[Haskell-cafe] Problem on existential type.

[Magicloud Magiclouds]

Thank you all guys. This explained so much.

On Sun, Sep 6, 2009 at 2:53 AM, Ryan Ingram<ryani.spam at gmail.com> wrote:
> On Thu, Sep 3, 2009 at 11:05 PM, Magicloud
> Magiclouds<magicloud.magiclouds at gmail.com> wrote:
>> data GridWidget = forall widget. (WidgetClass widget) => GridWidget widget
>>
>> liftGW :: (GridWidget widget) -> (widget -> t) -> t
>> liftGW (GridWidget label) f = f label
>> liftGW (GridWidget textView) f = f textView
>
> The type signature on liftGW is wrong.  Also, as mentioned elsewhere,
> the two matches overlap; the second case never gets called.
>
> The correct type signature for "liftGW" is:
>
> liftGW :: GridWidget -> (forall widget. WidgetClass widget => widget -> t) -> t
>
> Note that the "f" passed in has to accept *any* widget type, so it's
> possible that existential types aren't what you want.
>
>  -- ryan
>



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