如何让这个Haskell ADT派生出Show？

Question

ADT是免费的monad:

data Free f r = Free (f (Free f r)) | Pure r

我希望它能够得到它,Show以便我可以在使用它时将其打印出来.例如,如果我有以下内容:

data T next = A next | B next deriving (Show)
aa = Free $ A $ Free $ B $ Pure ()

就像现在一样,如果我添加deriving (Show)到FreeADT ,我会收到以下错误:

No instance for (Show (f (Free f r)))
      arising from the first field of ‘Free’ (type ‘f (Free f r)’)
    Possible fix:
      use a standalone 'deriving instance' declaration,
        so you can specify the instance context yourself
    When deriving the instance for (Show (Free f r))

我想要得到show aa一个可打印的字符串.这可能吗？

Answer 1

正如错误消息所暗示的那样,您需要使用一个名为的扩展StandaloneDeriving,它允许您明确指定派生实例上的约束.您还需要启用UndecidableInstances以支持您实际需要的约束.

{-# LANGUAGE StandaloneDeriving, UndecidableInstances #-}

deriving instance (Show r, Show (f (Free f r))) => Show (Free f r)