何时（如果曾经）可以部分应用类型同义词？ [英] When (if ever) can type synonyms be partially applied?

问题描述

显然有点心不在焉，我写了类似于以下内容：

  { - ＃LANGUAGE Con​​straintKinds＃ - } 
 { - ＃LANGUAGE TypeFamilies＃ - } 
 
 class Foo f where 
 type Bar f :: * 
 retbar :: Bar f  - > IO f 
 
类型Baz f =（Foo f，Eq f）
 
  -  WithBar :: *  - > （*  - >约束） - > *  - >约束
类型WithBar bcf =（cf，Bar f〜b）
 
实例Foo（）其中
类型Bar（）=（）
 retbar = return 
 
 naim :: WithBar（）Baz u =>你呃 - 为什么我可以这样做？ 
 naim = retbar（）
 
 main = naim :: IO（）
  

只有成功编译之后，我才意识到不应该实际工作： Baz 被定义为具有一个参数的类型同义词，但我在这里没有直接的论据。通常，GHC会咆哮我类型同义词'Baz'应该有1个参数，但当我尝试类似的时候没有给出。

现在，请不要误解我的意思：我真的很希望能够编写它，并且很容易看出它在这个特定示例中的工作原理（简单地将 WithBar 会产生签名 naim ::（Foo u，Bar u〜（））=> IO u ，这当然很好），但是我不明白为什么它实际上就是这样工作的。它可能只是 ghc-7.8.2 中的一个bug来允许这个吗？


解决方案

您的文件在GHC 7.8中编译，但在GHC 7.10中出现错误：
$ b

类型同义词'Baz'应该有1参数，但没有给出

在'naim'的签名中：naim :: WithBar（）Baz u => IO u

添加 -XLiberalTypeSynonyms 修复错误。因此，这是7.8中的一个错误。

Apparently a bit absent-mindedly, I wrote something like the following:

{-# LANGUAGE ConstraintKinds     #-}
{-# LANGUAGE TypeFamilies        #-}

class Foo f where
  type Bar f :: *
  retbar :: Bar f -> IO f

type Baz f = (Foo f, Eq f)

  -- WithBar :: * -> (*->Constraint) -> * -> Constraint
type WithBar b c f = (c f, Bar f ~ b)

instance Foo () where
  type Bar () = ()
  retbar = return

naim :: WithBar () Baz u => IO u  -- why can I do this?
naim = retbar ()

main = naim :: IO ()

Only after successfully compiling, I realised this shouldn't actually work: Baz is defined as a type synonym with one argument, but here I use it without a direct argument. Usually GHC barks at me Type synonym ‘Baz’ should have 1 argument, but has been given none when I try something like that.

Now, don't get me wrong: I would really like being able to write that, and it's easy enough to see how it could work in this particular example (simply inlining WithBar would yield the signature naim :: (Foo u, Bar u ~ ()) => IO u, which is certainly fine), but what I don't understand why it actually works just like that here. Is it perhaps only a bug in ghc-7.8.2 to allow this?

解决方案

Your file compiles in GHC 7.8, but in GHC 7.10 I get an error:

Type synonym ‘Baz’ should have 1 argument, but has been given none

In the type signature for ‘naim’: naim :: WithBar () Baz u => IO u

Adding -XLiberalTypeSynonyms fixes the error. Therefore, this is a bug in 7.8.

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