Re: [Haskell-cafe] type versus data declarations

Hola Emilio! On 6/7/07, Emilio Jesús Gallego Arias wrote:

Hello,

I'm wondering why you can write

...

data FSet a = Show a => M (a -> Double)

a :: FSet Double a = M $ \x -> 0

and it works, but

...

type FSet a = Show a => (a -> Double)

type only works for redefinitions (i.e. adding the |Show a| constraint makes FSet a different type to (a -> Double)). In addition you seem to be trying to pack a dictionary with the type (something you cannot do in Haskell without existentials). This is just a guess, but it seems that a definition using existentials is what you're looking for. data FSet a = forall a. Show a => FSet (a -> Double) Cheers, PD: Saluda Ángel de mi parte :)