May 2007 - Haskell-Cafe

Topoi, the categorical Analysis of Logic
by Philippe de Rochambeau 06 May '07

06 May '07

Hello, has anybody on this list read Goldblatt's "Topoi, the categorical Analysis of Logic"? Did reading that book make you a better Haskell programmer? If so, how? Cheers, phiroc

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Any Haskellers in St Louis, MO?
by Aditya Siram 06 May '07

06 May '07

Hi there.. Are there any Haskellers in St Louis , MO? Is there an active group here? Want to start one? Thanks... Deech _________________________________________________________________ Exercise your brain! Try Flexicon. http://games.msn.com/en/flexicon/default.htm?icid=flexicon_hmemailtaglineap…

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Re: [Haskell-cafe] Chaos
by Andrew Coppin 06 May '07

06 May '07

Just so people know... The version in Darcs now has strict Colour and Vector constructors. (Makes it run a few percent faster.)

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[IO Int] -> IO [Int]
by Phlex 06 May '07

06 May '07

Hello all, I'm trying to learn haskell, so here's is my first newbie question. I hope this list is appropriate for such help requests. I'm trying to write a function with the signature [IO Int] -> IO [Int] Here is my first attempt : conv :: [IO Int] -> IO [Int] conv l = do val <- (head l) return (val : (conv (tail l))) This does not work as I'm consing an Int to an (IO [Int]). So I tried this : conv2 :: [IO Int] -> IO [Int] conv2 l = do val <- (head l) rest <- (conv2 (tail l)) return (val : rest) That works, but it won't work for infinite lists. How could I achieve the desired result ? Thanks in advance, Sacha

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The Functional Pearls
by dons＠cse.unsw.edu.au 05 May '07

05 May '07

I've created a wiki page collecting the 'functional pearl' papers that have appeared in JFP and ICFP and other places over the last 20 odd years. http://haskell.org/haskellwiki/Research_papers/Functional_pearls Lots of lovely functional programs there. There's also a list on that page of pearls that don't appear to be online. If you know where they live, please add the links! Cheers, Don

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Displaying infered type signature of 'offside' functions
by Georg Sauthoff 05 May '07

05 May '07

Hi, I like the strong static type system of Haskell for various reasons. One reason is, that it makes easier to understand new code. I.e. when I read code I type ':t foo' in ghci/hugs from time to time, to check my own idea of the type signature, if it is not included in the source code. Well, ':t ' doesn't work withwith non-top-level/local functions (or does it?). Thus I searched for a way to get this information for functions, which are defined offside (of the main indentation level). So, what tools do you use to get the inferred type signature of local functions? I first looked at the GHC-API, but I found this comment in http://darcs.haskell.org/ghc/compiler/main/GHC.hs : - NOTE: -- - things that aren't in the output of the typechecker right now: [..] -- - type signatures Hm, is this comment in sync with the current GHC source? Then I looked at the ghc options. '-ddump-types' only dumps the signatures of global functions ... But '-ddump-tc' includes type signatures of offside defined functions. I.e. something like ghc -ddump-tc Foo.lhs 2>&1 \ | awk '/^[A-Z].+lhs/, ! // { print $0; }' \ | awk '/ :: / , /\[\]/ { if (!($0 ~ /\[\]/)) print $0; }' \ | less does the trick. But as the haskell/ghc wiki mentions, the -ddump-* output is optimized for human reading and not for some kind of 'automatic use'. For automatic use one should look at the GHC-API ... Well, I mention this, because I would like to integrate some lookup feature (for type signatures) into vim (if it doesn't exist yet). Best regards Georg Sauthoff

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Problems with Hs-Plugins
by Philipp Volgger 05 May '07

05 May '07

I tried following very simple program: > module Main where > import System.Eval.Haskell > > main = do i <- eval "1 + 6 :: Int" [] :: IO (Maybe Int) > if isJust i then putStrLn (show (fromJust i)) else return () I compile it with "ghc -c Main.hs" and everything seems fine. When I run it with "ghc Main.o" I get following message: > Main.o(.text+0x48):fake: undefined reference to > `AltDataziTypeable_zdfTypeableInt_closure' > Main.o(.text+0x4d):fake: undefined reference to > `SystemziEvalziHaskell_eval_closure' > Main.o(.text+0x36f):fake: undefined reference to > `__stginit_SystemziEvalziHaskell_' > Main.o(.rodata+0x0):fake: undefined reference to > `SystemziEvalziHaskell_eval_closure' > Main.o(.rodata+0x4):fake: undefined reference to > `AltDataziTypeable_zdfTypeableInt_closure' > collect2: ld returned 1 exit status When I run it with "runhaskell Main" I get: > > GHCi runtime linker: fatal error: I found a duplicate definition for > symbol > _GHCziWord_fromEnum1_closure > whilst processing object file > c:/ghc/ghc-6-4-2/ghc-6.4.2/HSbase1.o > This could be caused by: > * Loading two different object files which export the same symbol > * Specifying the same object file twice on the GHCi command line > * An incorrect `package.conf' entry, causing some object to be > loaded twice. > GHCi cannot safely continue in this situation. Exiting now. Sorry. So I did probably something totally wrong, but what?

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a question concerning type constructors
by Eric 05 May '07

05 May '07

Hi all, In Haskell, is it possible to declare a type constructor with a variable number of type variables e.g. data Tuple * .... allowing the following declarations: t: Tuple u: Tuple Bool v: Tuple Bool Int w: Tuple Bool Int Char ? E.

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Detect Either Windows or Linux environment
by SevenThunders 05 May '07

05 May '07

Is there a simple way to detect what operating system a given Haskell program is running under? It would help to make some programs that might have to interact with the operating system more portable. I haven't been able to quite figure out a simple way to do this. -- View this message in context: http://www.nabble.com/Detect-Either-Windows-or-Linux-environment-tf3695277.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: Monad definition question
by oleg＠pobox.com 05 May '07

05 May '07

Ilya Tsindlekht wrote > > It may be useful to relate to imperative programming: > > m1 >>= (\x -> m2) > > is > > let x = m1 in m2 > The analogy is not always straight-forward - try the list monad. This equivalence holds even for the List Monad. Here is an example of non-determinism: http://caml.inria.fr/pub/ml-archives/caml-list/2007/02/1d4df471019050b89e20… and here's an excerpt from that message, showing lets (aka bind): let numbers = List.map (fun n -> (fun () -> n)) [1;2;3;4;5];; let pyth () = let (v1,v2,v3) = let i = amb numbers in let j = amb numbers in let k = amb numbers in if i*i + j*j = k*k then (i,j,k) else failwith "too bad" in Printf.printf "got the result (%d,%d,%d)\n" v1 v2 v3;; Here, 'amb' is like 'msum' and any error is mzero. If we replace 'let' with 'do', and '=' with '<-' in some places, it looks almost like Haskell. Although the scheduler (the `run' function of the `monad') described in the message accumulates all possible worlds, it returns as soon as one `thread' made it successfully to the end. It is trivial to get the scheduler to run the remaining `treads' searching for more answers (by threads I certainly don't mean OS threads). The user code above stays as it is. The implementation of amb has been extended to annotate choices (and so possible worlds) with probabilities, which propagate in due course. The result not merely lists the answers but also their computed probabilities. > > Filinski then showed that the latter seemingly missing aspect > > indeed only appears to be missing. > Would this require some kind of macros doing extensive pre-processing > of the code? Hopefully the code above showed that no macros and no preprocessing required. In that code, addition, multiplication, if-then-else and all other OCaml operations remain as they have always been, with no modifications or redefinitions whatsoever. Delimited continuations suffice for everything, as Filinski proved in his paper.

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