在下面的代码中,
{-# LANGUAGE OverloadedLabels #-}
module Foo where
data R = R { x :: Int }
g :: Int
g = #x (R { x = 1 })
我希望这能进行类型检查,但我得到:
foo.hs:7:5: error:
• No instance for (GHC.OverloadedLabels.IsLabel "x" (R -> Int))
arising from the overloaded label ‘#x’
(maybe you haven't applied a function to enough arguments?)
• In the expression: #x
In the expression: #x (R {x = 1})
In an equation for ‘g’: g = #x (R {x = 1})
鉴于Overloaded Record Fields提案,我希望有IsLabel "x" (R -> Int)
的内置实例。还是这样吗?或者实施是否偏离提案?
在(至少当前)基础上,IsLabel
没有OverloadedLabels
实例(参见讨论here)。您可能需要使用一些定义孤立实例的库,例如generic-lens。当然,您可以自己定义:
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
import GHC.Records
import GHC.OverloadedLabels
instance HasField x r a => IsLabel x (r -> a) where
fromLabel = getField @x