Thank you all guys. This explained so much. On Sun, Sep 6, 2009 at 2:53 AM, Ryan Ingram<ryani.s...@gmail.com> wrote: > On Thu, Sep 3, 2009 at 11:05 PM, Magicloud > Magiclouds<magicloud.magiclo...@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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