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

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 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 wrote: Giuseppe Maggiore 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@gmail.com IvanMiljenovic.wordpress.com

-- Giuseppe Maggiore Ph.D. Student (Languages and Games) Microsoft Student Partner Mobile: +393319040031 Office: +390412348444

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-- Giuseppe Maggiore Ph.D. Student (Languages and Games) Microsoft Student Partner Mobile: +393319040031 Office: +390412348444