- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] confusion about 'instance'....
by Miguel Mitrofanov 10 Jan '08

10 Jan '08

> class A a > type T = (forall x.Num x=>x) > instance A T "type" declares a synonym, like #define in C - but working only on types. So, essentially, you wrote instance A (forall x.Num x => x) which is not very Haskelly. > I am simply trying to state that all members of typeclass Num are of > typeclass A.... You can't do that. But, if there would not be any other instances of A, then you don't need it at all, you can just use Num class. And if there are some, for example data D = D instance A D then it can happen (well, it's unlikely, but possible) that you or some other developer working on your code would declare instance Num D where ... After that, you would have two instances of A for the type D, one defined explicitly and one derived from the Num instance. That's not a problem for this empty class, but if class A is not empty, say class A x where a :: x then the compiler would be unable to decide which instance (and which "a") to choose. So allowing that leads to non-obvious bugs. However, if you trust yourself enough, you can do what you want to in this way: {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} class A a instance Num a => A a

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Re: [Haskell] Problems with Unicode Symbols as Infix Function Names in Propositional Calculus Haskell DSL
by Simon Marlow 10 Jan '08

10 Jan '08

Cetin Sert wrote: > I want to design a DSL in Haskell for propositional calculus. But > instead of using natural language names for functions like or, and, > implies etc. I want to use Unicode symbols as infix functions ¬, ˅, ˄, > →, ↔ But I keep getting error messages from the GHC parser. Is there a > way to make GHC parse my source files correctly? If it is not possible > yet, please consider this as a “feature request”. GHC supports unicode source files encoded using UTF-8 by default. It should be possible to use many unicode symbols for infix symbols. Note that when -XUnicodeSyntax is on, certain symbols have special meanings (e.g. → means ->). Without more information I can't tell exactly what problem you're encountering. If you supply the source code you're trying to compile, we might be able to help. Also, note that glasgow-haskell-users(a)haskell.org is a better forum for GHC-specific issues. Cheers, Simon

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Re: [Haskell-cafe] 0/0 > 1 == False
by Yitzchak Gale 10 Jan '08

10 Jan '08

Cristian Baboi wrote: > I think this should be put this way: > Bottom is a part of the semantic domain which is not Haskell. Rather, something outside Haskell that describes what Haskell programs mean. Yes. > In the semantic domain there is one bottom. > In Haskell there are many expressions that represent bottom. > One cannot test those for equality. Yes. > The result of a Haskell function applied to some arguments cannot be > bottom. I think you mean that they cannot be bottom if you want to compare them for equality. Yes. -Yitz

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Problem with own written monad
by Michael Roth 10 Jan '08

10 Jan '08

Hello list, while trying to learn the secrets of monads, I decided to write a simply monand for pure educational purpose. But it turned out that it isn't as easy as I thought... I circumnavigate quite a number of hurdles but now I reached a point where I'm at a loss. :-( The source: #! /usr/bin/env ghc data Stack a = Stack { run :: [a] -> (a, [a]) } push :: a -> Stack a push x = Stack f where f xs = ( x, x:xs ) pop :: Stack a pop = Stack f where f (x:xs) = ( x, xs ) top :: Stack a top = Stack f where f (x:xs) = ( x, x:xs ) instance Monad Stack where return x = Stack f where f xs = ( x, xs ) (>>=) stack g = Stack f where f s0 = (x2, s2) where (x1, s1) = run stack s0 (x2, s2) = run (g x1) s1 The errors: ./mymonad.hs:16:24: Couldn't match expected type `b' (a rigid variable) against inferred type `a' (a rigid variable) `b' is bound by the type signature for `>>=' at <no location info> `a' is bound by the type signature for `>>=' at <no location info> Expected type: [b] -> (b, [b]) Inferred type: [a] -> (b, [b]) In the first argument of `Stack', namely `f' In the expression: Stack f ./mymonad.hs:19:28: Couldn't match expected type `b' (a rigid variable) against inferred type `a' (a rigid variable) `b' is bound by the type signature for `>>=' at <no location info> `a' is bound by the type signature for `>>=' at <no location info> Expected type: [b] Inferred type: [a] In the second argument of `run', namely `s1' In the expression: run (g x1) s1 I think the problem is that my operator (>>=) is of type: Stack a -> (a -> Stack a) -> Stack a but should be: Stack a -> (a -> Stack b) -> Stack b But, I have simply no clue how to fix that. :-( Can anybody give my a hint? Thank you in advance. Michael Roth

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Problems with Unicode Symbols as Infix Function Names in Propositional Calculus Haskell DSL
by Cetin Sert 10 Jan '08

10 Jan '08

I want to design a DSL in Haskell for propositional calculus. But instead of using natural language names for functions like or, and, implies etc. I want to use Unicode symbols as infix functions ¬, ˅, ˄, →, ↔ But I keep getting error messages from the GHC parser. Is there a way to make GHC parse my source files correctly? If it is not possible yet, please consider this as a “feature request”. Best Regards, Cetin Sert INF 521, 4-6-2 69120 Heidelberg Germany http://www.corsis.de

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Refactoring status
by Peter Verswyvelen 10 Jan '08

10 Jan '08

Hi all, Is any work being done on Haskell refactoring support, like HaRe or others? Is anyone actively using refactoring? When using C#, I used Resharper a lot, and ever since, I'm really hooked to refactoring, so I miss it a lot when doing Haskelling. (I never seem to get a function name or signature right the first time. is it just me? J) I'm currently using Emacs with Haskell Mode (which does not offer refactoring support) but I think many of you use VIM (which does support it?) Can one use refactoring outside of an editor? This does not really sound practical, but maybe it works? Thank you, Peter PS: IMHO I don't think text should be the source format of our files. I think we should use a standarized decorated AST as the source, from which we can derive a textual (but also graphical) view and editor. Any comments on that? J

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Data.Binary and serialising doubles in IEEE format
by allan 10 Jan '08

10 Jan '08

Dear all I have what I think should be a relatively simple problem: I have a C program which reads in data from a binary file, it is a specific format so it knows that there are 3 integers followed by three doubles or whatever. So I'm trying to serialise the data from within a Haskell program and write it out to a file such that the C program can read this. I've gotten the integers to work no problem but I'm having difficulty with the doubles. My method is to use the Data.Binary library to write the data out as a bytestring and then simply write this out to disk. The problem is that the instance of Binary for Double does not serialise the data in the format which is expected by the C program (standard IEEE format using fread and a cast). Now there was a thread on (essentially) this very same topic on haskell-cafe back in April, see: http://www.haskell.org/pipermail/haskell-cafe/2007-April/024596.html There was some discussion about the 'right' thing to do and that the library intended to be split into two separate portions; one for users that wish to serialise their own data and have it readable by the same haskell program, and those that wish to communicate with programs written in other languages. From the above thread: > Perhaps we just don't care about ARM or other arches where GHC runs that > do not use IEEE formats, I don't know. If that were the case we'd say > something like: > > instance Binary Double where > put d = assert (isIEEE (undefined :: Double)) $ do > write (poke d) Now for my specific problem I'd be happy if I could write a function: putDouble :: Double -> Put and so obviously I tried: putDouble :: Double -> Put putDouble d = assert (isIEEE (undefined :: Double)) $ do write (poke d) However I seem to be stuck at this point, where does the 'write' function come from? Essentially then, is there anyone that can write my 'putDouble' function or give some hint as to how I might do it. For the moment assume that I'm not really concerned with portability across platforms at least for the time being (I certainly don't think that the C program I'm attempting to communicate with is particularly portable anyway). kind regards allan

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GHC 6.8.2 as a library on Windows and GHCi
by Peter Verswyvelen 09 Jan '08

09 Jan '08

I while I ago I sent an email regarding hs-plugins not working on windows. I now tried to directly use GHC API, but I also failed. The following program (which might be buggy, I copy/pasted from various sources) import DynFlags import GHC main = defaultErrorHandler defaultDynFlags $ do session <- newSession (Just "d:/app/ghc-6.8.2") flags <- getSessionDynFlags session (flags, _) <- parseDynamicFlags flags [] GHC.defaultCleanupHandler flags $ do setSessionDynFlags session flags{ hscTarget=HscInterpreted } prelude <- findModule session (mkModuleName "Prelude") Nothing setContext session [] [prelude] runStmt session "let n = 2 + 2" RunToCompletion -- n is bound runStmt session "n" RunToCompletion -- 4 is printed (note "it" is bound) gives the following error when run from GHCi *Main> :load "l:/test.hs" [1 of 1] Compiling Main ( l:/test.hs, interpreted ) Ok, modules loaded: Main. *Main> main GHCi runtime linker: fatal error: I found a duplicate definition for symbol _forkOS_entry whilst processing object file d:/app/ghc-6.8.2/lib\base-3.0.1.0/HSbase-3.0.1.0.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. Process haskell finished Is my code incorrect, or is this a (known?) bug in GHC 6.8.2 on Windows? I haven't tried the Linux version yet. NOTE: The program *does* run fine when compiling via GHC --make. Thanks, Peter

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Trouble with function with two clauses
by Fernando Rodriguez 09 Jan '08

09 Jan '08

Hi, I have the following type and function: data ConsCell a = Nil | Cons a (ConsCell a) deriving Show head' Nil = Nothing head' (Cons a _) = Just a Works fine, however, what's wrong with the following function? head'' | Nil = Nothing | Cons a _ = Just a Thanks!

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US Homeland Security program language security risks
by Galchin Vasili 09 Jan '08

09 Jan '08

Hello, https://buildsecurityin.us-cert.gov/daisy/bsi/articles/knowledge/coding/295… I stumbled across this page. It seems that Haskell and other strongly typed functional languages like Ml/OCaml will fare much, much better, e.g. buffer overrun. Thoughts . .... comments. Vasili

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