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Questions about lambda calculus
by Patrick LeBoutillier 04 Nov '10

04 Nov '10

Hi all, I've been reading this article: http://perl.plover.com/lambda/tpj.html in which the author talks about the lambda calculus and uses examples in Perl (here's a link directly to the code: http://perl.plover.com/lambda/lambda-brief.pl) I'm a total newbie with respect to the lambda calculus. fA = \x -> 'A' fB = \x -> 'B' x -=- y = (show $ x id) == (show $ y id) I tried (naively) to port these examples to Haskell to play a bit with them: fTRUE = \a -> \b -> a fFALSE = \a -> \b -> b fIF = \b -> \x -> \y -> b x y fPAIR = \a -> \b -> \f -> f a b fFIRST = \p -> p fTRUE fSECOND = \p -> p fFALSE fZERO = fPAIR fTRUE fTRUE fSUCC = \x -> fPAIR fFALSE x fIS_ZERO = \x -> fFIRST x fPRED = \x -> fSECOND x fONE = fSUCC fZERO fTWO = fSUCC fONE fADD = \m -> (\n -> fIF (fIS_ZERO m) n (fADD (fPRED m) (fSUCC n))) but I couldn't get fADD to compile: Occurs check: cannot construct the infinite type: t = (t1 -> t1 -> t1) -> t Probable cause: `fADD' is applied to too many arguments In the third argument of `fIF', namely `(fADD (fPRED m) (fSUCC n))' In the expression: fIF (fIS_ZERO m) n (fADD (fPRED m) (fSUCC n)) I think its because in these Perl examples all functions are treated as being of the same type (number or type of args doesn't matter), but this is not the case in Haskell. Is there any way to create code similar to this in Haskell? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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pred (succ n)
by ben 04 Nov '10

04 Nov '10

Hello, this question is somewhat similar to the one I've asked some hours ago: I would like to have a predecessor operator PRED for the untyped lambda calculus that has the property that PRED (SUCC n) --> n (I) for a suitable successor function SUCC. So far I've tried with SUCC := S := \nfx.f (n f x) and PRED := P := \nfx.n (\gh.h (g f)) (\u.x) (\u.u) for which I can prove beta equivalence P (S n) == n by induction on n, but not the property (I). Could anybody give me a pointer? Greetings, ben ps. Please keep me in CC, since I'm not currently subscribed to the list.

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fixpoint operator for ULC with specific reduction property
by ben 04 Nov '10

04 Nov '10

Hello, I'm looking for a fixpoint operator Z for the untyped lambda calculus that has the property Z g --> g (Z g) for any g. The best-known fixpoint operator Y := \f. (\x. f (x x)) (\x. f (x x)) does not have this property, but only Y g --> T <-- g (Y g) for some T. Does anybody know such a Z I'm looking for? Greetings, ben ps. Please keep me in CC, since I'm not currently subscribed to the list.

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Case statements shorter
by Angel Alonso 02 Nov '10

02 Nov '10

Hi, I have the following piece of code: n = 3 case n of 3 -> "Something" 4 -> "Something" _ -> "Other" Is it possible to shorten the case statement into something like this?: n = 3 case n of 3 or 4 -> "Something" _ -> "Other" Thanks. -- Angel Alonso

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Re: [Haskell-beginners] Case statements shorter
by uu1101＠gmail.com 02 Nov '10

02 Nov '10

Nice distinction! Thank you for the correction. Il giorno 02/nov/2010, alle ore 09.11, Roel van Dijk <vandijk.roel(a)gmail.com > ha scritto: > On Mon, Nov 1, 2010 at 7:23 PM, uu1101(a)gmail.com <uu1101(a)gmail.com> > wrote: >> * _ is not a special keyword in haskell. It's a regular variable so >> will >> always match. It's usually used when you don't use it's value in >> the right >> hand side or when it adds no information. In this case its value >> will be n >> so there is no reason to introduce another name. > > This is incorrect. _ is a pattern not a variable. > > I can write > > f (_:_) = "bla" > > but not > > f (x:x) = "bla" > Conflicting definitions for `x' In the definition of `f' > > So _ is a pattern that always matches but is never bound to a value. > > Regards, > Roel

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How to Install Gtk2Hs on Windows?
by John Smith 01 Nov '10

01 Nov '10

I tried following the directions on http://jystic.com/2010/10/20/installing-gtk2hs-on-windows/, but when I run cabal install gtk, I get * Missing C libraries: z, cairo * Missing C libraries: gobject-2.0, glib-2.0, intl I seem to have everything asked for in the PATH. pkg-config reports that Gtk+ 2.16.2 is installed.

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error when unregistering a package
by Ashish Agarwal 28 Oct '10

28 Oct '10

I always have trouble with conflicting packages when I do a fresh install of the Haskell Platform on an Ubuntu machine. Last time, I unregistered lots of packages, and let the necessary ones get recompiled. That fixed things. However, this time I'm having trouble with the unregister step. For many packages I get an error about a file not existing, e.g. $ sudo ghc-pkg unregister QuickCheck ghc-pkg: /var/lib/ghc-6.12.1/package.conf.d/QuickCheck-2.1.0.3-d4ae7d01dc2504e1785e18cd555e91ed.conf: removeLink: does not exist (No such file or directory) However, for many other packages, it works fine. Anyone know how to unregister a package when you get this error?

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Help-me read file
by Gilmara Pompelli 28 Oct '10

28 Oct '10

Hello I have a file with 100 lists, with 100 ints. I have to read the file and apply the map and sort functions on lists. II did it to read file: learquivo :: FilePath -> IO ([[Int]]) learquivo s = do conteudo <- readFile s return (read conteudo) But now applying the sort, the error appears: <interactive>:1:5: Couldn't match expected type `[a]' against inferred type `IO [[Int]]' how can I fix? Thanks

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Best practice for catching pure errors in IO?
by John Smith 27 Oct '10

27 Oct '10

I have a pure value which may throw errors when it is evaluated in an IO monad. My code to handle it looks like this: temp <- try $ evaluate value case temp of Left (error::ErrorCall) -> scoldTheUserAndTryAgain Right correct -> doSomething Can this be done without the temporary variable, or with less plumbing? I tried lifting either, but GHC was fussing about ambiguous types, appending a signature to a pattern match is no longer allowed(?), and it didn't look that much cleaner.

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read
by Russ Abbott 27 Oct '10

27 Oct '10

Hi, I'm having a problem with the read function. I thought it took a string and converted it into a value (if there is a conversion). But it doesn't seem to work for me on GHCi Prelude> read "123" *** Exception: Prelude.read: no parse Prelude> read (show 123) *** Exception: Prelude.read: no parse There must be something trivial that I'm missing. Thanks. *-- Russ Abbott*

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