我正在使用 GHC 8.2.1 。我有以下模块:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Moat (Moat,moat) where
import GHC.Records (HasField(..))
newtype Moat r = Moat r
moat :: r -> Moat r
moat = Moat
instance HasField s r v => HasField s (Moat r) v where
getField (Moat r) = getField @s r
另一个:
module Foo (Foo(..)) where
data Foo a = Foo { getDims :: (Int, Int), getData :: [a] }
我的问题是,当我导入了两个模块时,我尝试执行以下操作:
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE DataKinds #-}
import Moat
import Foo
import GHC.Records
oops :: (Int,Int)
oops = getField @"getDims" (moat (Foo (5,5) ['c']))
我收到此错误:
No instance for (HasField "getDims" (Moat (Foo Char)) (Int, Int))
arising from a use of ‘getField’
为什么HasField
实例未被解析?
答案 0 :(得分:4)
通过在定义{-# LANGUAGE PolyKinds #-}
实例的Moat
模块中启用HasField
来解决问题。
我认为这与多边形HasField
类型类有关:
λ :info HasField
class HasField k (x :: k) r a | x r -> a where
getField :: r -> a
这允许我们像这样定义HasField
个实例,其中字段选择器是非Symbol
类型:
import GHC.Records
data A = A B
data B = B deriving Show
instance HasField B A B where
getField (A b) = b
在ghci:
λ> getField @B (A B)
B