外部绑定类型声明没有ScopedTypeVariables [英] Outer bound type declarations without ScopedTypeVariables

问题描述

  { - ＃LANGUAGE ScopedTypeVariables＃ - } 
模块Control.Retry其中
 
导入前导隐藏（catch）
导入Control.Exception 
导入Control.Concurrent 
 
重试[]动作=动作
重试（i：is）action = catch action processError 
 where 
 processError（e :: IOException）= threadDelay i>>重试是行动
  

出于好奇，我想知道如何在不使用<$ c的情况下重新实现$ c> ScopedTypeVariables 编译指示，或者我是否可以，以及 processError 的推断类型声明实际上是什么，因为指定 processError :: IOException - >> IO a 使它不可编译。

解决方案

如果你想避免 ScopedTypeVariables ，你可以在大部分时间使用 asTypeOf 。

  retrying [] action = action 
 retrying（i：is）action = catch action processError 
 where 
 processError e = snd（e`asTypeOf`（undefined :: IOException）， threadDelay i>> retrying is action）
  

undefined :: IOException 是一个表达式类型的签名，这是标准允许的。 asTypeOf 要求例外 e 为 IOException 。

尽管如此，我还是倾向于使用 ScopedTypeVariables 。

With

  retrying :: [Int]  - > IO a  - > IO a 
  

类型的 processError 被推断作为

  processError :: IOException  - > IO a 
  

与 a 这里是与重试签名中的变量相同。但是，在没有 ScopedTypeVariables 扩展名的情况下，该类型可以在Haskell中指定，因为在书写签名中的类型变量默认是全局量化的。

I have a following working definition:

{-# LANGUAGE ScopedTypeVariables #-}
module Control.Retry where

import Prelude hiding (catch)
import Control.Exception
import Control.Concurrent

retrying [] action = action 
retrying (i:is) action = catch action processError
  where 
    processError (e :: IOException) = threadDelay i >> retrying is action

Just out of curiosity I am wondering how I could reimplement this without utilizing the ScopedTypeVariables pragma, or whether I could at all, and what the inferred type declaration of processError actually is, because specifying processError :: IOException -> IO a makes it uncompilable.

解决方案

If you want to avoid ScopedTypeVariables, you can most of the time use asTypeOf.

retrying [] action = action 
retrying (i:is) action = catch action processError
  where 
    processError e = snd (e `asTypeOf` (undefined :: IOException), threadDelay i >> retrying is action)

The undefined :: IOException is an expression type signature, and that is allowed by the standard. The asTypeOf requires the exception e to be an IOException.

I would prefer ScopedTypeVariables here, though.

With

retrying :: [Int] -> IO a -> IO a

the type of processError is inferred as

processError :: IOException -> IO a

with the a here being the same type variable as in the signature of retrying. That type can, however, not be specified in Haskell without the ScopedTypeVariables extension, since type variables in written-down signatures are by default universally quantified.

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