Haskell Constraint Kinds - 默认实现的默认约束

Question

标题:我想为通过约束参数化的类方法提供默认实现，该实现使用该约束的默认实例。

考虑以下因素:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE TypeFamilies #-}

import GHC.Exts (Constraint)

class Foo a where
  type Ctx a :: Constraint
  type Ctx a = Show a

  foo :: (Ctx a) => a -> String
  foo = show

main :: IO ()
main = putStrLn "Compiles!"

编译失败，错误如下:

Could not deduce (Show a) arising from a use of ‘show’
from the context (Foo a)

从我的角度来看，它应该使用 Show 的默认约束，这将让其编译。有什么原因这行不通吗？或者有人可以提出一个实现这一目标的好方法吗？

Answer 1

您可以使用DefaultSignatures来实现此目的:

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DefaultSignatures #-}

import GHC.Exts (Constraint)

class Foo a where
  type Ctx a :: Constraint
  type Ctx a = Show a

  foo :: (Ctx a) => a -> String

  default foo :: Show a => a -> String
  foo = show

main :: IO ()
main = putStrLn "Compiles!"

From my perspective, it should be using the default constraint of Show, which would let this compile.

您的方法不起作用的原因是您的类(class)的用户应该能够覆盖任意数量的默认值。如果有人尝试覆盖 Ctx 但不覆盖 foo，您的代码就会中断。