haskell - 如何在组合类型类的函数时添加中间值的类型注释？

Question

背景:我正在研究声明式编译器。在本类(class)中，我将编写一个类来构造一个中间数据结构。构建数据结构后，可以从数据结构中重新呈现输出。为了简化 stackoverflow，我创建了以下代码:

module Main where

import qualified Data.Word as W
import qualified Octetable as Oct

main :: IO ()
main = 
    do
        print (buildNRender "123")

data MyData = MyData Integer

data Construction model = Contains model | Error String
    deriving Show

class Builder g where
    build :: String -> (Construction g)
    render :: (Construction g) -> [W.Word8]
    buildNRender :: String -> [W.Word8]
    buildNRender = render . build

instance Builder MyData where
    build s = Contains (MyData (read s :: Integer))
    render (Contains (MyData n)) = Oct.toOctets n
    render (Error _) = []

明显的问题是，'buildNRender' 不能成为 Builder 的一部分，因为根本没有使用类型参数 g。

现在，对我来说很明显类型类不能像这样工作，其中两个或多个函数组合的中间值具有类型参数。

以下代码使中间类型显式并且有效 - 但没有 buildNRender。

...

main :: IO ()
main = 
    do
        print (render ((build "123") :: (Construction MyData))

...

但是，是否有一种优雅的方式来定义类的此类 DEFAULT 方法(如“buildNRender”)，并在调用方的上下文中指定中间类型，如以下代码所示？

...

main :: IO ()
main = 
    do
        print ((buildNRender "123") :: ?(Construction MyData)?)

...

Answer 1

The obvious problem is, buildNRender cannot be part of Builder, because type parameter g is not used at all.

好吧，曾经 是个问题(具体来说，g 会模棱两可)，但现在不是了，因为 GHC 现在具有允许使用此类参数的扩展。

{-# LANGUAGE <a href="https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/ambiguous_types.html" rel="noreferrer noopener nofollow">AllowAmbiguousTypes</a>, <a href="https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/type_applications.html" rel="noreferrer noopener nofollow">TypeApplications</a>, <a href="https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/scoped_type_variables.html" rel="noreferrer noopener nofollow">ScopedTypeVariables</a>, UnicodeSyntax #-}
    
module Main where

import qualified Data.Word as W
import qualified Octetable as Oct

main :: IO ()
main = 
    do
        print (buildNRender @MyData "123")

data MyData = MyData Integer

data Construction model = Contains model | Error String
    deriving Show

class Builder g where
    build :: String -> (Construction g)
    render :: (Construction g) -> [W.Word8]

buildNRender :: ∀ g . Builder g => String -> [W.Word8]
-- ∀ (forall) g . introduces the type variable g into scope
-- needs extension AllowAmbiguousTypes
buildNRender = render . build @g -- @g is a Type Application

instance Builder MyData where
    build s = Contains (MyData (read s :: Integer))
    render (Contains (MyData n)) = Oct.toOctets n
    render (Error _) = []

或者，不使用 UnicodeSyntax:

{-# LANGUAGE AllowAmbiguousTypes, TypeApplications, ScopedTypeVariables #-}

...

buildNRender :: forall g . Builder g => String -> [W.Word8]