问题描述
假设我们具有以下newtype定义:
Suppose we have the following newtype definition:
newtype A = A { _run :: Monad m => A -> [Int] -> m Int }
这不能与GHC 8.0.2一起编译:
This does not compile with GHC 8.0.2:
error: Not in scope: type variable ‘m’
用像IO或[]这样的具体类型类替换m确实可以编译，就像我期望的那样.既然可以，为什么GHC不允许上述签名?在此newtype内部添加类型类约束有什么问题?
Replacing m with a concrete typeclass like IO or [] does compile, as I would expect. Given that this is ok, why does GHC not allow the signature above? What is wrong with adding a typeclass constraint inside of this newtype?
推荐答案
这是可能的:
{-# LANGUAGE RankNTypes #-}
newtype A = A { _run :: forall m. Monad m => A -> [Int] -> m Int }
很难说出你想做什么，但这不是很有用.任何类型的A值都需要适用于所有monad(您无法选择).
It's hard to tell what you want to do, but this isn't very usable. Any value of type A needs to work for all monads (you don't get to choose).
在同样的限制下，这也是可能的:
This is also possible, with the same restrictions:
{-# LANGUAGE GADTs #-}
data A where A :: Monad m => (A -> [Int] -> m Int) -> A
但是也许你的意思更像是
But perhaps you mean something more like
newtype A m = A { _run :: A m -> [Int] -> m Int }
这允许使用不同的monad的不同类型的A值.
This allows for values of different types of A using different monads.
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