June 2007 - Haskell-Cafe

newbie question on Parsers from "Programming In Haskell"
by Juozas Gaigalas 05 Jun '07

05 Jun '07

Hello, I am a somewhat experienced programmer and a complete Haskell newbie, so I hope this is the correct ML for my question. I have decided to learn Haskell and started with Graham Hutton's book. Everything was going nicely until section 8.4, on sequencing functional parsers. I am trying to write an elementary parser that produces the 1st and 3d elements from a string. I am using the code from the book. --------------------- type Parser a = String -> [(a, String)] return :: a -> Parser a return v = \inp -> [(v, inp)] failure :: Parser a failure = \inp -> [] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x, xs)] parse :: Parser a -> String -> [(a, String)] parse p inp = p inp (>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v, out)] -> parse (f v) out p :: Parser (Char, Char) p = do x <- item item y <- item return (x, y) -- LINE 34 -------------------- BUT, when I try to :load parse.hs from hugs I get the following error: ERROR "parse.hs":34 - Last generator in do {...} must be an expression I have no idea what I am doing wrong and why Hugs is complaining. I hope this question is not too simply for this mailing list, but I have honestly googled for an answer and had found nothing. Juozas Gaigalas

7 6

QuickCheck Arbitrary - infinite recursion on recursive GADTs?
by Marc Weber 05 Jun '07

05 Jun '07

example: data A = A INt | B [A] instace Arbitrary A where arbitrary = oneof [ liftM A arbitrary , liftM B arbitrary ] But now QuickCheck will propably create a test value A ( B [ A ( B [ A .... -> no end Is there an easy QuickCheck way to prevent this? I can think of: a) using sized to pass when to stop (decrement the value ..) b) using some type hackary data Recursive a = R a then use arbitrary :: R ( R ( a )) and remove one R each time.. c) pass a recursion indicator manually and decrement that... Which would you choose? Marc

2 1

Java Monads?
by Greg Meredith 04 Jun '07

04 Jun '07

HaskellyCaffeinated, i noticed that there was a JavaMonad lib kicking around on the web, but all the links i can find are stale. Does anybody have a live pointer to this lib? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

1 0

Re: Has anyone looked into adding subtyping to Haskell?
by oleg＠pobox.com 04 Jun '07

04 Jun '07

Polymorphic extensible records with subtyping are already expressible in Haskell. There is nothing needs to be added: http://homepages.cwi.nl/~ralf/HList/ http://homepages.cwi.nl/~ralf/OOHaskell/ The full code is available via darcs http://darcs.haskell.org/OOHaskell/ As to polymorphic variants, they too are implementable in Haskell right now. No new extensions are required. http://www.haskell.org/pipermail/haskell/2006-July/018172.html http://darcs.haskell.org/HList/src/VariantP.hs The code shows that the expression problem is solved in Haskell as it is.

2 3

Idiomatically using lists
by kevin birch 04 Jun '07

04 Jun '07

Hello all, I have an implementation question that I hope someone can help out with. Say I have a fixed-size list: [1, 2, 3, 4, 5] that I want to treat as circular in a function in order to rotate one of the elements n positions. So rotating the second element 2 positions would result in: [1, 3, 4, 2, 5], or rotating the fourth element 2 positions would result in: [1, 2, 4, 3, 5]. There are two cases: if the element can be moved w/o rotating and where the element must be inserted into the list at the front. Is there an idomatic way to handle both of these cases in a function? Thanks, kevin

2 1

Fwd: [Haskell-cafe] Optimising UTF8-CString -> String marshaling, plus comments on withCStringLen/peekCStringLen
by Alistair Bayley 04 Jun '07

04 Jun '07

(this time include the list, too) ---------- Forwarded message ---------- From: Alistair Bayley <alistair(a)abayley.org> Date: 04-Jun-2007 16:13 Subject: Re: [Haskell-cafe] Optimising UTF8-CString -> String marshaling, plus comments on withCStringLen/peekCStringLen To: Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> On 04/06/07, Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> wrote: > > Are you sure fromUTF8Ptr is strict in its ptr arg? Try with a ! pattern > on that arg. You'll need -fbang-patterns. Is there some semantic advantage to bang-patterns, or is it just a syntactic convenience? 'cause once the guard is in there, it's not much bother to maintain. > You can also get ghc to tell you what strictness it inferred for your > functions. It's shown in the .hi file. Use ghc --show-iface UTF8.hi. I tried -ddump-hi and got: $wfromUTF8Ptr :: base:GHC.Prim.Int# -> base:GHC.Ptr.Ptr base:GHC.Word.Word8 -> base:GHC.Base.String -> base:GHC.Prim.State# base:GHC.Prim.RealWorld -> (# base:GHC.Prim.State# base:GHC.Prim.RealWorld, base:GHC.Base.String #) {- Arity: 4 Strictness: LSSL -} Assuming the LSSL tokens are in the same order as the function arguments, the first argument is lazy - GHC.Prim.Int#. hmmm.... some cognitive dissonance here. I'm a bit puzzled as to why it's in the .hi file at all - it's not an exported function. (I also see quite a bit of code in the hi-dump. Presumably this permits cross-module inlining, for those selected functions?) Alistair

1 0

RE: [Haskell] boilerplate boilerplate
by Simon Peyton-Jones 04 Jun '07

04 Jun '07

[Redirecting to haskell-cafe; the main list is a good place to open discussions but not so good for continuing them] | > derive (Ord,Eq,Read,Show,Typeable) (BlogEntry Name Title Body Emai) | | according to the most recent status report, the syntax is not quite | settled yet, so what is in ghc head might still change. more info here: | | http://haskell.org/haskellwiki/GHC/StandAloneDeriving Indeed... but nothing much is happening at the moment because there's not been much feedback so it's drifted off the radar (so to speak). Refining the wiki and evolving a consensus would be great. Simon

4 10

small hack on hasktags to make it collect class names
by Marc Weber 04 Jun '07

04 Jun '07

You can download the modified version from http://mawercer.de/marcweber/hasktags.hs Marc Weber

2 1

enumFrom* strangeness on GHC?
by Felipe Almeida Lessa 04 Jun '07

04 Jun '07

Hello =), I'm puzzled, and maybe someone can help me out. Why does this happens? $ time ghci -e "last $ take 1000000 $ [1..1000000]" 1000000 real 0m0.673s user 0m0.554s sys 0m0.024s $ time ghci -e "last $ take 1000000 $ [1..]" *** Exception: stack overflow real 0m1.305s user 0m1.057s sys 0m0.062s $ ghci -V The Glorious Glasgow Haskell Compilation System, version 6.6 Looking at http://darcs.haskell.org/ghc-6.6/packages/base/GHC/Enum.lhs , I still have no clue of what may be happening. Any ideas? Should I file a bug on GHC? Thanks, -- Felipe.

4 7

Just for a laugh...
by Andrew Coppin 04 Jun '07

04 Jun '07

If you're bored... can you come up with a solution to this? http://warp.povusers.org/ProgrammingChallenge.html (Obviously a pretty silly challenge, but hey.) My first instinct was to use Data.Bits - but I see no instance for Double. (Presumably because performing bitwise operations on a Double is a pretty odd thing to want to do.) So my next guess is to do some bizzare type system hackery that allows you to transform a Double into... whichever integer type is the same size, and proceed from there. Does anybody know how to do that? (Otherwise... wasn't there some library somewhere for serialising values in binary?)

16 32