haskell - 使用注释和 pretty-print

Question

我正在使用 pretty-print Data.Text.Prettyprint.Doc 。现在我想以不同的方式设计我的输出，并认为语义注释的想法非常适合这里。考虑以下示例

{-# 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"

此操作失败并出现类似错误

/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") <+> "]"
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

我几乎不明白。我的目的是注释文档，后来我想为注释提供不同的驱动程序。例如，我想要一个映射文档 X was here 的驱动程序至<strong>X was here</strong>和另一位司机到 <i>X was here</i> 。也许我错误地理解了整个注释部分。有人可以启发我或为我提供示例链接吗？

Answer 1

Pretty 类型类需要可以与任何类型的注释一起使用的东西。就您而言，它们仅适用于您的类型 Ann。这样的方法必须是它自己的东西，因为它对 pretty 无效。