27 Mar
2008
27 Mar
'08
12:45 p.m.
The current implementation is wrong, as it permits
type S a b = a type family F a :: * -> * type instance F a = S a
Why do we need to forbid this type instance? Because it breaks the confluence of equality constraint normalisation. Here are two diverging normalisations:
(1)
F Int Bool ~ F Int Char
==> DECOMP
F Int ~ F Int, Bool ~ Char
==> FAIL
(2)
F Int Bool ~ F Int Char
==> TOP
S Int Bool ~ S Int Char
==> (expand type synonym)
Int ~ Int
==> TRIVIAL
This does mean that a program such as
type FList a = Either One ((,) a) type instance F [a] = FList a will be disallowed in further versions? Doesn't this problem occur only for type synonyms that ignore one or more of the parameters? If so, this could be checked... hugo