There are two types of datatype contexts; haskell'98 contexts (which I
think are terrible), and GHC existential contexts (which I like):
class C a where runC :: a -> Int
data C a => T1 a = D1 a
All this does is add a context to the D1 *constructor*; that is:
-- D1 :: C a => a -> T1 a
But extracting a value of this type does nothing:
foo :: T1 a -> Int
foo (D1 a) = runC a -- compile error
However, putting the context on the RHS as you have done works in GHC
and does "the right thing"; pattern matching on that constructor now
brings the class into scope. You can think of the datatype has having
another field which is "proof that a is a member of C":
{-# LANGUAGE ExistentialQuantification #-}
data T2 a = C a => D2 a
-- D2 :: C a => a -> T2 a -- same as D1
bar :: T2 a -> Int
bar (D2 a) = runC a -- works
-- ryan
On Mon, Jul 26, 2010 at 7:48 AM, Gregory Crosswhite
I agree with prior discussion on this list that adding contexts to datatype declarations seems to be more trouble than its worth, since these contexts just have to be added again to every function using the datatype. However, I have often wondered: why do function *have* to have these contexts? What would it affect in the language if we made the contexts be implicit, so that if we have
data Datatype a = Context a => Datatype a
then for function declarations
f :: D a -> ...
the context "Context a" is automatically asserted by the compiler?
Cheers, Greg
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