所描述的类型家庭扩展不起作用 [英] Type Families extension does not work as described

问题描述

在类型族的Haskell wiki页面中，有以下例子：

  type family F a :: * 
 type instance F [Int] = Int  -  OK！ 
类型实例F String = Char  -  OK！ 
类型实例F（F a）= a  -  WRONG：类型参数提到一个类型系列
类型实例F（a。（a，b））= b  -  WRONG：出现类型在类型参数中
类型实例F Float = forall aa  -  WRONG：右手边可能不是所有类型
类型实例其中 -  OK！ 
 F（Maybe Int）= Int 
 F（Maybe Bool）= Bool 
 F（也许a）= String 
类型实例其中 -  WRONG：与早期实例冲突以下）
 F Int =浮动
 F a = [a] 
 
类型系列G ab :: *  - > * 
类型实例G Int =（，） -  WRONG：必须是两个类型参数
类型实例G Int Char Float =双重 -  WRONG：必须是两个类型参数
 类型实例，其中在此扩展名下是有效的语法。但是下面的代码在GHC 7.4.2中没有为我编译： 
 
 
  { - ＃LANGUAGE TypeFamilies＃ - } 
 
类型家庭F a :: * 
类型实例其中
 F（Maybe Int）= Int 
 F（也许Bool）= Bool 
 F（也许a） =字符串
  
错误信息是： 
 
 
 
 
 test.hs：4：15：解析输入错误'where'
 
 
由于这是一个解析错误，它看起来像不支持语法，所以我错过了必要的扩展，或者是其他东西不对？ 
 
解决方案
这似乎是过早记录的情况。根据此博客文章，此语法是最近添加到GHC HEAD中的功能的一部分，但它尚未在GHC的任何发布版本中有效。
 
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?
解决方案
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.

                        
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