- Beginners - Haskell.org

Applicative Composition
by edgar klerks 01 Oct '10

01 Oct '10

Hi All, I was wondering, why there isn't a composition operator for applicative functors. Of course it is rather trivial to implement, but it is a useful feature: {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} module ApplicativeComposition where import Control.Applicative class (Applicative f) => ApplicativeComposition f where (<.>) :: f (b -> c) -> f (a -> b) -> f (a -> c) instance (Applicative f) => ApplicativeComposition f where (<.>) f g = pure (.) <*> f <*> g Can this be added to later versions of haskell? Greets, Edgar

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Fwd: [Haskell-beginners] Applicative Composition
by Antoine Latter 01 Oct '10

01 Oct '10

Forwarding to list - it looks like I forgot to reply-all. ---------- Forwarded message ---------- From: Antoine Latter <aslatter(a)gmail.com> Date: Thu, Sep 30, 2010 at 7:17 PM Subject: Re: [Haskell-beginners] Applicative Composition To: edgar klerks <edgar.klerks(a)gmail.com> On Thu, Sep 30, 2010 at 6:47 PM, edgar klerks <edgar.klerks(a)gmail.com> wrote: > Hi All, > > I was wondering, why there isn't a composition operator for applicative > functors. Of course it is rather trivial to implement, but it is a useful > feature: > > {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} > module ApplicativeComposition where > import Control.Applicative > > class (Applicative f) => ApplicativeComposition f where > (<.>) :: f (b -> c) -> f (a -> b) -> f (a -> c) > > instance (Applicative f) => ApplicativeComposition f where > (<.>) f g = pure (.) <*> f <*> g > > Can this be added to later versions of haskell? > Here's the last time this topic came up on the lists: http://www.haskell.org/pipermail/libraries/2010-August/013992.html Here's the corresponding library proposal ticket: http://hackage.haskell.org/trac/ghc/ticket/4189 There were a few folks against it for a few different reasons. Antoine

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Heterogenous list implemented with GADT
by Ken Overton 30 Sep '10

30 Sep '10

Hello fellow beginners (and the rest of you, too), I was reading http://en.wikibooks.org/wiki/Haskell/GADT and I really liked the simple eval example. But I reached the end and was confused by the following definition of a heterogenous list: data TG2 where MkTG2 :: Show b => [b] -> TG2 I had thought a heterogeneous list was something like [ 5 , 'Z', 3.14, False ] How can the TG2 definition support the list I've given above? Or am I just wrong about what a heterogeneous list is? Thanks, -- kov This email and any attachments may contain information which is confidential and/or privileged. The information is intended exclusively for the addressee and the views expressed may not be official policy, but the personal views of the originator. If you are not the intended recipient, be aware that any disclosure, copying, distribution or use of the contents is prohibited. If you have received this email and any file transmitted with it in error, please notify the sender by telephone or return email immediately and delete the material from your computer. Internet communications are not secure and Lab49 is not responsible for their abuse by third parties, nor for any alteration or corruption in transmission, nor for any damage or loss caused by any virus or other defect. Lab49 accepts no liability or responsibility arising out of or in any way connected to this email.

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Haskell.Exts parse failed: FlexibleContexts not enabled
by Daniel Lincke 30 Sep '10

30 Sep '10

Hi, when parsing a file containing a function signature with a context like test :: (Functor f) => ... I get the follewing failure: ParseFailed "Malformed context: FlexibleContexts not enabled" Actually I enables flexible contexts whereever I could ... any ideas? Greets, daniel

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Guards as extensions of patterns
by Russ Abbott 30 Sep '10

30 Sep '10

I never realized that a guard can be used as an extension of a pattern. Is this recommended coding? elem n xs asks whether n is an element of xs elem :: (Eq a) => a -> [a] -> Bool elem _ [] = False elem n (x:_) | n == x = True elem n (_:xs) = elem n xs -- Russ Abbott ______________________________________ Professor, Computer Science California State University, Los Angeles Google voice: 424-242-USA0 (last character is zero) blog: http://russabbott.blogspot.com/ vita: http://sites.google.com/site/russabbott/ ______________________________________

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Why are the types of the result different?
by Russ Abbott 30 Sep '10

30 Sep '10

testWhere1 = a where b = 1 a = b+1 > testWhere1 2 testWhere2 = a where b = 1 c = 1/(b - b) a = b+1 > testWhere2 2.0 The same thing happens with let. testLet1 = let a = b+1 b = 1 in a > testLet1 2 testLet2 = let a = b+1 c = 1/(b - b) b = 1 in a > testLet2 2.0 -- Russ Abbott ______________________________________ Professor, Computer Science California State University, Los Angeles Google voice: 424-242-USA0 (last character is zero) blog: http://russabbott.blogspot.com/ vita: http://sites.google.com/site/russabbott/ ______________________________________

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The Data Parallel Haskell example from the Haskell Wiki won't work
by João Paulo Pizani Flor 29 Sep '10

29 Sep '10

Hello dear Haskell People! I'm currently very interested in doing some parallel programming in Haskell, specially in using the Data Parallel Haskell stuff... So as a first step I tried to emulate the vector multiply example found in the Haskell Wiki: http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell#A_simple_examp… The code I've written (two files) for this example is in the following pastebins: DPHVecMul.hs: http://paste.org/pastebin/view/22899 Main.hs: http://paste.org/pastebin/view/22900 I've compiled successfully both files and linked them, exactly as described in the Wiki. But when I run the resulting executable, the following error shows up: "dotp: Prelude.undefined" It's seems (from the tests I've done) that the call to the function *fromPArrayP *of the module *Data.Parallel.Array.Prelude *results always in undefined. In fact, I've taken a look at the function definition in the docs, and in fact it always returns undefined. It doesn't make ANY SENSE to me :P Has someone been able to successfully compile and run this example code?? If so, what am I doing wrong? Is there some obvious error I'm incurring in? :D Regards, João Paulo Pizani Flor joaopizani(a)gmail.com Computer Science - 2007/1 Federal University of Santa Catarina - Brazil

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ignature for a list of a set length
by Martin Tomko 29 Sep '10

29 Sep '10

Dear all, I want to have a function that acts only on lists of length 3 (I have a function that filters a list of lists and returns only those of that length). I guess I could change them into tuples (is there a way?), but anyway. Is there a way to specify in the signature that the function should only match such lists, or do I have to do pattern mattching in order to exclude other possibilities?: getTernaryRelationship :: [a] -> [a] getTernaryRelationship ls = if (length ls /= 3) then error "not a ternary rel" else ... thanks Martin

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step by step binding recursion
by Nehemiah Clark 28 Sep '10

28 Sep '10

Hi, If anyone is familar with Prolog, then you might have used the "trace" or "gtrace" function. I was wondering which function can I use to show the bindings in the recursion. It would be helpful for me to see what is going. Thanks

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removing duplicate tuples (including symmetrical ones)
by Martin Tomko 28 Sep '10

28 Sep '10

Hi all, I apologize for spamming this forum so frequently, but there is noone I can turn to around here... I have a list of (a,a) tuples, and am trying something like nub, but also matching for symmetrical tuples. I implemented it using the template from delete from Prelude. Seems like my typse signature has some troubles (Paarse error in pattern) but I am not sure where the problem is. removeDuplTuples :: [(a,a)] -> [(a,a)] removeDuplTuples [] = [] removeDuplTuples [b] = [b] -- using the syntactic sugar for single element in list removeDuplTuples x:xs = nub (if elem (snd x,fst x) xs then removeDuplTuples xs else [x] ++ removeDuplTuples xs) I assume the problem lies in elem (snd x,fst x) xs but I am not sure how to rewrite it. Thanks for all help, Martin

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