如何在Haskell中对类型类进行分组

Question

目的是使类型类约束代码更清晰。

type CanThrowDice = (Monad m, MonadIO m, Random a)
throwDice :: CanThrowDice m a => (a, a) -> m a
throwDice (r1, r2) = ...

代替

throwDice ::  (Monad m, MonadIO m, Random a) => (a, a) -> m a
throwDice (r1, r2) = ...

我记得在什么地方见过这个用法，但想不起来了。上面的代码警告我添加一些额外的pragma，如ConstraintKinds，我不能使它工作。

Answer 1

唯一缺少的是类型参数m和a以及ConstraintKinds语言pragma [ghc-doc]:

{-# LANGUAGEConstraintKinds#-}
--                ↓ ↓ type parameters
type CanThrowDicema= (Monad m, MonadIO m, Random a)
throwDice :: CanThrowDice m a => (a, a) -> m a
throwDice (r1, r2) = …