definition of take
Hi, I don't know much about the internals of GHC, but I like look around once in a while to learn a few things. I was wondering about the definition of take, which is, for Ints: - takeUInt :: Int# - [b] - [b] takeUInt n xs | n =# 0# = take_unsafe_UInt n xs | otherwise = [] take_unsafe_UInt :: Int# - [b] - [b] take_unsafe_UInt 0# _ = [] take_unsafe_UInt m ls = case ls of [] - [] (x:xs) - x : take_unsafe_UInt (m -# 1#) xs - Wouldn't pattern matching against 1# instead of 0#, like in the following definition, be better in terms of garbage collecting the list or what it depends on? --- takeUInt :: Int# - [b] - [b] takeUInt 0# _ = [] takeUInt n xs | n # 0# = take_unsafe_UInt n xs | otherwise = [] take_unsafe_UInt :: Int# - [b] - [b] take_unsafe_UInt 1# (x:xs) = [x] take_unsafe_UInt m ls = case ls of [] - [] (x:xs) - x : take_unsafe_UInt (m -# 1#) xs -- My guess would be that, if we're consuming a (take n xs) lazily, with the first definition garbage collecting would happen when the last element had been consumed and with the second, when the last element had been produced - (which is when we find out there are no more elements to consume in each case). Thanks, and sorry if the question is to naive, J.A. ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [Haskell-cafe] mathematical notation and functional programming
Things I'm unhappy about are for instance f(x) \in L(\R) where f \in L(\R) is meant F(x) = \int f(x) \dif x where x shouldn't be visible outside the integral O(n) which should be O(\n - n) (a remark by Simon Thompson in The Craft of Functional Programming) f(.) which means \x - f x or just f All of these are the same notation abuse, sometimes f x is meant to be interpreted as \x-f x In some cases it would be really tedious to add the extra lambdas, so the expression used in its definition is used to denote the function itself. a b c which is a short-cut of a b \land b c Both, ambiguity and complex notation, can lead to (human) parsing problems, which is what we are trying to minimise here. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting from Int to Double
How can I convert an Int into a Double? You don't convert to, you convert from :-) The function 'fromIntegral' is probably what you want. And what function can I use to convert from Double to Int (the inverse of fromIntegral) ? Use the functions in the RealFrac class. http://www.haskell.org/onlinereport/standard-prelude.html#$tRealFrac class (Real a, Fractional a) = RealFrac a where properFraction :: (Integral b) = a - (b,a) truncate, round :: (Integral b) = a - b ceiling, floor :: (Integral b) = a - b J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
On Tuesday 25 January 2005 02:25, Jan-Willem Maessen wrote: On Jan 24, 2005, at 8:53 PM, Jorge Adriano Aires wrote: And it would say nothing about things like: return 4 return 5 ==?== return 5 I can live with it. I feel obliged to point out (because the repeated references to the question are driving me up the wall) that this simple equality holds in every monad: return 4 return 5 ... return 5 Opss, of course... J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] using Map rather than FiniteMap
Just did a search after my last post and learned that FiniteMap is bad. Discoverd that Data.Map is the intended replacement. Downloaded it and modified it to work with 6.2. Blazingly fast! Yay. Hi, just curious, How much trouble was getting it to work with ghc 6.2 and adapting your program to use the API of Data.Map instead of Data.FiniteMap? J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
Right, but we are dealing with the type system here. Remember Haskell monoids are functors on types, not on values ... (ie the base objects the 'category theory' is applied to are the types not the values)... Therefore we only consider the types when considering Monads. How so? Functors map morphisms and objects from one category into another. class Functor f where fmap :: (a-b) - f b - f a We have the two maps there. - The type constructor, maps the objects (types). - The fmap higher order function, maps the morphisms (function between types). Monads are, in particular, functors. So again, the type constructor maps the objects (types) and the mapping on morphisms (functions from one type to the other) is given by liftM (that is, fmap = liftM). Like Ashley Yakeley said, we can have many different functions (morphism) between two types, namely IO a types. As such if you wished to consider the examples you gave distinct, the type system would need to distinguish side effects... Why? I don't see how side effects make any difference here... How do you distinguish morphisms f and g: f,g :: Int - Int f n = 2*n g n = 2+n J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
We face a severe problem here, not only that IO a is not an instance of Eq, which takes this whole discussion outside the realm of Haskell, on top of that we find the horrible fact that x /= x may be true in the IO Monad, consider x = getLine = putStrLn or anything similar -- actually we already have getChar /= getChar This isn't obvious to me. So x is an action, and it does not always produces the same side effects when executed. But why should that make x/=x? It is the same action, it gets one line from the input, and then prints it... In fact, I do not agree. See Rant 2 below. The sad truth is that IO actions in general aren't well defined entities (unless we index them with the space-time-coordinates of their invocation). So I suggest ending the discussion by agreeing that the question whether or not x mzero == mzero holds in the IO-Monad is meaningless (at least, it's fruitless). If you cannot agree, I have another question: is return 4 return 5 == return 5 true in the IO-Monad? Yeap, I thought about it too, have no idea, and cannot afford to spend much time thinking about it now either, since I got work to do... :-/ --- Rant 1 My gut feeling would be no. I think my intuitive reasoning is too just consider that, every IO action is equal to itself, then take the closure with respect to function application, and assume all others cannot be proved. That is, x === x, for all x :: IO a f x === g x, for all x :: a and f,g :: a - IO b, such that f === g Where equality between functions is defined the usual way. --- Rant 2 Nope, we have to have getChar === getChar. I think you'll agree if I say that we have: 1. return === return 2- return 5 === return 5 return 5 return 5 === return 5 return 5 Because this has nothing to do with IO. 1. We have that, return :: a-IO a is a function, not an action, so it must be equal to itself. 2. We have that, return :: a-IO a is a function, not an action, so it must return the same value when applied to the same element. 3. () :: (Monad m) = m a - m b - m b It is also a function, so () x === () x. And by the same reasoning () x y === () x y. So from 2 we have 3. A constant c :: a is just morphism(function) c : 0 - a, where 0 is the initial object (empty set). So we must have c === c. Which means getChar === getChar. In other words, by questioning wether you can have x ==/= x for x :: IO a, you are questioning wether we really have f === f for all functions f::a-b. --- return hello = putStrLn this does not only have the same type as putStrLn hello, and return the same value (), but it also carries out exactly the same actions. But does it really? hugs thinks otherwise: Prelude putStrLn hello hello () (28 reductions, 55 cells) Prelude return hello = putStrLn hello () (31 reductions, 56 cells) Prelude putStrLn hello hello () (26 reductions, 50 cells) even the same input does not necessarily lead to exactly the same actions, depending on whether hello is already known or not. This I don't agree with, I think you are using the word actions for two different things, the elements of type IO a, and their execution. What you just showed is that those IO () elements (actions) when executed, always created different side effects in the real world. Not that the actions themselves are different. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
A constant c :: a is just morphism(function) c : 0 - a, where 0 is the initial object (empty set). --- Rant2 correction Opss I messed up here. Should be terminal should 1- a (terminal object/unit set). At least that's how I usually think of constants in haskell 1 is ()... so I think I don't know what is a constant in Haskell... Anyway, stopping now. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
This isn't obvious to me. So x is an action, and it does not always produces the same side effects when executed. But why should that make x/=x? It is the same action, it gets one line from the input, and then prints it... OK, but then the different side-effects could not be used to distinguish putStrLn hello mzero and mzero. So I still believe, if you say these two are different, because they produce different output, you cannot easily insist on x === x. Agree. But I don't say that. This I don't agree with, I think you are using the word actions for two different things, the elements of type IO a, and their execution. What you You're right, but one of my problems is to identify elements of type IO a. If the returned value isn't the thing, the execution must matter, but which parts of the execution are to be taken into account? How can we tell if functions f === g? They must have the same domain, codomain and return the same result for every element of the domain. This is just the mathematical definition. For any two arbitrary functions f,g, can you tell if they are the same or not? As a definition, I'd be happy to have, x,y :: IO a are the same if, given the same, real world, they produce the same effects and return the same result. Now I'm not saying we can derive that x === x, for x ::IO, from that, but it is certainly consistent with that point of view, so we can take it as an axiom. Which I think we already do. We also have that if f === g than f x === g x. That includes functions of type f,g :: a - IO b. All seems consistent. Any other equality relation should include that one. Is it enough? It's enough to be able to tell that: putStrLn hello return 3 === putStrLn (he++llo) =\ _ return (1+2) And it would say nothing about things like: return 4 return 5 ==?== return 5 I can live with it. To prove that two functions are in deed the same, we may use, say, number theory knowledge, which falls outside the scope of haskell. I find it sensible to do the same with actions. Maybe (not sure) it is sensible to specify return::(a - IO a), as an action with no side effects such that return x === return x iff x === x. If we add that to our knowledge of IO, along with an appropriate specification for (=), then we would have: return 4 return 5 === return 5 J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
(Sorry about the recurrent self answers) Maybe (not sure) it is sensible to sapecify return::(a - IO a), as an action with no side effects such that return x === return x iff x === x. return x === return y iff x === y-- this is what I meant to write. But even that is not enough, should be: return x is an action with no side effects and that always returns x The previous specification is be a consequence of this one, and it failed to specify that the returned value was always x. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
What would happen if this was the definition? instance MonadPlus [] where mzero = [] mplus a b | a == [] = b | otherwise = a Isn't the above a monoid as well? Yes. Is there only on correct definition of a monad/monoid on lists - or does anything that satisfies the monad laws count? I got the impression you could define anthing you liked for mzero and mplus - providing the laws are upheld? I'm not arguing that definition would be wrong. It is a monoid. This is the instance for (): instance MonadPlus() where mzero = () mplus a b = () And this would be correct too: instance MonadPlus Maybe where mzero = Nothing mplus a b = Nothing instance MonadPlus [] where mzero = [] mplus a b = [] Which are not really useful. I'm claiming that the fact that Maybe is a trivial Monoid doesn't mean we should dumb down other instances, like the one on lists. The usual definition of Monoid on lists is [] as identity and ++ as the monoid operation. That is how it's defined in class monoid, and I expect this relation to hold: instance MonadPlus m = Monoid (m a) where mempty = mzero mappend = mplus Then, I'd say you're not thinking of monadic sums, but of catching errors, and the appropriate place for that is the class MonadError. I am thinking about how some monads are summed - like Maybe and the Parser monad. But, this is not how monadic parsers are summed. Just look into the instace of MonadError for Text.ParserCombinators.ReadP.P. Again it would be the case for parsers that would return just one possible parsing, but not for parsers that return [(a,String)]. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
One common example is using MonadPlus for some backtracking algorithm, then instantiatiating it to Maybe or List instance depending on wether you just want one solution or all of them. Backtracking only works with the first kind, even if you're only interested in the first solution. This must hold: (mplus a b) = c = mplus (a = c) (b = c) Not really. If the recursive call is something like msum [all_possible_paths], then you are backtracking. The difference is that by using Maybe you'll stop as soon as you succeed, and with list you will find all possible paths. I don't have a small, self-contained, example at hand so I'll use one by Carsten Schultz that I once saw posted in comp.lang.functional. Hope he doesn't mind. http://groups-beta.google.com/group/comp.lang.functional/msg/d7ac1fe1684ef840 -- --- -- knapsack problem module Subset2 where import Control.Monad sss :: MonadPlus m = Int - [Int] - m [Int] sss n [] | n0 = mzero sss n (x:xs) = case compare n x of LT - mzero EQ - return [x] GT - liftM (x:) (sss (n-x) xs) `mplus` sss n xs -- --- sss 40 [3, 8, 9, 13, 14, 15, 17, 19] :: [[Int]] [[3,8,14,15],[3,9,13,15],[8,13,19],[8,15,17],[9,14,17]] sss 40 [3, 8, 9, 13, 14, 15, 17, 19] :: Maybe [Int] Just [3,8,14,15] [*] For instance, I've missed Maybe being an instance of MonadError. You could define your own instance, of course. Yes of course, and I did ;) But I think it should be provided nontheless, and I even find the fact that it isn't is playing a big part in all this confusion. Since there is no MonadError instance for Maybe, we end up using its MonadPlus instance, which just happens to be the same. But it cannot be generalized, for other types. Lets forget about lists, think (Either e). It's usual to move from (Maybe a) to (Either e a) to consider more than one kind of error. But, there is no natural instance of MonadPlus for (Either e a). What would mzero be? Yet, the Maybe like, mplus operation makes perfect sense in (Either e) or any other MonadError, though. You don't need Monoids at all, what you need is the concept of error. Just define it as: skipError x y = catchError x (\_-y) J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
I think it would be helpful if all these classes came with their laws prominently attached in their Haddock documentation or wherever. Agree. The trouble with MonadPlus is that the precise set of associated laws is either unspecified or not the most useful (I assume there's a paper on the class somewhere). I think everyone can agree on these: mplus mzero a = a mplus a mzero = a mplus (mplus a b) c = mplus a (mplus b c) snip (mplus a b) = c = mplus (a = c) (b = c) I just checked the paper, A monadic Interpretation of Tatics, by Andrew Martin and Jeremy Gibbons http://web.comlab.ox.ac.uk/oucl/work/jeremy.gibbons/publications/tactics.pdf And in deed, these are the listed laws for MonadPlus. On the other hand, Maybe is said to be an instance of MonadPlus. So now I'm lost. Cheers, J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
Am Sonntag, 23. Januar 2005 15:58 schrieb Jorge Adriano Aires: I'm not arguing that definition would be wrong. It is a monoid. This is the instance for (): instance MonadPlus() where mzero = () mplus a b = () Maybe I'm stupid, but: class Monad m = MonadPlus m where mzero :: m a mplus :: m a - m a - m a How does () fit into this, () isn't of kind * - *, as far as I know () Int is meaningless -- just checked, gives Kind Error. Nope, I am. Sorry! I was alternating between monoids and monadplus, and came up with that nonsense. I was obviously thinking about Monoid () and not MonadPlus (). And this would be correct too: instance MonadPlus Maybe where mzero = Nothing mplus a b = Nothing instance MonadPlus [] where mzero = [] mplus a b = [] Both aren't correct, since mzero `mplus` x == x doesn't hold (they're syntactically correct, though). Yeap. You are right again. Sorry for this terrible example, please ignore it. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
But only some instances (such as []) satisfy this: (mplus a b) = c = mplus (a = c) (b = c) Other instances (IO, Maybe) satisfy this: mplus (return a) b = return a I think mplus should be separated into two functions. How would we implement the first kind in the Maybe instance of MonadPlus? J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
snip Only the monoid Maybe a is not very nice (nor is the monoid IO a),since the second argument of the composition is in general ignored. snip So I think, rather than separating mplus, one should think about whether it is sensible to make Maybe and IO instances of MonadPlus in the first place. I don't know nearly enough of the innards of Haskell to form a valuable opinion of that, but perhaps somebody could enlighten me? My humble opinion follows. It's still a Monoid, being boring should be no reason not to include it [*]. By taking advantage of typeclasses, we can easily alternate between more elaborate approaches and dull ones (if we have the dull ones available too). One common example is using MonadPlus for some backtracking algorithm, then instantiatiating it to Maybe or List instance depending on wether you just want one solution or all of them. [*] For instance, I've missed Maybe being an instance of MonadError. Greetings, J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Implicit parameters:
Isn't it just the monomorphism restriction at work? This works fine: f () = do a - get_unique putStr (showInt a \n) b - get_unique putStr (showInt b \n) c - get_unique putStr (showInt c \n) get_unique :: (?global_counter :: IORef Int) = IO Int get_unique = readIORef ?global_counter J.A. ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [Haskell-cafe] Top Level etc.
Perhaps one could have top-level implicit parameters (or top-level contexts in general): module (?myvar :: IORef Int) = Random where Hi! I suggested something very similar to this some months ago, syntax and all. Nice to see I'm not the only one thinking along this lines. http://www.mail-archive.com/haskell%40haskell.org/msg14884.html module Main where import MyMain -- mymain :: (?myvar :: IORef Int) = IO () -- outside main = do var - newIORef 1 -- initialisers in the order you want let ?myvar = var in mymain By then I also suggest that maybe we could also bind the implicit on import, something like: module (?par :: Parameter) = A where ... module B where import A -- simple, ?par unbound import qualified A as Ak where ?par = k -- ?par bound to k import qualified A as Am where ?par = m -- ?par bound to m Seemed fine as long as the parameters didn't depend on the imported modules. But on hindsight, making an import depend on valued defined in the body of the module is probably quite clumsy, unfortunately (right?). Still, import qualified A as Ak where ?par = 1 or import qualified A as Ak where ?par = newIORef or even import C(k) import qualified A as Ak where ?par = k Doesn't sound that bad though. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Some random newbie questions
On Friday 07 January 2005 12:03, Ketil Malde wrote: Naive use of foldl. I tend to think the default foldl should be strict (ie. replaced by foldl') -- are there important cases where it needs to be lazy? Hi, One simple example would be, reverse = foldl (flip (:)) [] J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Some random newbie questions
On Friday 07 January 2005 12:03, Ketil Malde wrote: Naive use of foldl. I tend to think the default foldl should be strict (ie. replaced by foldl') -- are there important cases where it needs to be lazy? Hi, One simple example would be, reverse = foldl (flip (:)) [] A better example would be building some other lazy structure that is strict on it's elements... J.A. --- module Test where import Data.List data L = E | !Int :+: L deriving Show -- my head h (x:+:xs) = x h E= error ops -- rev1 = foldl (flip (:+:)) E rev2 = foldl' (flip (:+:)) E l= [error , error , 1::Int] -- *Test h (rev1 l) 1 (0.00 secs, 264560 bytes) *Test h (rev2 l) *** Exception: (0.01 secs, 264524 bytes) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Some random newbie questions
No, it would work with strict foldl too. In fact in the absence of optimization it would work better (uses less time and space). The optimization required is inlining and strictness analysis. Is this also true if your just going to use the first few elements after reversing it? A function which requires lazy foldl for correctness would have to be sometimes lazy in its first argument, and at the same time some partial results would have to be undefined. By function I mean the first argument of foldl, treated as a binary function. Here this doesn't apply because flip (:) x y is always defined. And another common case for foldl, sum, doesn't apply because (+) is usually strict on both arguments (although in principle it does not have to be true because of overloading, which implies that a compiler can only optimize particular specializations of sum, not generic sum). I don't know of any real-life example. Yes you are right, my bad. I was thinking as if lists were not lazy on their elements, therefore my second example... But yes, now it is not a common example anymore. Perhaps there are cases where evaluating partial results is correct but inefficient. I don't know such case either. Just replace the errors on my second example by some big computations... J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Some random newbie questions
On Sunday 09 January 2005 21:30, Marcin 'Qrczak' Kowalczyk wrote: Jorge Adriano Aires [EMAIL PROTECTED] writes: No, it would work with strict foldl too. In fact in the absence of optimization it would work better (uses less time and space). The optimization required is inlining and strictness analysis. Is this also true if your just going to use the first few elements after reversing it? Yes. A strict fold would evaluate cons cells of the result while they are constructed, not list elements. They are all defined (flip (:) x y is always defined), so a strict foldl is correct. Yes, now I was refering to the efficiency issue only. Making a cons cell should be not more expensive than making a thunk which will make a cons cell when evaluated. Ok, I wasn't sure about this... Well, unless the implementation doesn't inline flip and thus making these thunks is actually faster than running them. In this case a lazy foldl is more efficient than a strict foldl, as long as a sufficiently small part of the result is used; it's always less efficient if the whole result is examined. Right. J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Some random newbie questions
(+) is usually strict on both arguments (although in principle it does not have to be true because of overloading, which implies that a compiler can only optimize particular specializations of sum, not generic sum). Since you mention it, there was some talk about this in the #haskell channel, and I wondered why aren't sum and product members of Num with default instances, just like mconcat is also a member of Data.Monoid.Monoid. From the docs: mconcat :: [a] - a Fold a list using the monoid. For most types, the default definition for mconcat will be used, but the function is included in the class definition so that an optimized version can be provided for specific types J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Class Synonyms - example 2
Jorge,
Besides the case where 'a' is the same as 'b', there is also another
interesting case. That is when you have both, Foo A B and Foo B A.
This is a
known property (named DoubleFoo) [...]
Again, with -fallow-undecidable-instances:
\begin{code}
class (Foo a b, Foo b a) = DoubleFoo a b
instance (Foo a b, Foo b a) = DoubleFoo a b
\end{code}
Ah! Got it.
Thanks Stefan,
J.A.
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[Haskell-cafe] Class Synonyms
Hello! I got a multi-parameter type class: class Foo a b | a - b where foo_method1 :: ... foo_method2 :: ... ... And some particular cases are important on their own, like the one where 'a' and 'b' are the same, I call elements with this property, Bar. So I defined: class Foo a a = Bar a where This is nice, now I can replace (F a a) for (Bar a) in the context of many functions. Less typing and it's more readable. Still, I have to define instances as: instance Foo A A where foo_method1 = bla1 foo_method2 = bla2 instance Bar A where I'd like to achieve something better than this. I was looking for something like class synonyms. So if I declared an instance Foo A A, I automagicaly had Bar A, because they'd be the same. Or even better, I could declare instance Bar A where foo_method1 = bla1 foo_method2 = bla2 Is anything like this possible at all? Thanks, J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Class Synonyms - example 2
Maybe I should have included a more interesting example in the previous mail. So I had this class: class Foo a b | a - b where foo_method1 :: ... foo_method2 :: ... ... Besides the case where 'a' is the same as 'b', there is also another interesting case. That is when you have both, Foo A B and Foo B A. This is a known property (named DoubleFoo), so I'd like to type contexts as, DoubleFoo a b = instead of, (Foo a b, Foo b a) = so I tried: class (Foo a b, Foo b a) = DoubleFoo a b where This works fine if I'm going to define functions which need both instances of Foo. Something like: testDouble :: DoubleFoo a b = a - b - c testDouble a b = foo_method1 a b ... foo_method1 b a but it doesn't help me with: testDouble2 :: DoubleFoo a b = a - b - c testDouble2 a b = foo_method1 a b ... testDouble2 b a now I need DoubleFoo b a as well. Seems to me like there is no way of saying: Foo a b , Foo b a = DoubleFoo a b right? J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell] why no strictness annotations in labelled fields?
Is there a good reason one can't do:
data Foo = Foo {bar::!String}
Just add a space after the ::
{bar:: !String}
J.A.
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Re: [Haskell-cafe] foldlWhile
(opss just noticed I did a reply-to) The following is closer to the original, but doesn't work when the whole list is folded (i.e., p always satisfied): foldlWhile f p a = head . dropWhile p . scanl f a Serge's version returns the last 'a' that satisfies 'p', while yours Not really. returns the first 'a' that does not satisfy 'p'. This should be an equivalent version: Yeap, just like his, foldlWhile :: (a - b - a) - (a - Bool) - a - [b] - a foldlWhilefp abs = snip (_, False) - a snip It tests the accumulator with p, and returns it on false. J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] FiniteMap-like module for unordered keys?
Hello, A hash-table becomes rather useless without mutable state AFAICS. Without it, one might almost just as well use a list of pairs... Could you please elaborate? Is there motive why an Hash Table, implemented as FiniteMap of Lists, for instance, wouldn't be worth to using over a simple List? (This wouldn't help G. Klyne of course). I've always wondered why a non-monadic version of is not provided in the GHC libs... J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] QuickCheck - Extracting data values from tests
The generate function is exported: generate :: Int - StdGen - Gen a - a Thanks, that's the one I was missing. J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] QuickCheck - Extracting data values from tests
Hello all, When using Quickcheck, is there some way to extract generated data values to the IO Monad? I know I can collect and print information about test cases, but that's not enough. Data may be pretty complex, and there may be no parsers for it. If a test suddenly goes wrong, just having it displayed doesn't seem that useful. You may be interested in a QuickCheck hack of mine that saves the offending data value to use immediately in the next test run. Nice! It's different from what I was looking for but also quite usefull. You can get the current version with darcs get http://thunderbird.scannedinavian.com/repos/quickcheck; I've only used this for my own code, so I'd be interested in any feedback. Ok. In some cases it's a lot easier to generate a value from a seed and size rather than saving the value in some way that you can restore (ie functions). I've been investigating doing test-driven-development with QuickCheck, saving failing test cases is one step towards that goal. If you have more ideas on that topic, I'd like to hear about it. Well, returning (part of) the generated data is one of them :) Also, even when I'm implementing a generator, I want to see how it is working. Running a verboseCheck on some dummy property helps, but I may want to analyse the data, or some parts of it better - for instance, for many data structures I have alternative show functions that take parameters as arguments. This isn't clear to me, can you give other examples? Not sure which part is not clear... I'll just try to explain each of them. Lets say I'm implementing a generators for Graphs. Also, even when I'm implementing a generator, I want to see how it is working. I want to check if the generated Graphs are like I intended them to be. Running a verboseCheck on some dummy property helps, but I may verboseCheck by default prints all the data. I can run it on a dummy function that always returns True to see what kind of data I'm getting. want to analyse the data, or some parts of it better May want to print the 'actual graphs' on the screen (ASCII art, or maybe using some function that calls Gnuplot). Then I may want to check in more detail the info in contained in some of the nodes. Then I may decide to run some functions on it. many data structures I have alternative show functions that take parameters as arguments. Like I just said, I may want to show the graph in many ways. But there are more possibilities. Why limitate the usefulness of QuickCheck? Suppose I just implemented generators for a few kinds of terms and formulas to test some properties. Now I want to benchmark a couple of different unification functions... I'd expect to be able to use my generator for that. Unless I'm missing something, I cannot. Am I right? J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] QuickCheck - Extracting data values from tests
Hello all, When using Quickcheck, is there some way to extract generated data values to the IO Monad? I know I can collect and print information about test cases, but that's not enough. Data may be pretty complex, and there may be no parsers for it. If a test suddenly goes wrong, just having it displayed doesn't seem that useful. I'd expect quickCheck to have type: quickCheck :: forall a. (Testable a) = a - IO [a] Show I could just get the offending data with: please_be_empty - quickCheck prop_foo Also, even when I'm implementing a generator, I want to see how it is working. Running a verboseCheck on some dummy property helps, but I may want to analyse the data, or some parts of it better - for instance, for many data structures I have alternative show functions that take parameters as arguments. Thanks in advance, J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Problem with Unboxed Types
Just to be sure, I've just compiled and run this code with GHC 6.2.1, on
a Linux system. I can't account for your difficulty I'm afraid.
Simon
Thanks, and nevermind, I think I figured out what was happening. Working now.
J.A.
{-# OPTIONS -fglasgow-exts #-}
module Main where
import GHC.Exts
showUnboxedInt :: Int# - String
showUnboxedInt n = (show $ I# n) ++ #
main = print (showUnboxedInt 3#)
| -Original Message-
| From: [EMAIL PROTECTED]
[mailto:glasgow-haskell-users-
| [EMAIL PROTECTED] On Behalf Of Jorge Adriano Aires
| Sent: 27 August 2004 02:55
| To: [EMAIL PROTECTED]
| Subject: Problem with Unboxed Types
|
| Hello,
| I'd like to try using Unboxed types, but I always get a parse error
on input
| `#' error. To make sure I wasn't making some mistake I tried the
example in
| the wiki:
|
| http://www.haskell.org/hawiki/UnboxedType
|
| module Main where
| import GHC.Exts
|
| showUnboxedInt :: Int# - String
| showUnboxedInt n = (show $ I# n) ++ #
|
|
| Even tried adding fglasgow-exts, nothing works though. I'm using GHC
6.2.1 on
| a SuSE 8.2 system (glibc 2.3) installed using the available RPMs. Any
idea?
| Thanks in advance,
| J.A.
|
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Problem with Unboxed Types
Hello, I'd like to try using Unboxed types, but I always get a parse error on input `#' error. To make sure I wasn't making some mistake I tried the example in the wiki: http://www.haskell.org/hawiki/UnboxedType module Main where import GHC.Exts showUnboxedInt :: Int# - String showUnboxedInt n = (show $ I# n) ++ # Even tried adding fglasgow-exts, nothing works though. I'm using GHC 6.2.1 on a SuSE 8.2 system (glibc 2.3) installed using the available RPMs. Any idea? Thanks in advance, J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: How do I specify the source search path?
On Wednesday 30 June 2004 16:15, S. Alexander Jacobson wrote: I have the working code for my Haskell App Server Framework in ~/HAppS/HAppS.hs. If I want to start working on an app that uses this framework in e.g. ~/MyApp, how do I tell GHCi to resolve import HAppS? Note: The HAppS code is in active development so I don't want to put it in the system GHC lib hierarchy. I just want to use it add hoc from the command line. Using the -i flag. Unfortunatly relative paths (using '~') don't seem to work, so you have to type the full path: ghci -i/home/user/HAppS/ As far as I know, there is also no way to specify the search path in an environment var, so my solution was to add to my .alias: alias ghci='ghci -i/home/jadrian/Program/Haskell/MyLibs' alias ghc='ghc -i/home/jadrian/Program/Haskell/MyLibs' J.A. J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [Haskell] Initialisation without unsafePerformIO
What ideas do people have for getting rid of unsafePerformIO? Hope my suggestion is not too naive. I get along quite fine using implicit parameters in many cases, it's just tedious explicitly typing them in every function context. I'd be pretty happy if it was possible to define the 'scope' of some implicit parameters in a module and/or define their scope as being a whole module. The 2nd option would be something like: module (?par :: Parameter) = A where ... Functions in A could have ?par in their context without having it explicitly typed. Now the import of A could be done with: module B where import A -- simple, ?par unbound import A as Ak where ?par = k -- ?par bound to k import A as Am where ?par = m -- ?par bound to m ... k :: Parameter k = ... m :: Parameter m = ... ... Also, module (?par :: Parameter) = C where import A -- both A and C paremeterised by ?par Since both modules share the same parameter, instantiation on ?par in the import of C would propagate to the import of A. At first glance it seems simple syntactic sugar and therefore doable. Along with some options in the interpreter to hide/show (this kind of) implicit parameters when displaying signatures, check module context, etc. probably also quite usable. J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Namespace trouble
Jorge Adriano Aires [EMAIL PROTECTED] writes: I have the following structure: MyProgram/A.hs MyProgram/Aux/B.hs MyProgram/Aux/C.hs You have already received replies to your question, so let me make a different point. If you ever intend your program to work on Windows, do not use Aux as a file or directory name! The libraries mailing list has some recent experience of this. (Apparently Aux is a reserved filename on Windows and you get strange behaviour if you try to use it for anything else.) Thanks for the tip, I'm working in linux though and Aux was really just an example :) J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Namespace trouble
Hello, I have the following structure: MyProgram/A.hs MyProgram/Aux/B.hs MyProgram/Aux/C.hs and: A imports C B imports C Can I make this work using namespaces only (i.e. no -i flag)? I expected this to work: MyProgram/A.hsname: Aimport Aux.C MyProgram/Aux/B.hsname: Aux.Bimport C MyProgram/Aux/C.hsname: Aux.C But complains when importing C from B since its name is Aux.C. What is the most elegant way to deal with such cases? J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Namespace trouble
I expected this to work: MyProgram/A.hsname: Aimport Aux.C MyProgram/Aux/B.hsname: Aux.Bimport C MyProgram/Aux/C.hsname: Aux.C But complains when importing C from B since its name is Aux.C. What is the most elegant way to deal with such cases? Answering myself, this works: MyProgram/A.hsname: Aimport Aux.C MyProgram/Aux/B.hsname: Aux.Bimport Aux.C MyProgram/Aux/C.hsname: Aux.C I just have to load B from MyProgram when testing it in ghci, instead of loading it in MyProgram/Aux. Thanks to Lunar for the tip. J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [Haskell-cafe] Monadic Composition
So, yes, it is useful, but should it be included in a standard Monad module? After all, this module contains mostly trivial functions ;) BTW. You can write this function using foldM: compM l a = foldM (#) a l where # is an often used reverse application operator: x # f = f x Right. Now that I look at it, someone probably tried to give me this advice before but I failed to understand... (sorry monotonom!). It's all clear now. One more question. Isn't the foldM description a bit misleading? From the Report and also in GHC documentation: The foldM function is analogous to foldl, except that its result is encapsulated in a monad.(...) foldM f a1 [x1, x2, ..., xm ] == do a2 - f a1 x1 a3 - f a2 x2 ... f am xm After reading this I expected left associativity, just like in my first definition. That'd mean a fail wouldn't stop a computation immediately but instead be passed from function to function. By checking its definition in the report I can see this is not the case though. Cheers, J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Monadic Composition
Hello, I needed to compose monadic functions, since I couldn't find anything in Control.Monad so I defined my own. I first came up with a left associative version: compLM [f1,..., fn] a = (f1 a = f2) = ... = fn compLM:: Monad m = [a-m a] - (a-m a) compLM mfs a = foldl (=) (return a) mfs Later a right associative version, which I believe to be more efficient, particularly when some computation returns fail: compRM [f1,..., fn] a = \x-f1 x = (\x-f2 x = ... = \x-fn x) a compRM :: Monad m = [a-m a] - (a-m a) compRM = foldr (\f g- (\x -f x = g)) return This higher order function seems quite useful to me and not trivial, so I expected it to be available. When this happens I wonder if I'm missing something. Am I complicating things? Is it actually available (or some other that does the trick) ? Or is it just me who finds this usefull enough to be in the libs? J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: int to float problem
Mike T. Machenry [EMAIL PROTECTED] writes: I recently desided I wanted a bunch function to return float instead of Int. [...] I found fromInteger but it didn't seem to work on the return value of the cardinality function for instance. Try fromIntegral, which works for Int and Integer, too. Casting an Integral value to a Fractional value to perform arithmetic operations, is a very common need and I don't like adding fromIntegral everywhere, so ended up writing a (very simple) module with generalized arithmetic operators (see attachment). The » next to the operations indicate a cast from an Integral to a Fractional value. J.A. module CrossTypeOps where -- Addition (+«) :: (Fractional a, Integral b)= a - b - a (+«) x n = x+fromIntegral n (»+) :: (Integral a, Fractional b)= a - b - b (»+) n x = fromIntegral n + x (»+«) :: (Integral a, Fractional b)= a - a - b (»+«) m n = fromIntegral m+fromIntegral n -- Difference (-«) :: (Fractional a, Integral b)= a - b - a (-«) x n = x-fromIntegral n (»-) :: (Integral a, Fractional b)= a - b - b (»-) n x = fromIntegral n - x (»-«) :: (Integral a, Fractional b)= a - a - b (»-«) m n = fromIntegral m-fromIntegral n -- Multiplication (*«) :: (Fractional a, Integral b)= a - b - a (*«) x n = x*fromIntegral n (»*) :: (Integral a, Fractional b)= a - b - b (»*) n x = fromIntegral n * x (»*«) :: (Integral a, Fractional b)= a - a - b (»*«) m n = fromIntegral m*fromIntegral n -- Division (/«) :: (Fractional a, Integral b)= a - b - a (/«) x n = x / fromIntegral n (»/) :: (Integral a, Fractional b)= a - b - b (»/) n x = fromIntegral n / x (»/«) :: (Integral a, Fractional b)= a - a - b (»/«) m n = fromIntegral m / fromIntegral n -- Priorities infixl 6 +«, »+, »+«, -«, »-, »-« infixl 7 *«, »*, »*«, /«, »/, »/«
Pattern matching with implicit par. bug
There it goes,
J.A.
type CTPar = ([Double],Int,Int)
us :: (?ctPar :: CTPar) = [Double]
us = let (us',_,_) = ?ctPar in us'
ghc-5.04: panic! (the `impossible' happened, GHC version 5.04):
tcSplitTyConApp
{?ctPar{-r2kd-} :: ([GHCziFloat.Double{-3u-}],
GHCziBase.Int{-3J-},
GHCziBase.Int{-3J-})}
Please report it as a compiler bug to [EMAIL PROTECTED],
or http://sourceforge.net/projects/ghc/.
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Pattern matching, implict par. question
Hello, When trying type CTPar = ([Double],Int,Int) ctPar ::(?ctPar::CTPar)=CTPar ctPar@(us,n,j) = ?ctPar I got this error message in ghci is: Illegal overloaded type signature(s) in a binding group for ctPar, us, n, j that falls under the monomorphism restriction When generalising the type(s) for ctPar, us, n, j Failed, modules loaded: CrossTypeOps. Should the monomorphis restriction really apply here? The values of us,j,n do depend on the context, but not their types (right?). J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Very simple question
On style, yes. A concrete example of what I asked before. The following doesn't type check for obvious reasons. In your opinion what is the most elegant way to fix it? Like I said, I'm starting to feel like defining my own operators is the way to go. On the other hand, this kind of situation seems quite common and there are no such operators in the standard libraries, so I might be wrong here. Opinions anyone? J.A. -- Source Code: -- module TypeCheck where import Data.Array import Data.Ix bigR :: Double bigR = sum[a!(i,j)*(r!i - n*b!i)*(r!j-n*b!j) | (i,j)-range(bounds a)]/n a :: Array (Int,Int) Double b :: Array Int Double r :: Array Int Int n :: Int a=undefined b=undefined r=undefined n=undefined ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Ints as ints and floats - Question on style
Hello, Lately I've been coding functions where I have to use Int elements, both as Ints (e.g. as an array or list index) and Doubles/Floats (e.g. when performing arithmetic operations with Doubles/Floats). How do you usually deal with this? Some options I can think of are, 1. using fromIntegral when needed (some formulas become unredable). 2. duplicating all those ints to float elements. (not very readable either, you end up with two distinct elements that mean the same thing). 3. creating new operators perform arithmetic operations with Ints/Integers and Float/Doubles. Cheers, J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Global variables?
Hello, Is it even possible to make a global variable in Haskell? If yes, how? Thanks. (short answer, no time now...) Look here: http://www.haskell.org/pipermail/haskell-cafe/2002-January/002589.html Hope it helps ;) J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Ambiguous type variable
Hi all, This works fine, as expected f :: (Num a, Random a) = Int - [a] f = randomRs (0,1).mkStdGen If I skip the type signature, though, I get the following error messages: Main.hs:14: Ambiguous type variable(s) `a' in the constraint `Random a' arising from use of `randomRs' at Main.hs:14 In the first argument of `(.)', namely `randomRs (0, 1)' In the definition of `f': (randomRs (0, 1)) . mkStdGen Main.hs:14: Ambiguous type variable(s) `a' in the constraint `Num a' arising from the literal `1' at Main.hs:14 In the first argument of `randomRs', namely `(0, 1)' In the first argument of `(.)', namely `randomRs (0, 1)' Why exctly can't ghci figure how the type of f? This just happens when loading the code from a file, :t randomRs (0,1).mkStdGen in ghci works fine. J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: ANNOUNCE: GHC version 5.04.2 released
Hi, Are there going to be SuSE rpms available anytime soon? Anyone working on them? Thanks, J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: ANNOUNCE: GHC version 5.04.2 released
Hi, Are there going to be SuSE rpms available anytime soon? Anyone working on them? Thanks, J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: slide: useful function?
Doesn't seem that usefull to me, you can get the several consecutive ones by applying tails to your list. I want to propose the following function slide, which is like map, but depends not on one value of a list, but on several consecutive ones. slide :: ([a] - b) - [a] - [b] slide f [] = [] slide f xs = f xs : slide f (tail xs) slide f = map f.init.tails The init is needed because you don't apply f to the empty List. If you did, then it would be just. slide :: ([a] - b) - [a] - [b] slide f [] = f [] --- slide f xs = f xs : slide f (tail xs) slide f = map f.tails J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Why no findM ? simple Cat revisited
Simple Cat (revisitied)
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { v - x; if f v then return (Just v) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h
\end{code}
Seems to me like the name findM could be misleading
mapM :: (Monad m) = (a - m b) - [a] - m [b]
filterM :: (Monad m) = (a - m Bool) - [a] - m [a]
These take a monadic function and a list of elements. Yours works the other
way around (takes a function and a list of 'monadic elements').
I'd expect the definition of findM to be:
findM' :: (Monad m) = (a - m Bool) - [a] - m (Maybe a)
findM' f [] = return Nothing
findM' f (x:xs) = do { b - f x; if b then return (Just x) else findM' f xs }
This one doesn't serve your purpose though.
J.A.
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Re: Why no findM ? simple Cat revisited
Simple Cat (revisitied)
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { v - x; if f v then return (Just v) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h
\end{code}
Seems to me like the name findM could be misleading
mapM :: (Monad m) = (a - m b) - [a] - m [b]
filterM :: (Monad m) = (a - m Bool) - [a] - m [a]
These take a monadic function and a list of elements. Yours works the other
way around (takes a function and a list of 'monadic elements').
I'd expect the definition of findM to be:
findM' :: (Monad m) = (a - m Bool) - [a] - m (Maybe a)
findM' f [] = return Nothing
findM' f (x:xs) = do { b - f x; if b then return (Just x) else findM' f xs }
This one doesn't serve your purpose though.
J.A.
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library of monadic functions [was: Why no findM ? simple Cat revisited]
I appreciate your comment.
I agree that the type of findM should be the one you suggested,
and it still fits my original purpose. It's no more than a step arout.
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { b - f x; if b then return (Just x) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (=return.isLeft) (hCat stdin)
where hCat h = try (hGetLine h=putStrLn) : hCat h
\end{code}
Yes, you are right.
It was enough because, you don't really care about what you found, you just
want to search and stop when you do find something. You are returning the
action that returned an element that satisfied your
condition, not the actual element like before.
I expetct the next Haskell Library Report includes findM.
It's obviously useful.
I think both versions can be very useful:
findM :: (Monad m) = (a - m Bool) - [a] - m (Maybe a)
findM' :: (Monad m) = (a - Bool) - [m a] - m (Maybe a)
Same can be said for,
takeWhileM :: (Monad m) = (a - m Bool) - [a] - m [a]
takeWhileM' :: (Monad m) = (a - Bool) - [m a] - m [a]
both would be usefull for different purposes.
Oh and since we're on it I also miss,
iterateM :: (Monad m) = (a - m a) - a - m [a]
untilM :: (Monad m) = (a - m a) - a - m [a]
etc etc...
I've just been coding them as I need them, like many of you I suppose.
J.A.
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Re: Bug? [was: Implicit params]
Now fixed in the HEAD, and will be in 5.04.2 Thanks for pointing it out. Simon Thanks :) J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Bug? [was: Implicit params]
Now fixed in the HEAD, and will be in 5.04.2 Thanks for pointing it out. Simon Thanks :) J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Bug? [was: Implicit params]
On Thursday 14 November 2002 18:47, Iavor S. Diatchki wrote:
hello,
Well, actually you must be right since the pure field defines a pure
(projection) function... Hmmm, ok, can someone explain this to me,
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
m:: Int
}
-- this is fine, obviously...
pure :: E s - Int
pure e = m e
-- but this is not...
pure2 :: (?e :: E s) = Int
pure2 = m (?e)
Why exactly isn't this allowed? What is the workaround?
Error msg:
Ambiguous constraint `?e :: E s'
At least one of the forall'd type variables mentioned by the
constraint
must be reachable from the type after the '='
In the type: forall s. (?e :: E s) = Int
While checking the type signature for `pure2'
Failed, modules loaded: none.
-
-
this seems like a bug in GHC. Hugs is happy with it. The s in the
pure2 signature is not ambiguous because it is determined when you give
the value of the implicit parameter. in fact the way i think about
implicit parameters is simply as a nice notation for computations in the
environment monad, so in my mind the above two definitions are pretty
much the same.
Thanks Iavor!
GHC people, can you confirm this, Is it a bug? I'm using Ghc 5.0.4 SuSE rpms.
Is there a workaround?
Thanks,
J.A.
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Bug? [was: Implicit params]
On Thursday 14 November 2002 18:47, Iavor S. Diatchki wrote:
hello,
Well, actually you must be right since the pure field defines a pure
(projection) function... Hmmm, ok, can someone explain this to me,
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
m:: Int
}
-- this is fine, obviously...
pure :: E s - Int
pure e = m e
-- but this is not...
pure2 :: (?e :: E s) = Int
pure2 = m (?e)
Why exactly isn't this allowed? What is the workaround?
Error msg:
Ambiguous constraint `?e :: E s'
At least one of the forall'd type variables mentioned by the
constraint
must be reachable from the type after the '='
In the type: forall s. (?e :: E s) = Int
While checking the type signature for `pure2'
Failed, modules loaded: none.
-
-
this seems like a bug in GHC. Hugs is happy with it. The s in the
pure2 signature is not ambiguous because it is determined when you give
the value of the implicit parameter. in fact the way i think about
implicit parameters is simply as a nice notation for computations in the
environment monad, so in my mind the above two definitions are pretty
much the same.
Thanks Iavor!
GHC people, can you confirm this, Is it a bug? I'm using Ghc 5.0.4 SuSE rpms.
Is there a workaround?
Thanks,
J.A.
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Re: Record of STRefs better than STRef to a Record?
If I use an STRef to a record, will a new record be created
each time I want
to update a single field? Or can I expect GHC to optimize it
and have the field of the record updated in place?
You'll get a new record for each update. This might not be so bad
though, depending on the number of fields in your record.
One of them has 4, the other 3, but they might grow bigger...
Here's another trick if you use this route:
added some parenthesis
data E s = E{
refi :: !(STRef s Int),
refc :: !(STRef s Char)
}
and compile with -funbox-strict-fields. This will eliminate the boxing
of the STRefs.
Nice, thanks :)
One more question.
I'm passing that 'record' around as an implicit value. The record as STRefs
that I use to collect info, but it also has some pure fields with 'read-only'
info. Something like,
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
max :: Int
}
In some functions I might need only some pure fields, and none of the STRefs,
but since I pass something of type 'E s' around, everything ends up beeing
monadic. Is using two records (on with STRefs and one with pure fields) the
only/best way to avoid this?
I would like to use two records, doesn't seem natural, but I also don't like
to end up using monadic functions when they are, in fact, pure...
Thanks,
J.A.
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Re: Record of STRefs better than STRef to a Record?
I'm passing that 'record' around as an implicit value. The record as
STRefs that I use to collect info, but it also has some pure fields with
'read-only' info. Something like,
opss let me change 'max' to 'm'
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
m:: Int
}
In some functions I might need only some pure fields, and none of the
STRefs, but since I pass something of type 'E s' around, everything ends
up beeing monadic.
there is no reason why that should be the case. you only need to be
within the ST monad when you read the references, so if in some function
you only look at the pure values, it can have a pure type.
hope this helped
iavor
That's what I thought, untill I tried it.
Well, actually you must be right since the pure field defines a pure
(projection) function... Hmmm, ok, can someone explain this to me,
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
m:: Int
}
-- this is fine, obviously...
pure :: E s - Int
pure e = m e
-- but this is not...
pure2 :: (?e :: E s) = Int
pure2 = m (?e)
Why exactly isn't this allowed? What is the workaround?
Error msg:
Ambiguous constraint `?e :: E s'
At least one of the forall'd type variables mentioned by the
constraint
must be reachable from the type after the '='
In the type: forall s. (?e :: E s) = Int
While checking the type signature for `pure2'
Failed, modules loaded: none.
--
Thanks,
J.A.
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Re: Record of STRefs better than STRef to a Record?
If I use an STRef to a record, will a new record be created
each time I want
to update a single field? Or can I expect GHC to optimize it
and have the field of the record updated in place?
You'll get a new record for each update. This might not be so bad
though, depending on the number of fields in your record.
One of them has 4, the other 3, but they might grow bigger...
Here's another trick if you use this route:
added some parenthesis
data E s = E{
refi :: !(STRef s Int),
refc :: !(STRef s Char)
}
and compile with -funbox-strict-fields. This will eliminate the boxing
of the STRefs.
Nice, thanks :)
One more question.
I'm passing that 'record' around as an implicit value. The record as STRefs
that I use to collect info, but it also has some pure fields with 'read-only'
info. Something like,
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
max :: Int
}
In some functions I might need only some pure fields, and none of the STRefs,
but since I pass something of type 'E s' around, everything ends up beeing
monadic. Is using two records (on with STRefs and one with pure fields) the
only/best way to avoid this?
I would like to use two records, doesn't seem natural, but I also don't like
to end up using monadic functions when they are, in fact, pure...
Thanks,
J.A.
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Record of STRefs better than STRef to a Record?
Hi all,
If I use an STRef to a record, will a new record be created each time I want
to update a single field? Or can I expect GHC to optimize it and have the
field of the record updated in place?
Right now I'm using a record of STRefs, like:
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char
}
but it can get a little messy, since thread s propagates to every datatype
that uses the record type in it's definition.
Thanks,
J.A.
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Re: producing and consuming lists
But like I just showed, sometimes paring them may not be a natural approach though... Yeah, I understand what you mean. In the examples you give, you could always try to make the appended stuff (the zs, etc) the same length by appending Nothings, but probalby not a general solution. Yes maybe, I was hoping I could find a more elegant solution though. I wonder if, given you original function which returns ([Int],[Int]), you couldn't do something really ugly like using unsafeInterleaveIO on writing the first list, and then write the second list by hand. Just a thought. If consuming is just writing stuff to files I think Trace (which uses unsafeInterleaveIO) would be just fine, but consuming might be more than that. In that case I'd have to fuse the functions that consume each of the streams, with the function that produces them. J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Dealing with configuration data
Evening,
I'm trying to write a utility that reads in some user preferences from
a pre-determined file, does some work, and exits. Sounds simple enough.
The problem I'm having is with the preferences: How do I make it
available throughout the entire program? (FWIW, most of the work is
effectively done inside the IO monad.) I could explicitly pass the
record around everywhere, but that seems a trifle inelegant.
My current solution is to use a global ('scuse my terminology, I'm not
sure that's the right word to use here) variable of type IORef Config
obtained through unsafePerformIO. It works, but strikes me as a rather
barbaric solution to a seemingly tame enough problem...
Intuition tells me I should be able to `embed', if you will, the config
record somehow within or alongside the IO state, and retrieve it at
will. (Is this what MonadState is for?) However it also tells me that
this will /probably/ involve lots of needless lifting and rewriting of
the existing code, which makes it even less enticing than passing
everything around explicitly.
This is how I usually do it:
http://www.mail-archive.com/haskell@haskell.org/msg10565.html
(ignore the last part of the post...)
J.A.
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Re: mutable records
Hi, thx for this reply. Is there any overhead using this mutable? I just thought I should point out that Mutable is not an haskell type. You can see in the Utils module that it is just a type synonim for IORef: http://icfpcontest.cse.ogi.edu/simulator/pfe.cgi?Utils#Mutable http://haskell.cs.yale.edu/ghc/docs/latest/html/base/Data.STRef.html I think that sometimes STRef might be a better choice... Are you also using Templates ? With this mutable I can adapt a lot of software from Ocamel. I think this John Hughes paper Global Variables in Haskell might be usefull to you: http://www.math.chalmers.se/~rjmh/Globals.ps But the gnawing question remains: Shall it be possible to be almost as efficient (in native code) as Ocamel's code, I refer here to Doug Bagley's comparison of programming languages. Remarkably the fibonacci numbers test scores almost as well. But the code is not the same. Comparable code would have been to use the same code of Ocamel, to be more specific: how efficiently is recusivity implemented in Haskell. I cannot compare this on my Windows XP since I need MSVC6.0 on this machine which I don't have. On the other hand I have cygwin installed now. Unfortunately I can't make makefiles. Probably on the web I can find an explanation. In this way I can recompile Ocamel with Cygwin and compare the results a bit. There are also 5(?) failures of Haskell programms . Is there a flaw in these programms? P.S. does anyone know a good Haskell IDE for Windows XP? I don't really use windows but I know there is support for the windows version of JCreator: http://www.students.cs.uu.nl/people/rjchaaft/JCreator/ You can always use Xemacs :) http://www.xemacs.org/Download/win32/ http://www.haskell.org/haskell-mode/ J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: A problem with haskell-mode
Hi, all, I installed the latest version of Haskell mode for emacs. Whenever I load haskell major mode by opening a haskell file, Xemacs gives me the following error message: (1) (error/warning) Error in `post-command-hook' (setting hook to nil): (void-variable imenu--index-alist) My Xemacs version is 21.4.8. It works well under GNU emacs though. Anybody has an idea how to get around it? Thanks. Deling I had the same problem, and I found a fix somewhere. Don't reember what exactly I had to change so I'll just send you my Haskell section of xemacs init.el. By the way I get an error whenever I type undefined. Anyone else noticed it? (1) (error/warning) Error in `post-command-hook' (setting hook to nil): (wrong-type-argument listp a ) J.A. --- (setq load-path (cons /usr/share/emacs/site-lisp/haskell-mode-1.41 load-path)) (setq auto-mode-alist (append auto-mode-alist '((\\.[hg]s$ . haskell-mode) (\\.hi$ . haskell-mode) (\\.l[hg]s$ . literate-haskell-mode (autoload 'haskell-mode haskell-mode Major mode for editing Haskell scripts. t) (autoload 'literate-haskell-mode haskell-mode Major mode for editing literate Haskell scripts. t) ;(add-hook 'haskell-mode-hook 'turn-on-haskell-ghc) (add-hook 'haskell-mode-hook 'turn-on-haskell-font-lock) (add-hook 'haskell-mode-hook 'turn-on-haskell-decl-scan) (require 'imenu) (add-hook 'haskell-mode-hook 'turn-on-haskell-doc-mode) (add-hook 'haskell-mode-hook 'turn-on-haskell-indent) ;(add-hook 'haskell-mode-hook 'turn-on-haskell-simple-indent) ;(add-hook 'haskell-mode-hook 'turn-on-haskell-hugs) ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Docs missing?
The doc RPM package for Red Hat 7.3 suffers the same problem as the SuSE one. Could someone please give a hand-holding guide so that we can fix it ourselves? Please? Please? It's easy ;) Go to http://haskell.cs.yale.edu/ghc/documentation.html and in the downloadable/printable documentation section you'll find tar.gzs with the HTML files missing. Just untar/gunzip them in the appropriate dir (the on that contains the main index.html file) J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: DeepSeq
On Friday 19 July 2002 12:10, George Russell wrote: Would it be possible to bring the DeepSeq library into the libraries distributed with GHC? (I think Dean Herington is responsible for it.) Of course it's easy enough to drop it into one's own program (I am just about to do this) but (1) It is fairly common to want to force deeper evaluation. (2) DeepSeq is simple enough to be dropped in the GHC distribution, without it causing much trouble or making it much bigger. (3) At the same time, it is not so simple that it can be reimplemented in a couple of lines. Agree. Beeing able to derive instances of DeepSeq would be nice too. J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: State monads don't respect the monad laws in Haskell
One may ask the question: what is seq useful for not in conjunction with unsafePerformIO, other than efficiency. That, I don't know the answer to. Here is an example. main::IO() main=do time1 - getCPUTime w - return $! calcSomething time2 - getCPUTime ... J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: SUGGESTION: haskell-announce mailing list
On Saturday 11 May 2002 11:41, Jose Romildo Malaquias wrote: I Would also be moderately in favor of merging the haskell and haskell-cafe lists back into one, mainly because I always felt the distinction was somewhat arbitrary, who knows what discussions will turn out to be too long or have interesting tangents until it is too late and everyone has said everything on haskell? but this is a different issue. I wonder how many addresses are on haskell and not haskell-cafe or vice versa. I am in favor of both sugestions. I like the actual haskell/haskell-cafe situation. J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: updating labelled fields
On Tuesday 07 May 2002 02:07, John Meacham wrote:
DrIFT which i am now maintaining can derive such utility functions out
of the box. just add a {-!deriving: update -} to get update functions
for every labeled field in a datatype. quite useful, I have not updated
the web page yet, but the new DrIFT homepage will be at
http://homer.netmar.com/~john/computer/haskell/DrIFT/
Very nice :)
I'll check it out.
J.A.
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Re: updating labelled fields
I often create structures like:
data MyData = MyData { foo :: ..., bar :: ..., }
That makes 2 of us :-)
and most of the time i do one of two things:
1) read values from the structure, as in:
let x = (foo myData) in ...
2) update values in the structure, as in:
let myData' = myData { foo = (foo myData)+1 }
1)
I've used datatypes with labeled fields mostly to pass around implicit values.
If that is your case then there is a way around it.
Declare the datatype as
data MyData = MyData { foo_ :: fooType, bar_ :: ..., }
and then declare
foo :: (?implicitdata :: MyData)= fooType
foo = foo_ ?yourdata
So when you work in a contex that depends on some implicit data you can just
use foo. I've used this *a lot* lately.
2)
Yes. My method now is declaring set and apply functions to every field of my
data structure.
fooAp f ni=ni{foo=f(foo ni)}
fooSet x = fooAp (const x)
Only very rarely (usually only during intializization) do I actually put
values into the structure that *don't* depend on their previous value. I
end up with expresions like:
... myData { foo = (foo myData) + 1 ;
bar = (bar myData) ++ bar ;
ick = (ick myData) ! n ; ... }
Yeap quite ugly isn't it? :-)
I was wondering if there existed any sort of update syntax. Obviously
Nope, not that I know of.
not real update, but enough to get rid of the (foo myData) parts of my
epxression which really serve to just clutter up with expression. Perhaps
something like:
... myData { foo - (+1) ; bar - (++bar) ; ick - (!n) ; ... }
Yes looks nice, thought about something like that before too.
or the like, where x { ... y - e ... } is translated to x { ... y = e
(y x) ... } (i only use - because that seems to be the default
extension symbol, i guess because we don't want to trample symbols people
might actually use.)
Anyway I'd prefer to have some way to 'derive' apply and set functions.
Something like
data MyData = MyData { foo :: fooType, bar :: ..., }
deriving (Set, Apply)
Using the keyword deriving would probably be a bad idea though :)
The set and apply functions could be derived with a standard postfix or maybe
prefix... fooAp or apFoo.
Maybe we could introduce sintax to specify it...
deriving (Set with set, Apply with ap)
I don't know... I'm just brainstorming right now.
Having actual functions is important. I don't think I have to explain why to
people in this mailing list :-)
Anyway, does such a thing exist, and, if not, is there any chance it could
exist, or is it just syntactic salt to too many people? :)
I whish you better luck than I've had so far whenever making posts about this
same issue ;)
J.A.
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Re: updating labelled fields
On Monday 06 May 2002 23:28, Hal Daume III wrote: I wouldn't at all mind making this addition if I had a sense that it would actually be accepted and that people weren't going to go crazy over the syntax. Would something like - be preferred or something like $=? I'd still prefer having some way to automaticly derive 'apply' functions. There is already nice syntax for setting a field value and I always end up defining 'set' functions to each and every field because I want to pass them as arguments. Imagine you have an STRef to a labeled datatype, lets call it stdata. You want to apply some function g to field foo of that structure. modifySTRef (fooAp g) stdata Changing its value to x modifySTRef (fooSet x) stdata With syntatic sugar only you'd have to read the reference, apply the function to the field and then update it. IMO, 'set field' and 'apply to field' functions are as usefull as the 'field projection' functions that are derived from the definition of the labeled datatype. Anyway I agree that it would be nice to have special syntax for updates. I'll use it if I have it available. On Monday 06 May 2002 23:42, David Feuer wrote: Why not $= ? Yeap very nice in deed. I'd vote for this one. J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Syntax highlighting for KDE's Kate
Dear KDE users,
I've hacked syntax highlighting files for Kate, KDE's editor.
Feel free to use or to modify them.
http://www.informatik.uni-bonn.de/~ralf/software.html#syntax
Great!
To bad the literate haskell mode doesn't work with the LaTeX kind of literate
programming - \begin{code} ... \end{code} - that's how I've been coding all
my haskell programs lately :)
I'll probably hack it myself if/when I have the time.
J.A.
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Re: Read File
- [moving to haskell-cafe] I want to read some data from a text file. I accomplished it but I want to read data from file as a Integer list for example text file has [1,2,3,5] - Wait you actually have [1,2,3,5] - and when I read this data from file I handeled it as string. What can I do to get it as integer list. Hint: map read [1,2,3] ::[Int] --- this would be helpfull if you had 1 2 3 5. It's easier then Hint: read [1,2,3,5]::[Int] J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: order of computation in 'do' notation
NOTE: Even the examples are correct in the H98 libraries documentation in the IO module section, they are incorrect in the H98 Report: http://www.haskell.org/onlinereport/io-13.html Scroll down, to 7.2. snip when I put the same code through ghc, it waits for a command first, and then when the command has been entered it displays the prompt, which is just silly. Surely the 'do' notation was designed to sequence computations, but it obviously isn't behaving quite right here! The problem is that stdout is buffered by default and *not flushed when you're waiting for input in stdin*, IMO it should be. IIRC this was a change that happened between the somwhere between 5.00.1 and 5.02.2. I remember suddenly all my programs not working correctly anymore... I'd be very grateful for any suggestions as to how to fix this. thanks, One way is to *import IO* and set stdout to NoBuffering mainLoop :: IO TRS - IO () mainLoop t = do putStr \n hSetBuffering stdout NoBuffering -- HERE!!! (c,a) - getCommand 0 case c of help - do putStr help mainLoop t load - do trs - load a putStr mainLoop (return trs) show - ...and so on... You may also flush stdout every time you want using hflush. Check the haskell libraries documentation for the IO module for more info. J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: defining (- Bool) as a set
On Monday 22 April 2002 23:31, Hal Daume III wrote: I'd like to be able to define something like instance Eq a = Coll (- Bool) a where empty= \_ - False single x = \y - if x == y then True else False union a b = \x - a x || b x insert s x = \y - x == y || s y and the like However, this seems to be impossible. Is this the type lambda restriction that's been discussed recently on the mailing list? - Hal Hi Hal, I'd do it like this, hope it helps. -- module Test where newtype BinClass a = BC (a-Bool) class Coll c a where empty :: c a single :: a-c a union :: c a-c a-c a insert :: c a-a-c a instance Eq a = Coll BinClass a where empty= BC(\_-True) single x = BC(\y - if x == y then True else False) union (BC a) (BC b) = BC(\x - a x || b x) insert (BC s) x = BC(\y - x == y || s y) ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: defining (- Bool) as a set
class Collection e ce | ce - e where empty :: ce insert :: e - ce - ce member :: e - ce - Bool instance Eq a = Collection a (a - Bool) where empty = (\x - False) insert e f = (\x - if x == e then True else f x) member e f = f e This is way better than my solution... I had never used multi-parameter classes before, so I forgot the functional dependency (right name? the |ce-e), and there was obviously no need for my extra constructor. J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Bugs?
I've sent this msg to the ghc main mailing list, but this one is more
adequate. I've tried this with both ghc 5.02.3/5.02.2, SuSE RPMs.
Thanks in advance,
J.A.
---
[1] Bug1?
This declaration:
data A = (,) Int Int
is accepted by ghci. Is this behaveour correct,
1. It kind of shadows (,) is defined in PrelTup meaning that you can no longer
use (,) prefix to refer to tuples - like (,) 1 2.
2. Seems to me like (,) is not correct syntax for a consym as defined in the
H98 Report so we shouldn't be able to redefine it.
Note: didn't check any other interpreter/compiler.
[2] Bug2?
- Step 1
Load this in ghci,
---
module Test where
data C = C ((,) Int Int)
data A = (,) !Int !Int
h :: A
h = (,) 1 2
{-
f :: C
f = C ((,) 3 4)
-}
- Step 2
Uncomment the f function. (you'll get an error)
- Step 3
To get rid of the error comment out the 'data A' declaration and function 'h'.
Now you should get this error message:
-
Failed to find interface decl for `Teste.A'
from module `Teste'
-
Quiting ghci and then reloading the file works fine.
J.A.
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Re: Bug?
On Wednesday 17 April 2002 03:15, Jorge Adriano wrote: [1] Bug1? This declaration: A = (,) Int Int Opsss cut and paste problems :) data A = (,) Int Int is accepted by ghci. Is this behaveour correct, This is what I meant. Anyway in Bug 2 I used the 'correct' declaration [2] Bug2? - Step 1 Load this in ghci, --- module Test where data C = C ((,) Int Int) data A = (,) !Int !Int -- --- There it is snip J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Bug?
On Wednesday 17 April 2002 03:15, Jorge Adriano wrote: [1] Bug1? This declaration: A = (,) Int Int Opsss cut and paste problems :) data A = (,) Int Int is accepted by ghci. Is this behaveour correct, This is what I meant. Anyway in Bug 2 I used the 'correct' declaration [2] Bug2? - Step 1 Load this in ghci, --- module Test where data C = C ((,) Int Int) data A = (,) !Int !Int -- --- There it is snip J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Bug?
[1] Bug1?
This declaration:
A = (,) Int Int
is accepted by ghci. Is this behaveour correct,
1. It kind of shadows (,) is defined in PrelTup meaning that you can no longer
use (,) prefix to refer to tuples - like (,) 1 2.
2. Seems to me like (,) is not correct syntax for a consym as defined in the
H98 Report so we shouldn't be able to redefine it.
Note: didn't check any other interpreter/compiler.
[2] Bug2?
- Step 1
Load this in ghci,
---
module Test where
data C = C ((,) Int Int)
data A = (,) !Int !Int
h :: A
h = (,) 1 2
{-
f :: C
f = C ((,) 3 4)
-}
- Step 2
Uncomment the f function. (you'll get an error function)
- Step 3
To get rid of the error comment out the 'data A' declaration and function 'h'.
Now you should get this error message:
-
Failed to find interface decl for `Teste.A'
from module `Teste'
-
[3] Strict Pairs question
I really miss them :-)
I know I can do something like
data A a b= A !a !b
But then you can't use zips fst etc etc...
One possible solution would be to add some Class Pair, with default instances
for these functions. Is this a bad idea for any particular reason?
It would also be nice to be able to generalize the idea syntatic sugar for
standar tuples and be able to define constructors like, say
(:,:) and (#:, , :#) etc. Is this feasable?
J.A.
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Bug?
[1] Bug1?
This declaration:
A = (,) Int Int
is accepted by ghci. Is this behaveour correct,
1. It kind of shadows (,) is defined in PrelTup meaning that you can no longer
use (,) prefix to refer to tuples - like (,) 1 2.
2. Seems to me like (,) is not correct syntax for a consym as defined in the
H98 Report so we shouldn't be able to redefine it.
Note: didn't check any other interpreter/compiler.
[2] Bug2?
- Step 1
Load this in ghci,
---
module Test where
data C = C ((,) Int Int)
data A = (,) !Int !Int
h :: A
h = (,) 1 2
{-
f :: C
f = C ((,) 3 4)
-}
- Step 2
Uncomment the f function. (you'll get an error function)
- Step 3
To get rid of the error comment out the 'data A' declaration and function 'h'.
Now you should get this error message:
-
Failed to find interface decl for `Teste.A'
from module `Teste'
-
[3] Strict Pairs question
I really miss them :-)
I know I can do something like
data A a b= A !a !b
But then you can't use zips fst etc etc...
One possible solution would be to add some Class Pair, with default instances
for these functions. Is this a bad idea for any particular reason?
It would also be nice to be able to generalize the idea syntatic sugar for
standar tuples and be able to define constructors like, say
(:,:) and (#:, , :#) etc. Is this feasable?
J.A.
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Re: about downloading
On Sunday 14 April 2002 17:34, julie madec wrote: where can I download a version of Haskell? Thank you http://www.haskell.org/implementations.html J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: GHC 5.02.3, SuSE rpms
On Tuesday 09 April 2002 08:38, Ralf Hinze wrote: I've uploaded SuSE 7.3 rpms for the patchlevel release of the Glasgow Haskell Compiler (GHC), version 5.02.3. http://www.informatik.uni-bonn.de/~ralf/ghc-5.02.3-1.src.rpm http://www.informatik.uni-bonn.de/~ralf/ghc-5.02.3-1.i386.rpm http://www.informatik.uni-bonn.de/~ralf/ghc-prof-5.02.3-1.i386.rpm Enjoy, Ralf Thanks for maintaning the Ghc SuSE Rpms :) Is there any chance to make this available in SuSEs ftp server? I ask this becouse it would be nice to have ghc ugrades listed in YOU (Yast Online Update) and apt (there is already apt for SuSE, I'm using it and it is great). This way every SuSE users of ghc would be able to keep their ghc version up to date easily even if they're not on haskell mailing lists. Thanks, J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: I need some help
[Obs: moving to haskell cafe] [beware, huge answer follows] Hello I am writing a whole program which is analyz a elegtromagnetic wave. This is ok but in the program I use a lot of constant for example pi,epsilon etc. I want to ask that how I use a constant in Haskell ? Defining constants it's easy, examples. -- with no type signature epsilon = 3.1 -- with explicit type signature epsilon2 :: Double epsilon2 = 3.1 -- specifying the type but no type signature epsilon3 = 3.1 :: Double (by the way, wouldn't it make sense not to get a warning about missing type signature in this last case. I know there is no signature there but still...) However the user of the program give some argument then program is starting. And I want to use this arguments for example at the end of the program. What will I do. Good question. I've had a bad time with this, some in this list proably got tired of me saying the same thing over and over again... those of you might want to skip the rest of my mail :-) If you want the user to be able to chose those values, then they are NOT constants, period. Yes their values won't chnage during the rest of the algorithm, but that doesn't makes it constants, so you can't declare them that way. What *can* you do. 1. One *awfull* option is to pass thos values as arguments to all your functions. Yes signatures will get monstruous, it will complicate the developing process etc, 2. Instead of passing all those variables, group them in a record - data type with named fields - and pass that record arround. (http://www.haskell.org/onlinereport/exps.html#sect3.15) Signatures will get better. It's kind of weird though, to always have to pass around that record. Things can get quite messy, IMO. -- foo f :: Double - Double - Double f x y = eta*x - epsilon^2*y -- foo2 f :: Options - Double - Double - Double f opt x y = (eta opt)* (epsilon opt)^2*y And this is just a very simple example, it can get much worst. 3. Implicit parameters. (At least in Ghc, what about others?) Now this is the closest to an elegant solution. *Read both*: http://citeseer.nj.nec.com/246042.html http://www.cs.chalmers.se/~rjmh/Globals.ps f :: (?eta::Double, ?epsilon::Double)= Double - Double - Double f x y = ?eta*x - ?epsilon^2*y Now you don't have to specify eta and epsilon in the type signature itself, but in the context - no passing around extra (not natural) parameters. It makes an huge diference IMO when developing your algorithms. Still signatures wil get even bigger than if you just passed them as parameters, you can't even type the context like: (?eta, ?epsilon::Double) So that is just awfull! Once again records will help. f :: (?opt::Options)= Double - Double - Double f x y = (eta ?opt) *x - (epsilon ?opt)^2*y It still lokes quite bad. I *hate* the field projection functions, and I hate the '?'!! So you could simply rename you data opt fields and declare, eta :: (?opt:: Options) = Double eta = etaF ?opt epsilon :: (?opt:: Options) = Double epsilon = epsilonF ?opt Now f could be written like: f :: (?opt::Options)= Double - Double - Double f x y = eta*x - epsilon^2*y Which differs from the first f function only in the context. It is *almost* good enough for me :-) Why the almost? you ask... Well, implicit parameters are not only non-standard features, but also experimental features (right?), so you they can change anytime. -- NOTE: some of you might wanna quit reading here, I keep complaining about this stuff, you are probably tired of it :-) -- In a couple of days you will also want to use more than one record, because grouping certain constants together doesn't feel right, and contexts will get bigger. Also, you will want to use global variables and instead of a record you will use an STRef to a record, and very small signatures will have in they're context something like: (?opt :: STRef s Opt). And then you will need to update some option fields, and you will want to use modifySTRef, and you will need to create an applyField function to every damn field of the record... So you will start asking 1. (when) will we have implicit contexts available? 2. wouldn't it be good to have, besides the data projection functions, some update and apply field functions, automaticly derived when declaring the data type? [yes am actually *asking* if *anyone agrees* with me? haven't had any feedback on this one, so I don't no wether it is a good or just plain dumb idea] IMVHO, it is of extreme importance and *urgent* to get implicit parameters right. This question about how to use this 'user difined constants' is a *simple* question and should have a *very simple* and standard answer. The same goes to global variables. We need them in certain situations and it is supposed to be, and it can be from my
Re: first-class modules
On Tuesday 26 March 2002 17:31, you wrote: A number of people have discussed the use of implicit parameters to mimic global variables in Haskell. I am wondering if any have done the same for a first-class module system such as that proposed by Shields and Jones. It seems to make a tremendous amount of sense to do it that way: 1. semi-constant values: Things that are calculated or input at the beginning of the program could be handled by Main.main, which would pass the results to the Second module, whose main function it could then call. This seems much more natural than the implicit parameter approach for this purpose. It seems nice for that *specific* purpose. What I like about implicit parameters is the fact that they are a more general approach, and that is IMO very important. You, might for instance, decide that maybe it would be a good idea to, under certain specific conditions, change one of those semi-constant values. Imagine some AI algortithm, you iterate some learning function 'learnF' over and over again. Now you realize that under certain conditions maybe the learning rate (used in learnF) should be changed. Chaging your implementation would be easy with implicit parameters. J.A. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: I need some help
However the user of the program give some argument then program is starting. And I want to use this arguments for example at the end of the program. What will I do. Good question. I've had a bad time with this, some in this list proably got tired of me saying the same thing over and over again... those of you might want to skip the rest of my mail :-) huh? what is this line doing here.. I thought I had cut/pasted it below, but instead I copy/pasted it (then changed it). Most of the is pretty readble, I just complain about stuff in the last part of the e-mail :-) J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
literate haskell (again)
Tried using Andrew Cookes haskell.sty and liked it a lot, but seems like the
(X)emacs haskell mode doesn't support the \begin{code} \end{code} literate
programming style, just the one where code is preceded by ' '.
Anyone has got an hack available for this?
J.A.
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Re: literate haskell (again)
On Friday 22 March 2002 16:31, you wrote:
Jorge Adriano [EMAIL PROTECTED] writes:
Tried using Andrew Cookes haskell.sty and liked it a lot, but seems like
the (X)emacs haskell mode doesn't support the \begin{code} \end{code}
literate programming style, just the one where code is preceded by ' '.
Anyone has got an hack available for this?
According to my haskell-mode.el:
(...)
So you probably want to change this as per the instructions.
Seems like it was not a .emacs (in fact .init.el here) problem.
I just noticed that if you got to add a line break *right after* the
\begin{code}
and right after the
\end{code}
If you add a space after the '{code}' it will not toggle comment/code modes.
Where should I report this bug?
Since I'm on haskell-mode bugs, when typing undefined in xemacs I get
(1) (error/warning) Error in `post-command-hook' (setting hook to nil):
(wrong-type-argument listp a )
Thanks,
J.A.
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literal haskell mode (nice :)
I got some tips from some people at #xemacs in openprojects.org and cam up
with the file in attachment. I know nothing about e-lisp so beware!!
I just have one major problem now. I wanted it to load x-symbol instead of
just LaTeX mode (and x-symbol-fontify after loading the mode). If anyone can
do that I'd be really gratefull :-)
J.A.
P.S.: put the attached file in the directory wher you have the .el and add
to you ~/.emacs (or ~/.xemacs/init.el), (require 'two-mode-mode).
(I think :)
;; two-mode-mode.el -- switches between tcl and sgml(html) modes
;; $Id: two-mode-mode.el,v 1.3 2001/12/12 16:18:53 davidw Exp $
;; two-mode-mode.el is Copyright David Welton [EMAIL PROTECTED]
;; 1999, 2000, 2001
;; two-mode-mode.el may be distributed under the terms of the Apache
;; Software License, Version 1.1
;; As I'm sure is painfully obvious, I don't know much about elisp,
;; but I thought it would be cool to do this mode, for mod_dtcl. So
;; if you have any comments or suggestions, please email me!
;; These same concepts could be used to do a number of neat 2-mode
;; modes, for things like PHP, or anything else where you have a
;; couple of modes you'd like to use.
;; Use of 'psgml-mode' is highly recommended. It is, of course, a
;; part of Debian GNU/Linux.
;; Modified by Marco Pantaleoni [EMAIL PROTECTED]
;; to allow execution of an hook on mode switching.
;; Also added a standard mode hook and some documentation strings.
;; Modified by Jorge Adriano [EMAIL PROTECTED]
;; to work with LaTeX and Literate Haskell modes instead of HTML/PHP
;; WARNING: I don't really know what I'm doing here... :-)
;; changes: 1. changed ? to \\begin{code} (and similar with ?
;; 2. changed (turn-on-font-lock-if-enabled)
;; to (font-lock-default-fontify-buffer)
;; the 1st one caused an error msg with my xemacs version
;; I thought that was the appropriate xemacs command
;;
;; If anyone can change this in order to load x-symbol and fontify it
;; instead of just LaTeX mode, please send the patch to me :-)
;; configure these:
(defvar two-mode-lmatch \\begin{code}
Open tag for `second' mode.)
(defvar two-mode-rmatch \\end{code}
Close tag for `second' mode.)
(defvar default-mode (list 'LaTeX-mode LaTeX)
Default mode.)
(defvar second-mode (list 'literate-haskell-mode Haskell)
Second mode: mode used inside special tags.)
;;
(defvar two-mode-update 0)
(defvar two-mode-mode-idle-timer nil)
(defvar two-mode-bool nil)
(defvar two-mode-mode-delay (/ (float 1) (float 8)))
;; Two mode hook
(defvar two-mode-hook nil
*Hook called by `two-mode'.)
(setq two-mode-hook nil)
;; Mode switching hook
(defvar two-mode-switch-hook nil
*Hook called upon mode switching.)
(setq two-mode-switch-hook nil)
(defun two-mode-mode-setup ()
(make-local-hook 'post-command-hook)
(add-hook 'post-command-hook 'two-mode-mode-need-update nil t)
(make-local-variable 'minor-mode-alist)
(make-local-variable 'two-mode-bool)
(setq two-mode-bool t)
(when two-mode-mode-idle-timer
(cancel-timer two-mode-mode-idle-timer))
(setq two-mode-mode-idle-timer (run-with-idle-timer two-mode-mode-delay t
'two-mode-mode-update-mode))
(or (assq 'two-mode-bool minor-mode-alist)
(setq minor-mode-alist
(cons '(two-mode-bool two-mode) minor-mode-alist
(defun two-mode-mode-need-update ()
(setq two-mode-update 1))
(defun two-mode-change-mode (to-mode)
(if (string= to-mode mode-name)
t
(progn
(save-excursion
(if (string= to-mode (cadr second-mode))
(funcall (car second-mode))
(funcall (car default-mode
(two-mode-mode-setup)
(if two-mode-switch-hook
(run-hooks 'two-mode-switch-hook))
(if (eq font-lock-mode t)
(font-lock-fontify-buffer))
(font-lock-default-fontify-buffer
(defun two-mode-mode-update-mode ()
(when (and two-mode-bool two-mode-update)
(setq two-mode-update 0)
(let ((lm -1)
(rm -1))
(save-excursion
(if (search-backward two-mode-lmatch nil t)
(setq lm (point))
(setq lm -1)))
(save-excursion
(if (search-backward two-mode-rmatch nil t)
(setq rm (point))
(setq rm -1)))
(if (and (= lm -1) (= rm -1))
(two-mode-change-mode (cadr default-mode))
(if (= lm rm)
(two-mode-change-mode (cadr second-mode))
(two-mode-change-mode (cadr default-mode)))
(defun two-mode-mode ()
Turn on two-mode-mode
(interactive)
(funcall (car default-mode))
(two-mode-mode-setup)
(if two-mode-hook
(run-hooks 'two-mode-hook)))
(provide 'two-mode-mode)
Re: literal haskell mode (nice :)
On Friday 22 March 2002 23:27, Jorge Adriano wrote: I got some tips from some people at #xemacs in openprojects.org and cam up with the file in attachment. I know nothing about e-lisp so beware!! Opss I forgot: The original file is Copyrighted by David Welton and it is located here: http://www.dedasys.com/freesoftware/ http://www.dedasys.com/freesoftware/files/two-mode-mode.el J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: literal haskell mode (nice :)
I got some tips from some people at #xemacs in openprojects.org and cam up
with the file in attachment. I know nothing about e-lisp so beware!!
Last e-mail today. Forgot to actually tell what was this for (need ti get
some sleep :)... It's easy to guess by looking at the file anyway the idea is
to change xemacs modes acording to context. I changed the orginal file to
work with literate haskell + latex. It works (for me at least) with the
\begin{code} \end{code} kind of lhs files.
J.A.
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Re: literate haskell
Thanks for all your answers :-) J.A. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
literate haskell
Why is it necessary to leave a blank line between comments and code?
I'm using LaTeX in my lhs files, with the code inside a verbatim
environment, and I'd rather start writing my code right after the
\begin{verbatim}.
I'd also like to know if anyone as changed is xemacs configuration file in
such a way that is easy to change from haskell to LaTeX mode from within lhs
files, and compile (latex compile) them.
I always have to change to latex mode, the change the syntax highlight, and
even then I can't compile the from the included lhs files - it always calls
view instead of compile, even if I specify the correct Master file.
Thanks in advance
J.A.
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Re: pattern-matching with labelled types
On Friday 08 March 2002 01:52, you wrote:
Andre W B Furtado writes:
| Of course, it is possible to do something like
|
| update :: MyType - Int - MyType
| update mt newValue = MT {x = newValue, y = oldValue}
| where oldValue = y mt
|
| but this really annoys me when MyType has too many fields. Suggestions?
update mt newValue = mt {x = newValue}
Since we are on the subject, I have some questions about labeled datatypes
updates.
You can in deed update like that mt {x = newValue}, but whenever I use
labeled datatypes I always end up with Andres problem, having to define not
only an 'update' function but also an 'apply' function for every field.
I need them becouse I want to pass them as arguments to other funtions.
This happens to me all the time, but one very good example IMO is having a
STRef to a labeled data type. Now you want to update some field and you want
to to be able to use,
modifySTRef (updField1 3)
or
modifySTRef (appField1 (+1))
It would be extremely usefull IMO to have not only field projection funtions,
but also apply and update funtions. That, and not beeing possible to specify
contexts partialy, makes it particulary complicated for me to follow John
Hughes strategy to simulate global variables in haskell
(http://www.cs.chalmers.se/~rjmh/Globals.ps)
J.A.
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Re: strange behavior with ghci version 5.02.2
After loading a module with :l Module you can't use the Prelude functions unqualified, you just get things like interactive:1: Variable not in scope: `show' I am pretty sure that this worked some days ago, and I was using the same version then. I feel totally confused. Has this happened to anyone else? Yes, for some modules it happens for others it doesn't. Kind of strange. SuSE 7.3 rpms here, v. 5.02.2 J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Sendind mail
Hi, I'd like to know if there's any Hugs compatible library with functionalities to send mail. I don't think so. If all you want to do is send mail from within an haskell program why not just make system calls. I did something like that a long time ago. I've sent you the module in attachment. (suposed to work in *nixes) J.A. import System Este ficheiro contem fucoes para eu enviar mails aos meus alunos type Mail = String Mails das Turmas P3 e P4 - posso ter que alterar para filtrar alunos mails_P3 :: [Mail] mails_P3 = [mp0 ++ show(n) ++ @mat.uc.pt | n - [301.. 336] ] mails_P4 :: [Mail] mails_P4 = [mp0 ++ show(n) ++ @mat.uc.pt | n - [401.. 407] ] Mails das minhas turmas Praticas mails_TP :: [Mail] mails_TP = mails_P3 ++ mails_P4 Enviar mails: type Subject = String type Path= String type Message = String Envia dados: - Subject da mensagem - Path do ficheiro com a mensagem - Endereco de e-mail Envia o mail com subj. e mensagens dadas para o endereco. sendmail :: Subject - Path - Mail - IO ExitCode sendmail sub msgfile e_mail = do --putStrLn(comando) system( comando ) where comando = mail -s \ ++ sub ++\ ++ e_mail ++++ msgfile send_to_list :: Subject - Path - [Mail] - IO [ExitCode] send_to_list sub msgfile e_mails = do sequence (map (sendmail sub msgfile) e_mails)
Re: efficiency question
On Monday 11 February 2002 02:10, Hal Daume III wrote: So instaed of storing 1 and 2 in the list, it stores pointers to them. The reason it does this is so that it makes it easy to generate code for polymorhpic functions. (,) being boxed means that instead of storing a pair of element, you're storing a pair of pointers. That means you get worse performance because you have to chase more pointers. (at least that's my impression) Thanks, I knew the concept but not the name. On Sunday 10 February 2002 18:48, Kirsten Chevalier wrote: I'd guess that it's not just that you have to apply the (,) constructor -- it also has to do with the fact that the tuples it's constructing here are boxed. This brings another question to my mind, isn't it toupling a standard technique used in functional programming? I'm pretty sure I've seen it focused in some papers/text books. I for one would not expect that folding the list twice would be more efficient... J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: efficiency question
On Sunday 10 February 2002 18:48, Kirsten Chevalier wrote: I'd guess that it's not just that you have to apply the (,) constructor -- it also has to do with the fact that the tuples it's constructing here are boxed. could you elaborate a little more on that (boxed / unboxed) or provide a link on the subject? thanks J.A. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
