[Haskell-cafe] ANN: exists-0.1
rendel at informatik.uni-marburg.de
Mon Feb 6 17:07:13 CET 2012
Gábor Lehel wrote:
> data E = forall a. C a => E a
> I don't know if anyone's ever set out what the precise requirements
> are for a type class method to be useful with existentials.
More than you seem to think. For example:
data Number = forall a . Num a => Number a
foo :: Number -> Number
foo (Number x) = Number (x * x + 3)
So the binary operation (*) can be used.
Note that from a type-checking perspective, the pattern match on (Number
x) also extracts the type, which is then available when checking the
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