- Haskell-Cafe - Haskell.org

instance of String illegal
by Adam Hupp 29 Sep '06

29 Sep '06

I am trying to create an instance of a class for String types. I have the following: class Foo a where mkFoo :: a -> String instance Foo String where mkFoo x = x and receive the following error: test.hs:9:0: Illegal instance declaration for `Foo String' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Foo String' Failed, modules loaded: none. I see that in general I am not able to create an instance of a class for any type [a], where a is a specific type. I can work around it by wrapping my Strings in some other data type but I'd like to understand why it's not possible. Any insight would be appreciated. -Adam

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Creating DLLs with GHC
by Matthew Bromberg 29 Sep '06

29 Sep '06

I am having some difficulty with creating a dynamic link library using GHC on windows XP. I am attempting to follow the example in http://www.haskell.org/ghc/docs/6.4/html/users_guide/win32-dlls.html though I have a binary build of ghc 6.5 My problem (I think) is that some of my Haskell source files link to an external C library also contained in a DLL. I generate my Haskell object files when I compile my Haskell executable. I then attempt to use the object files in a ghc compiler statement like this, ghc --mk-dll -o netsim.dll ExternLib.o ExternLib_stub.o dllNet.o src1.o src1_stub.o src2.o -optl-lmatrixstack -optl-L"." My external C library is in matrixstack.dll and it has a corresponding static link stub library matrixstack.lib in the same directory as all the sources. It has references in one of the sources (say src1.hs). Unfortunately I get a host of undefined references to basically all the functions in matrixstack.dll and also some undefined references of the form Parsefile.o:ghc2996_0.hc:(.text+0x130): undefined reference to `TextziParserCombinatorsziParsecziError_show_closure' Parsefile.o:ghc2996_0.hc:(.text+0x220): undefined reference to `TextziParserCombinatorsziParsecziChar_spaces_closure' Parsefile.o:ghc2996_0.hc:(.text+0x24a): undefined reference to `TextziParserCombinatorsziParsecziChar_spaces_closure' Parsefile.o:ghc2996_0.hc:(.text+0x29c): undefined reference to `TextziParserCombinatorsziParsecziChar_spaces_closure' ... This appears to be coming from unsatisfied references to Text.ParserCombinators.Parsec, which I thought was a standard library. I note that my stand alone haskell executable links just fine and runs perfectly. What am I missing to make this work?

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flip dot
by Greg Fitzgerald 28 Sep '06

28 Sep '06

Since there's talk of removal of the composition operator in Haskell-prime<http://hackage.haskell.org/trac/haskell-prime/wiki/CompositionAsDot>, how about this: Instead of: foo = f . g you write: foo = .g.f A leading dot would mean, "apply all unnamed parameters to the function on the right". A trailing dot would mean, "apply the result of the left to the function on the right". Prelude> notNull = .null.not => [a] -> Bool notNull [1,2,3] => True [1,2,3].notNull => True [1,2,3].null.not => True I like this because it has the same perks as the composition operator, yet it looks like OO code and the data flows nicely from left to right. It reads especially well when using the bind operator on the same line. Thoughts? Thanks, Greg

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Re: Curious Functor Class
by Ashley Yakeley 28 Sep '06

28 Sep '06

On Sep 28, 2006, at 00:38, Jeremy Gibbons wrote: >> Perhaps the key is that there exist types P and Q s.t. there's an >> isomorphism >> >> F a <=> (P -> a,Q) > > F is Naperian iff there's a P with F a = P -> a; but what's the Q for? > >> This seems to be intuitively Napierian: >> >> ln (P -> a,Q) = (P,ln a) | ln Q >> >> I can believe that Hoistables are in fact Idioms, though I know >> there are Idioms that are not Hoistables (Maybe and Either, for >> instance). > > That's right. Every Monad is an Idiom. So are constant functors (F > a = Int) - which I guess a Naperian anyway. Hoistables are not always Idioms, it turns out. I think this can be made a Hoistable, but not an Idiom (because it has a "Q"): type WithInt = (,) Int -- i.e. WithInt a = (Int,a) I don't know if that counts as Napierian or not. >> (Also I think "Idiom" is a better class name than "Applicative".) > > Me too! Can you tell Ross and Conor? I've tried... Hey Ross, Conor, "Idiom" is a better name than "Applicative". Pretty much everyone thinks so. > * > > I don't subscribe to haskell-cafe, so apparently I can't post. Feel > free to post my reply there, if you think it is useful. -- Ashley Yakeley

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source code for haskell web server?
by mvanier 28 Sep '06

28 Sep '06

Does anyone know where I could find the source code for the Haskell web server described in the papers "Tackling the Awkward Squad" by SPJ and "Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server" by Simon Marlow? Thanks in advance, Mike

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Eager global IO actions (per module initialization)
by Einar Karttunen 28 Sep '06

28 Sep '06

Hello I am needing a way to run initializers defined in various modules in an eager fashion before main. I am doing this to load deserialization functions for a Typeable function. Basically I have code like: $(inferDecoderAndRegisterItOnStartup ''MyType) which defines a class instance, but additionally I want to call 'registerDecoderForType "MyType" decodeMyType' automatically on startup. Calling registerDecodeForType for all types in main gets very tedious and error-prone when doing things by hand. Thus an automated solution would be very nice. - Einar Karttunen

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Re: [Haskell] BitSyntax for Haskell
by Einar Karttunen 28 Sep '06

28 Sep '06

On 26.09 10:01, Adam Langley wrote: > >For the decoding part: > >* Provide a monadic interface > > Are you suggesting a monad to pass in the input around, or that it > returns mzero on error? The latter makes more sense to me. Yes. Also make it possible for user supplied functions to fail in better ways than to produce Either or use error. > >* Add a test part to ReadType: > > Test :: ReadType a -> (a -> Bool) -> ReadType Test > > (or a -> m ()) in the monadic case. > > Again, I'm not clear what you are thinking of here? In some protocols I am using there are some fixed bytes which I want to ignore (no Haskell value produced), but check that they are valid in the data stream. - Einar Karttunen

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Expressing seq
by Chad Scherrer 28 Sep '06

28 Sep '06

I was reading on p. 29 of "A History of Haskell" (a great read, by the way) about the controversy of adding seq to the language. But other than for efficiency reasons, is there really any new primitive that needs to be added to support this? As long as the compiler doesn't optimize it away, why not just do something like this (in ghci)? Prelude> let sq x y = if x == x then y else y Prelude> 1 `sq` 2 2 Prelude> (length [1..]) `sq` 2 Interrupted. There must be a subtlety I'm missing, right? -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: Curious Functor Class
by Ashley Yakeley 28 Sep '06

28 Sep '06

Jeremy Gibbons wrote: > I haven't assimilated the forall here, but datatypes with only one shape > of data have been called "Naperian" by Peter Hancock (because they > support a notion of logarithm), and they're instances of McBride and > Paterson's "idioms" or "applicative functors". > > http://sneezy.cs.nott.ac.uk/containers/blog/?p=14 > http://www.cs.nott.ac.uk/~ctm/Idiom.pdf > > As for uses: Bruno Oliveira and I claim an application via The Essence > of the Iterator Pattern: > > > http://www.comlab.ox.ac.uk/jeremy.gibbons/publications/index.html#iterator Perhaps the key is that there exist types P and Q s.t. there's an isomorphism F a <=> (P -> a,Q) This seems to be intuitively Napierian: ln (P -> a,Q) = (P,ln a) | ln Q I can believe that Hoistables are in fact Idioms, though I know there are Idioms that are not Hoistables (Maybe and Either, for instance). (Also I think "Idiom" is a better class name than "Applicative".) -- Ashley Yakeley Seattle WA

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Re: [Haskell] Re: compiler-independent core libraries infrastructure
by Neil Mitchell 28 Sep '06

28 Sep '06

[Moving to cafe for follow up discussions] Hi Bulat, So, just to confirm in my mind what you are proposing: Compiler/Version specific Core: Yhc.Core, Hugs.Core, GHC.Core .... With a different version for each compiler version. Tied intimately to the compiler. The leveler: Core - which abstracts each version/variant of *.Core into one big pile. Contains versions of STM/Arrays in Pure Haskell which might not give the expected performance, but at least keep the semantics as best they can. Base: Pure Haskell which depends on Core, but NOT on *.Core. (Then after you split up base - useful, but personally not of massive importance to me ;) ) I think this is a great plan! Now you have a concrete idea of what you want to achieve in the end, how are you going to get there? It seems a rather ambitious project. Perhaps you might be best suited by getting base to compile with Cabal (and maybe even natively on Windows - without MingW), then you can go from there to the splitting in a Cabal friendly manner without hacking through makefiles? Anyway, once you have decided what you want, I'm sure the Yhc team will do their best to help you get to where you want with regards to that specific compiler. Just to try and keep your work more generally applicable, it would be nice if you kept the Hat project in the back of your mind, so that this work can be reused by them to get full haskell.org libraries. Thanks Neil

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