- Beginners - Haskell.org

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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Re: Beginners Digest, Vol 27, Issue 57
by Martin Tomko 28 Sep '10

28 Sep '10

Ahaaa, the heureka moment. Stupid me, I should have tried :t null Thanks Brent! On 9/27/2010 6:00 PM, beginners-request(a)haskell.org wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: matching the output of a standard function to my > function definition (non-exhaustive pattern problem) (Brent Yorgey) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 27 Sep 2010 12:21:08 -0400 > From: Brent Yorgey<byorgey(a)seas.upenn.edu> > Subject: Re: [Haskell-beginners] matching the output of a standard > function to my function definition (non-exhaustive pattern problem) > To: beginners(a)haskell.org > Message-ID:<20100927162108.GA5408(a)seas.upenn.edu> > Content-Type: text/plain; charset=us-ascii > > On Mon, Sep 27, 2010 at 06:15:22PM +0200, Martin Tomko wrote: > >> Dear Daniel >> that worked perfectly, thanks for the suggestion! >> >> One more question: how do you use null to check for empty lists? >> something like [] == null did not work ... just chekcing, I got my >> orignal code to work, thanks again! >> Martin >> > When in doubt, consult the types! > > null :: [a] -> Bool > > That is, null is a function for testing lists to see whether they are > empty. So rather than 'list == null' you write 'null list'. > > -Brent > > > ------------------------------ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > End of Beginners Digest, Vol 27, Issue 57 > ***************************************** > > -- Martin Tomko Postdoctoral Research Assistant Geographic Information Systems Division Department of Geography University of Zurich - Irchel Winterthurerstr. 190 CH-8057 Zurich, Switzerland email: martin.tomko(a)geo.uzh.ch site: http://www.geo.uzh.ch/~mtomko mob: +41-788 629 558 tel: +41-44-6355256 fax: +41-44-6356848

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matching the output of a standard function to my function definition (non-exhaustive pattern problem)
by Martin Tomko 27 Sep '10

27 Sep '10

Dear all I am sure this is a very beginner problem, but I am not sure how to solve it: I have a myFunct function: myFunct :: Int -> [a] -> a defined as: myFunct _ [] = error "empty list provided as arg" myFunct a [b] | length [x|x<-[b],fid x == a] == 0 = error "no match" | otherwise = head [x|x<-[b],fid x == a] Which works fine (may not be elegant, but works). Now, I am chaining it in a larger function, where at some stage I use a graph object from Data.Graph and check for its edges, resulting in [(Int,Int)] trying to run this fails: myFunct (fst (head (edges graph))) myList *** Exception: FunctStruct.hs:(136,1)-(138,48): Non-exhaustive patterns in function myFunct contrived example, but myList is a list of [a]). The problem arrises from the definition of the type of Edge as (Vertex,Vertex), but type Vertex = Int (http://hackage.haskell.org/packages/archive/containers/0.2.0.1/doc/html/Dat…) so that should be ok, or am I making a mistake somewhere and I should go from vertex to Integer explicitly? Thanks a lot! Martin

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