Re: [Haskell-cafe] ANN: exists-0.1

Hi, 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 right-hand side. Tillmann