Matthew Brecknell <haskell <at> brecknell.org> writes: > > Levi Stephen: > > I have a data type: > > > > > data T a = forall b. (Show b) => T b a > > > > and I want to use/extract 'b' from this. > > You can't. (Well, I believe you can if you have prior knowledge of the > actual type of the existentially wrapped "b", and you're willing to use > an "unsafe" coerce, but I've never tried.) > You can do type safe casts. IIRC Stephanie Weirich presented a paper on it at icfp 2000(?). Of course, as you point out, you would still need prior knowledge of the type.
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