest) = a : c : e : g : i :
matchPattern5 rest
matchPattern5 (a:_:c:_:e:_:g:_:rest) = a : c : e : g : rest
matchPattern5 (a:_:c:_:e:_: rest)= a : c : e : rest
matchPattern5 (a:_:c:_:rest) = a : c : rest
matchPattern5 (a:_:rest) = a : rest
matchPattern5 (rest) = rest
-- 13. total time =1.48 secs (74 ticks @ 20 ms)
-- total alloc = 346,006,752 bytes (excludes profiling overheads)
matchPattern10 (a:_:c:_:e:_:g:_:i:_:k:_:m:_:o:_:q:_:s:_:rest) =
a:c:e:g:i:k:m:o:q:s: matchPattern10 rest
matchPattern10 (a:_:c:_:e:_:g:_:i:_:k:_:m:_:o:_:q:_:rest) =
a:c:e:g:i:k:m:o:q:rest
matchPattern10 (a:_:c:_:e:_:g:_:i:_:k:_:m:_:o:_:rest) =
a:c:e:g:i:k:m:o:rest
matchPattern10 (a:_:c:_:e:_:g:_:i:_:k:_:m:_:rest) =
a:c:e:g:i:k:m:rest
matchPattern10 (a:_:c:_:e:_:g:_:i:_:k:_:rest) = a:c:e:g:i:k:rest
matchPattern10 (a:_:c:_:e:_:g:_:i:_:rest) = a:c:e:g:i:rest
matchPattern10 (a:_:c:_:e:_:g:_:rest) = a:c:e:g:rest
matchPattern10 (a:_:c:_:e:_: rest)= a:c:e:rest
matchPattern10 (a:_:c:_:rest) = a:c:rest
matchPattern10 (a:_:rest) = a:rest
matchPattern10 (rest) = rest
On Wed, Jun 9, 2010 at 11:47 PM, Markus Läll markus.l...@gmail.com wrote:
So out of curiosity i took the definitions given in this thread, and
tried to run timing-tests.
Here's what I ran:
ghc -prof -auto-all -o Test Test.h
Test +RTS -p
and then looked in the Test.prof file
Re: [Haskell-cafe] Haskell, Queries and Monad Comprehension
I would be intrested in seeing this! Could you paste or upload it somewhere? 2010/5/24 C. McCann c...@uptoisomorphism.net: 2010/5/23 Günther Schmidt gue.schm...@web.de: is there anybody currently using Haskell to construct or implement a query language? I've a half-baked, type-indexed (in HList style) implementation of relational algebra lying around somewhere, if that counts as a query language. I was experimenting with using it as a sort of abstract collection interface, which actually worked rather nicely I think, but I didn't have time to flesh it out completely. In particular, only very simple queries and limited kinds of relation composition were supported. Definitely just toy code, though, and dreadfully inefficient; if you're looking for an actual implementation meaning usable interface to an external persistence layer then disregard this. - C. ___ 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
