Dear friends: I am an ULTRA-ULTRA Haskell beginner, just started literally yesterday.   I am just toying around with Cordelia Hall and John O'Donnell excellent book on dicrete mathematics with Haskell. Well, they have a program, stdm, to accompany the book. It happens that it is in literate style. In theory, this should be very easy to work with, but after saving it with lhs extension, I try to load it, without success. Could somebody out there help me with this? Best regards, Francisco Gutiérrez From: "beginners-request@haskell.org" To: beginners@haskell.org Sent: Monday, April 16, 2012 2:45 PM Subject: Beginners Digest, Vol 46, Issue 23 Send Beginners mailing list submissions to     beginners@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@haskell.org You can reach the person managing the list at     beginners-owner@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:  splitAt implementation (using foldr) and infinite lists       (Lorenzo Bolla)   2. Re:  splitAt implementation (using foldr) and infinite lists       (Ozgur Akgun)   3. Re:  Design of Webmachine in Haskell (Petar Radosevic)   4. Re:  Design of Webmachine in Haskell (Michael Snoyman)   5. Re:  splitAt implementation (using foldr) and infinite lists       (Dmitriy Matrosov)   6.  Training tasks (Nikita Beloglazov)   7.  Cross-platform .hs files on Linux and Windows (Vinay Sajip)   8. Re:  Cross-platform .hs files on Linux and    Windows (Lorenzo Bolla) ---------------------------------------------------------------------- Message: 1 Date: Mon, 16 Apr 2012 15:21:55 +0100 From: Lorenzo Bolla Subject: Re: [Haskell-beginners] splitAt implementation (using foldr)     and infinite lists Cc: beginners@haskell.org Message-ID: <20120416142155.GC30186@dell> Content-Type: text/plain; charset=us-ascii On Mon, Apr 16, 2012 at 12:52:02PM +0400, Dmitriy Matrosov wrote:

Hi all.

If i implement take using foldr

take'      :: Int -> [a] -> [a] take' n    =  foldr (\x z -> if fst x <= n then snd x : z else []) []                 . zip [1..]

it'll work fine with infinite lists. But if i implement splitAt similarly

splitAt'    :: Int -> [a] -> ([a], [a]) splitAt' n  = foldr (\x (z1, z2) -> if fst x <= n then  (snd x : z1, z2)                                       else              ([], snd x : z2))                     ([], [])                     . zip [1..]

and call it like this

*Main> fst $ splitAt' 4 [1..] ^CInterrupted.

Try something like this: splitAt' n  = foldr (\x zs  -> if fst x <= n then  (snd x : fst zs, snd zs) else              ([], snd x : snd zs)) ([], []) . zip [1..] I'm no Haskell expert, but I suspect that when pattern-matching z2, it tries to evaluate it and it hangs... My version does not hang... hth, L. -- Lorenzo Bolla http://lbolla.info ------------------------------ Message: 2 Date: Mon, 16 Apr 2012 15:55:20 +0100 From: Ozgur Akgun Subject: Re: [Haskell-beginners] splitAt implementation (using foldr)     and infinite lists To: Lorenzo Bolla Cc: beginners@haskell.org Message-ID:     Content-Type: text/plain; charset="utf-8" You can also use lazy pattern matching. http://en.wikibooks.org/wiki/Haskell/Laziness#Lazy_pattern_matching On 16 April 2012 15:21, Lorenzo Bolla wrote:

...

splitAt'    :: Int -> [a] -> ([a], [a]) splitAt' n  = foldr (\x ~(z1, z2) -> if fst x <= n then  (snd x : z1, z2)                                       else              ([], snd x : z2))                     ([], [])                     . zip [1..]

Ozgur