Haskell 重载列表扩展不工作
GHC 7.8中的重载列表语言pragma非常吸引人,因此我决定尝试一下:Haskell 重载列表扩展不工作,haskell,language-extension,Haskell,Language Extension,GHC 7.8中的重载列表语言pragma非常吸引人,因此我决定尝试一下: {-# LANGUAGE OverloadedLists #-} import Data.Set (Set) import qualified Data.Set as Set mySet :: Set Int mySet = [1,2,3] 编译器给了我: No inst
{-# LANGUAGE OverloadedLists #-}
import Data.Set (Set)
import qualified Data.Set as Set
mySet :: Set Int
mySet = [1,2,3]
编译器给了我:
No instance for (GHC.Exts.IsList (Set Int))
arising from an overloaded list
In the expression: [1, 2, 3]
In an equation for ‘mySet’: mySet = [1, 2, 3]
No instance for (Num (GHC.Exts.Item (Set Int)))
arising from the literal ‘1’
In the expression: 1
In the expression: [1, 2, 3]
In an equation for ‘mySet’: mySet = [1, 2, 3]
Failed, modules loaded: none.
甚至发行说明中的示例也不起作用:
> ['0' .. '9'] :: Set Char
<interactive>:5:1:
Couldn't match type ‘GHC.Exts.Item (Set Char)’ with ‘Char’
Expected type: [Char] -> Set Char
Actual type: [GHC.Exts.Item (Set Char)] -> Set Char
In the expression: ['0' .. '9'] :: Set Char
In an equation for ‘it’: it = ['0' .. '9'] :: Set Char
['0'..'9']::设置字符
:5:1:
无法将类型“GHC.Exts.Item(Set Char)”与“Char”匹配
预期类型:[Char]->Set Char
实际类型:[GHC.Exts.Item(设置字符)]->设置字符
在表达式中:['0'..'9']::Set Char
在“it”的方程式中:it=['0'..'9']::Set Char
有人知道这里发生了什么吗?只定义了一个微不足道的实例。您可以为
数据定义自己的实例。使用以下方法设置:
{-# LANGUAGE TypeFamilies #-}
instance IsList (Set a) where
type Item (Set a) = a
fromList = Data.Set.fromList
toList = Data.Set.toList
请注意,IsList
仅适用于GHC>=7.8。它在错误中说:没有(GHC.Exts.IsList(Set Int))的实例。
Data.Set
不为IsList
定义实例。你可以很容易地自己编写这个实例,下面是一个类:很不幸,我原以为这些实例是内置的。还要注意,你也需要使用TypeFamilies
pragma才能工作。谢谢你指出这一点;我已相应地编辑了答案。