haskell:为特定类型专门化函数

Question

举个例子，我想实现这样的东西：

someWrite ::         IORef a -> a -> IO ()
someWrite = writeIORef
someWrite :: Eq a => IORef a -> a -> IO ()
someWrite ref value = do
    oldValue <- readIORef ref
    if oldValue == value
        then putStrLn "Did not write"
        else writeIORef ref value

显然，该代码是非法的。这样的事怎么能做？

Answer 1

那么，这个怎么样：

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, KindSignatures, FunctionalDependencies, UndecidableInstances, TypeFamilies, ScopedTypeVariables #-}
import Data.IORef
data HTrue
data HFalse
class SomeWrite a where
  someWrite :: IORef a -> a -> IO ()
class SomeWrite' f a where
  someWrite' :: f -> IORef a -> a -> IO ()
instance (Pred f a, SomeWrite' f a) => SomeWrite a where
  someWrite = someWrite' (undefined::f)
class Pred f a | a -> f where {}
instance (f ~ HTrue) => Pred f a
instance Pred HFalse Bool
instance Pred HFalse String
-- Add the rest here
instance SomeWrite' HTrue a where
  someWrite' _ = writeIORef
instance Eq a => SomeWrite' HFalse a where
  someWrite' _ ref value =
    do
      oldValue <- readIORef ref
      if oldValue == value
      then putStrLn "Did not write"
      else writeIORef ref value

https://wiki.haskell.org/GHC/AdvancedOverlap描述了执行此操作的方法。