haskell - 如何获取文本类型？

Question

我有以下代码

 {-# LANGUAGE OverloadedStrings, TypeSynonymInstances, FlexibleInstances #-}

module Lib where

import Data.Text (Text)

class DoSomething a where
  something :: a -> IO ()

instance DoSomething String where
  something _ = putStrLn "String"


instance DoSomething Text where
  something _ = putStrLn "Text" 

在 REPL 中，我尝试获取 Text 的实例。键入如下:

:t something ("hello" :: Text) 

编译器提示:

<interactive>:1:12: error:
    • Couldn't match expected type ‘Text’ with actual type ‘[Char]’
    • In the first argument of ‘something’, namely ‘("hello" :: Text)’
      In the expression: something ("hello" :: Text)

默认情况下，它将采用 String类型 :

:t something "hello"
something "hello" :: IO ()

如何获得 Text输入而不是 String类型？

Answer 1

您的代码没有任何问题。做这样的事情本身会导致这样的错误:

λ> import Data.Text
λ> let t = "hello world" :: Text

<interactive>:11:9: error:
    • Couldn't match expected type ‘Text’ with actual type ‘[Char]’
    • In the expression: "hello world" :: Text
      In an equation for ‘t’: t = "hello world" :: Text

但是你可以用 String 做到这一点:

λ> let t = "hello world" :: String

请注意，即使您的文件有 OverloadedString扩展名，当您在 repl 中加载该文件时，它不会被加载。您可以像这样看到当前加载的扩展:

λ> :show language
base language is: Haskell2010
with the following modifiers:
  -XNoDatatypeContexts
  -XNondecreasingIndentation

您可以使用 OverloadedStrings将其注释为 Text 的扩展名甚至 ByteString .

λ> :set -XOverloadedStrings
λ> :show language
base language is: Haskell2010
with the following modifiers:
  -XNoDatatypeContexts
  -XNondecreasingIndentation
  -XOverloadedStrings
λ> let t = "hello" :: Text
λ> import Data.ByteString
λ> let t = "hello" :: ByteString

在 repl 中设置上述扩展后，您的代码将起作用:

λ> :t something ("hello" :: Text) 
something ("hello" :: Text) :: IO ()
λ> something ("hello" :: Text) 
Text

OverloadedStrings 扩展添加了对重载字符串的支持。你可以找到更多关于它的细节 here .简短说明:通过将您的类型定义为 IsString 的实例typeclass，你可以通过字符串文字来表示它:

import GHC.Exts (IsString(..))

data MyFancyText =
  MyFancyText String
  deriving (Show, Eq, Ord)

instance IsString MyFancyText where
  fromString str = MyFancyText str

然后在 REPL 中:

λ> let xs = "hello" :: MyFancyText
λ> :t xs
xs :: MyFancyText