[Haskell-cafe] Code that doesn't compile - but should :)

[Chris Eidhof]

On 16 mrt 2010, at 10:58, Giuseppe Maggiore wrote:

> Well, first of all thanks!
> Second, why the need for explicit quantification?

I'm not sure, but I think it has to do with ambiguity. I think it's similar to the problem:

> readShow :: (Read a, Show a) -> String -> String
> readShow = show . read

We need to explicitly quantify over the type variables so that we can give an explicit type signature on the following line:

> let method = select s :: a -> f a

There might be an easier way to do this, but I'm not sure how exactly.

-chris


> 
> On Tue, Mar 16, 2010 at 2:39 AM, Chris Eidhof <chris at eidhof.nl> wrote:
> What about this?
> 
> > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
> >  UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
> >  TypeOperators, TypeSynonymInstances #-}
> >
> > data Data k = Pair Integer (() -> k)
> > data RecData = RecData (Data RecData)
> > mk_data x = RecData(Pair x (\() -> mk_data (x+1)))
> >
> 
> The I had to change the type of the Converter typeclass
> 
> > class Converter a f where convert :: f a -> a
> >
> > -- instance Converter RecData Data where
> > --  convert (RecData r) = r
> >
> > class Selector s a where select :: s -> a
> >
> 
> And explicitly quantify the type variables:
> 
> > f :: forall f s a . (Selector s (a->f a), Converter a f) => s -> (a->a)
> > f s =
> >  let method = select s :: a -> f a
> >  in (\x ->
> >        let res = method x
> >        in convert res)
> 
> -chris
> 
> On 16 mrt 2010, at 10:36, Giuseppe Maggiore wrote:
> 
> > The error message (obtained by loading the file with ghci) is:
> > GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
> > Loading package ghc-prim ... linking ... done.
> > Loading package integer ... linking ... done.
> > Loading package base ... linking ... done.
> > [1 of 1] Compiling Main             ( C:\Users\pulcy\Desktop\Papers\Monads\Objec
> > tiveMonad\HObject\Experiments\FunctorsProblems.hs, interpreted )
> >
> > C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
> > Problems.hs:18:15:
> >     Could not deduce (Selector s (f a -> a))
> >       from the context (Selector s (a1 -> f1 a1), Converter a1 f1)
> >       arising from a use of `select'
> >                    at C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObjec
> > t\Experiments\FunctorsProblems.hs:18:15-22
> >     Possible fix:
> >       add (Selector s (f a -> a)) to the context of
> >         the type signature for `f'
> >       or add an instance declaration for (Selector s (f a -> a))
> >     In the expression: select s
> >     In the definition of `method': method = select s
> >     In the expression:
> >         let method = select s in (\ x -> let res = ... in convert res)
> >
> > C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Experiments\Functors
> > Problems.hs:21:11:
> >     Couldn't match expected type `a1' against inferred type `f a'
> >       `a1' is a rigid type variable bound by
> >            the type signature for `f'
> >              at C:\Users\pulcy\Desktop\Papers\Monads\ObjectiveMonad\HObject\Expe
> > riments\FunctorsProblems.hs:16:18
> >     In the expression: convert res
> >     In the expression: let res = method x in convert res
> >     In the expression: (\ x -> let res = method x in convert res)
> > Failed, modules loaded: none.
> > Prelude>
> >
> >
> > On Tue, Mar 16, 2010 at 2:31 AM, Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com> wrote:
> > Giuseppe Maggiore <giuseppemag at gmail.com> writes:
> >
> > > Hi! Can anyone tell me why this code does not work? I cannot seem to
> > > figure why it is broken...
> >
> > The error message (and how you got it) would help...
> >
> > > {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
> > >   UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
> > >   TypeOperators, TypeSynonymInstances #-}
> >
> > You sure you have enough language extensions there? ;-)
> >
> > Barely :)
> >
> >
> > --
> > Ivan Lazar Miljenovic
> > Ivan.Miljenovic at gmail.com
> > IvanMiljenovic.wordpress.com
> >
> >
> >
> > --
> > Giuseppe Maggiore
> > Ph.D. Student (Languages and Games)
> > Microsoft Student Partner
> > Mobile: +393319040031
> > Office: +390412348444
> >
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> 
> 
> 
> -- 
> Giuseppe Maggiore
> Ph.D. Student (Languages and Games)
> Microsoft Student Partner
> Mobile: +393319040031
> Office: +390412348444
> 
>