Type Families extension does not work as described
haskell, type-families
Solution
This appears to be a case of premature documentation. According to this blog post, this syntax is part of a feature recently added to GHC HEAD, but it's not yet valid in any released version of GHC.
Problem
On the Haskell wiki page for Type Families, there is the following list of examples: ``` type family F a :: * type instance F [Int] = Int -- OK! type instance F String = Char -- OK! type instance F (F a) = a -- WRONG: type parameter mentions a type family type instance F (forall a. (a, b)) = b -- WRONG: a forall type appears in a type parameter type instance F Float = forall a.a -- WRONG: right-hand side may not be a forall type type instance where -- OK! F (Maybe Int) = Int F (Maybe Bool) = Bool F (Maybe a) = String type instance where -- WRONG: conflicts with earlier instances (see below) F Int = Float F a = [a] type family G a b :: * -> * type instance G Int = (,) -- WRONG: must be two type parameters type instance G Int Char Float = Double -- WRONG: must be two type parameters ``` This demonstrates that `type instance where` is valid syntax under this extension. However the following code does not compile for me with GHC 7.4.2: ``` {-# LANGUAGE TypeFamilies #-} type family F a :: * type instance where F (Maybe Int) = Int F (Maybe Bool) = Bool F (Maybe a) = String ``` The error message is: test.hs:4:15: parse error on input `where' Since this is a parsing error, it looks like that syntax is not supported, so am I missing a requisite extension, or is something else amiss?