haskell - 无法将类型 ‘[Char]’ 与 ‘Data.Text.Internal.Text’ 匹配

Question

这似乎是一个热门问题。我对 Haskell 一点都不熟悉，所以通过阅读类似问题的答案，我无法真正理解在我的案例中需要做什么。

我的脚本是这样的:

import Text.Pandoc.JSON

pagebreakXml :: String
pagebreakXml = "<w:p><w:r><w:br w:type=\"page\"/></w:r></w:p>"

pagebreakBlock :: Block
pagebreakBlock = RawBlock (Format "openxml") pagebreakXml

blockSwapper :: Block -> Block
blockSwapper (Para [Str "<div class=\"docxPageBreak\"></div>"])  = pagebreakBlock
blockSwapper blk = blk

main = toJSONFilter blockSwapper

编译它会导致这些错误:

$ ghc --make docx-page-filter.hs -package-db=/Users/eugene/.cabal/store/ghc-8.8.3/package.db
[1 of 1] Compiling Main             ( docx-page-filter.hs, docx-page-filter.o )

docx-page-filter.hs:7:35: error:
    • Couldn't match expected type ‘Data.Text.Internal.Text’
                  with actual type ‘[Char]’
    • In the first argument of ‘Format’, namely ‘"openxml"’
      In the first argument of ‘RawBlock’, namely ‘(Format "openxml")’
      In the expression: RawBlock (Format "openxml") pagebreakXml
  |
7 | pagebreakBlock = RawBlock (Format "openxml") pagebreakXml
  |                                   ^^^^^^^^^

docx-page-filter.hs:7:46: error:
    • Couldn't match type ‘[Char]’ with ‘Data.Text.Internal.Text’
      Expected type: Data.Text.Internal.Text
        Actual type: String
    • In the second argument of ‘RawBlock’, namely ‘pagebreakXml’
      In the expression: RawBlock (Format "openxml") pagebreakXml
      In an equation for ‘pagebreakBlock’:
          pagebreakBlock = RawBlock (Format "openxml") pagebreakXml
  |
7 | pagebreakBlock = RawBlock (Format "openxml") pagebreakXml
  |                                              ^^^^^^^^^^^^

docx-page-filter.hs:10:25: error:
    • Couldn't match expected type ‘Data.Text.Internal.Text’
                  with actual type ‘[Char]’
    • In the pattern: "<div class=\"docxPageBreak\"></div>"
      In the pattern: Str "<div class=\"docxPageBreak\"></div>"
      In the pattern: [Str "<div class=\"docxPageBreak\"></div>"]
   |
10 | blockSwapper (Para [Str "<div class=\"docxPageBreak\"></div>"])  = pagebreakBlock
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
$

起初，我接受了这里建议的可接受答案:Couldn't match expected type `Text' with actual type `[Char]' .它有助于将错误数量减少到一个错误。现在编译脚本会导致这个错误，我不知道如何修复:

$ ghc --make docx-page-filter.hs -package-db=/Users/eugene/.cabal/store/ghc-8.8.3/package.db -XOverloadedStrings
[1 of 1] Compiling Main             ( docx-page-filter.hs, docx-page-filter.o )

docx-page-filter.hs:7:46: error:
    • Couldn't match type ‘[Char]’ with ‘Data.Text.Internal.Text’
      Expected type: Data.Text.Internal.Text
        Actual type: String
    • In the second argument of ‘RawBlock’, namely ‘pagebreakXml’
      In the expression: RawBlock (Format "openxml") pagebreakXml
      In an equation for ‘pagebreakBlock’:
          pagebreakBlock = RawBlock (Format "openxml") pagebreakXml
  |
7 | pagebreakBlock = RawBlock (Format "openxml") pagebreakXml
  |                                              ^^^^^^^^^^^^
$

我应该如何更改我的代码以解决此问题？

Answer 1

在 Haskell 中有多种常用的字符串类型。默认情况下，字符串文字("stuff in quotes")的类型为 String(即 [Char])，但是您的库正在使用的期望值类型为 Text。

• 启用 OverloadedStrings 扩展，这样字符串文字也可以具有类型 Text

• 将类型签名从 String 更改为 Text； Text 可以从 text 库中的 Data.Text 模块导入(还值得一提的是，有两个 Text，另一个来自 Data.Text.Lazy，这可能是您将来不匹配的另一个来源)。

{-# LANGUAGE OverloadedStrings #-}   -- Add at the top of the file

...  -- imports
import Data.Text (Text)              -- Import the Text type

pagebreakXml :: Text                 -- from String to Text
pagebreakXml = "<w:p>..."