我正在使用Reflex.Dom 库,它定义了一组用于创建 HTML DOM 元素的函数
el
创建一个元素el'
创建并返回一个元素elAttr
创建具有给定属性的元素elAttr'
创建并返回具有给定属性的元素- ETC
我正在制作自己的小部件库,我不想为每个小部件定义所有这些变体。所以我写了一个类型类,它使用相同的名称,但是根据彼此定义所有函数,每个实例中只定义一个:
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
module ElMaker where
import Data.Map (Map)
import qualified Data.Map as Map
import qualified Reflex.Dom as D
-- el: type of element to create
-- input: input parameter
-- output: return value
class (D.MonadWidget t m) => ElMaker t m el input output where
el :: el -> input -> m output
el e = elAttr e Map.empty
elAttr :: el -> Map Text Text -> input -> m output
elAttr e attrs input = snd <$> elAttr' e attrs input
el' :: el -> input -> m (D.El t, output)
el' e = elAttr' e Map.empty
-- This is the only one to implement, yay!
elAttr' :: el -> Map Text Text -> input -> m (D.El t, output)
我创建了一个使用原始实例elAttr'
来测试它的实例。有效:
import Data.Text (Text)
import qualified Reflex.Dom as D
instance (D.MonadWidget t m) => ElMaker t m Text (m output) output where
elAttr' = D.elAttr'
然后我创建了一个Button
小部件实例,它在单击按钮时返回一个事件。有效:
data Button = Button
instance (MonadWidget t m) => ElMaker t m Button (m input) (Event t ()) where
elAttr' _ attrs contents = do
(e, _) <- D.el' "button" contents
return $ (e, D.domEvent D.Click e)
我希望能够编写小部件,所以我尝试重写Button
实例以使用Text
实例ElMaker
来创建元素。但它无法编译:
data Button = Button
instance (MonadWidget t m) => ElMaker t m Button (m input) (Event t ()) where
elAttr' _ attrs contents = do
(e, _) <- el' ("button" :: Text) contents
return $ (e, D.domEvent D.Click e)
编译器输出:
MDL.hs:119:15: error:
• Could not deduce (ElMaker t m Text (m input) output0)
arising from a use of ‘el'’
from the context: MonadWidget t m
bound by the instance declaration at MDL.hs:116:10-71
The type variable ‘output0’ is ambiguous
Relevant bindings include
contents :: m input (bound at MDL.hs:117:19)
elAttr' :: Button
-> Map.Map Text Text -> m input -> m (D.El t, Event t ())
(bound at MDL.hs:117:3)
These potential instance exist:
instance MonadWidget t m => ElMaker t m Text (m output) output
-- Defined in ‘ElMaker’
• In a stmt of a 'do' block:
(e, _) <- el' ("button" :: Text) contents
In the expression:
do { (e, _) <- el' ("button" :: Text) contents;
return $ (e, D.domEvent D.Click e) }
In an equation for ‘elAttr'’:
elAttr' _ attrs contents
= do { (e, _) <- el' ("button" :: Text) contents;
return $ (e, D.domEvent D.Click e) }
我认为这是因为该函数没有对限制其类型的值做任何事情,并且编译器确实希望它具有具体类型。但是这个类型类并不关心那个类型参数的值是什么。有什么办法可以编译这个吗?