Haskell: 导出修复类型的显示

Question

我正在尝试使用 recursion-schemes 实现递归数据类型。我希望能够打印出来。

import Data.Functor.Foldable

data T1F a = Foo deriving Show
type T1 = Fix T1F
data T2 = Bar T1 deriving Show -- error here

错误信息：

No instance for (Data.Functor.Classes.Show1 T1F)
  arising from the first field of ‘Bar’ (type ‘T1’)
Possible fix:
  use a standalone 'deriving instance' declaration,
    so you can specify the instance context yourself
When deriving the instance for (Show T2)

如何使 T1 派生 Show？

Answer 1

使用包 deriving-compat:

{-# LANGUAGE TemplateHaskell #-}
import Data.Functor.Foldable
import Text.Show.Deriving

data T1F a = Foo deriving Show
$(deriveShow1 ''T1F)
type T1 = Fix T1F
data T2 = Bar T1 deriving Show

Haskell: 导出修复类型的显示

Haskell: deriving Show for Fix types

haskell

typeclass

deriving

fixpoint-combinators