Re: [Haskell-cafe] Problem on existential type.

Thank you all guys. This explained so much. On Sun, Sep 6, 2009 at 2:53 AM, Ryan Ingram wrote:

On Thu, Sep 3, 2009 at 11:05 PM, Magicloud Magiclouds 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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