问使用注释和漂亮的打印机

EN

{-# LANGUAGE OverloadedStrings #-}

import Data.Text.Prettyprint.Doc

data Foo = X | Y
data Ann = AnnX | AnnY

instance Pretty Foo where
  pretty X = "[" <+> (annotate AnnX "X was here") <+> "]"
  pretty Y = "nothing"

/Example.hs:14:14: error:
    • Couldn't match type ‘ann’ with ‘Ann’
      ‘ann’ is a rigid type variable bound by
        the type signature for:
          pretty :: forall ann. Foo -> Doc ann
        at /Example.hs:14:3-8
      Expected type: Doc ann
        Actual type: Doc Ann
    • In the expression: "[" <+> (annotate AnnX "X was here") <+> "]"
      In an equation for ‘pretty’:
          pretty X = "[" <+> (annotate AnnX "X was here") <+> "]"
      In the instance declaration for ‘Pretty Foo’
    • Relevant bindings include
        pretty :: Foo -> Doc ann
          (bound at /Example.hs:14:3)
   |
14 |   pretty X = "[" <+> (annotate AnnX "X was here") <+> "]"
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^