Type Families extension does not work as described

Question

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?

Answer 1

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.