haskell - 类型系列扩展无法按描述工作

Question

关于the Haskell wiki page for Type Families ，有以下示例列表:

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

这表明 type instance where 是此扩展下的有效语法。但是，以下代码无法使用 GHC 7.4.2 进行编译:

{-# LANGUAGE TypeFamilies #-}

type family F a :: *
type instance where
  F (Maybe Int)  = Int
  F (Maybe Bool) = Bool
  F (Maybe a)    = String

错误信息是:

test.hs:4:15: parse error on input `where'

由于这是一个解析错误，看起来该语法不受支持，所以我是否缺少必要的扩展，或者是否有其他问题？

Answer 1

这似乎是一个过早记录的情况。根据this blog post ，此语法是最近添加到 GHC HEAD 的功能的一部分，但它在任何已发布的 GHC 版本中尚未有效。