11 Oct
2006
11 Oct
'06
6:43 a.m.
Hello oleg, Wednesday, October 11, 2006, 6:45:28 AM, you wrote:
instance (Monad m, HSequence m HNil HNil) => HSequence m HNil HNil where hSequence _ = return HNil
how can i use the goal of the declaration as one of the conditions without causing some sort of black hole in the type inference algorithm?
Very easily: the instance head is implicitly the part of its own context (so that a method can be recursive).
as an example: data T = C instance Eq T where C==C = True (/=) = not (==) instance declarations may be even mutually recursive: instance Eq T where a==b = compare a b == EQ instance Ord T where compare a b = EQ a