[Haskell-cafe] Ambiguous type with PolymorphicComponents

[Daniel Fischer]

Am Samstag, 7. März 2009 20:28 schrieb Maurí­cio:
> Hi,
>
> When reading this code in ghci, I get an
> ambiguous type at last line:
>
>
> {-# LANGUAGE PolymorphicComponents #-}
> {-# LANGUAGE RankNTypes #-}
> import Graphics.UI.Gtk
> data Test = Test (forall w. WidgetClass w => w)
> toAction (Test w) = toWidget w
>
>
> It's interesting that if I replace 'Integral' for
> 'WidgetClass' and 'toInteger' for 'toWidget', I
> get no error messages, and I see no essencial
> diference between both classes and both functions.
>
> Do you know what am I missing?

Type defaulting.

When you have 

data Test = Test (forall w. (C1 w, C2 w, ..., Cn w) => w)

and

function (Test w) = classmethod w,

there is no way to decide which instance to use, hence the type variable is 
ambiguous.
But if at least one of these classes is numeric and all of the classes are 
defined in the standard libraries, then (section 4.3.4 of the report, IIRC) 
the ambiguous type variable is defaultable. In the case of Integral and 
toInteger, defaulting kicks in and defaults the type to Integer (unless you 
specified other defaults):

{-# LANGUAGE PolymorphicComponents #-}
{-# LANGUAGE RankNTypes #-}

data Test = Test (forall w. (Show w, Integral w) => w)

toAction (Test w) = show w


$ ghci Polymorph -fwarn-type-defaults
GHCi, version 6.8.3: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( Polymorph.hs, interpreted )

Polymorph.hs:6:25:
    Warning: Defaulting the following constraint(s) to type `Integer'
             `Integral w' arising from a use of `w' at Polymorph.hs:6:25
    In the first argument of `show', namely `w'
    In the expression: show w
    In the definition of `toAction': toAction (Test w) = show w
Ok, modules loaded: Main.
*Main>

If you really want the contents of Test to bepolymorphic, before you use it, 
you must cast it to some specific type,

toAction (Test w) = toWidget (w :: SomeSpecificWidget)

should work.

>
> Thanks,
> Maurício

Cheers,
Daniel