如何获取文字类型? [英] How to get the text type?

问题描述

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 类型的实例，如下所示:

And in REPL, I tried to get an instance of Text type as the following:

:t something ("hello" :: Text) 

编译器抱怨:

<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:

By default, it will take the String type :

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

如何获取 Text 类型而不是 String 类型?

How to get Text type instead of String type?

推荐答案

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:

There is nothing problematic with your code. Doing something like this itself would result in such an error:

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

但是您可以使用 String :

λ> let t = "hello world" :: String

请注意，即使文件中具有 OverloadedString 扩展名，当您在repl中加载该文件时也不会加载该文件.您可以看到当前正在加载的扩展名，如下所示:

Note that even though your file has OverloadedString extension in it, it won't be loaded when you load that file in the repl. You can see the currently loaded extensions like this:

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

您可以使用 OverloadedStrings 扩展名将其注释为 Text 甚至是 ByteString .

You can use the OverloadedStrings extension to annotate it to Text or even 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中设置上述扩展名后，您的代码将起作用:

With the above extension set in repl, your code will work:

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

OverloadedStrings扩展增加了对重载字符串的支持.您可以在此处.简短说明:通过将类型定义为 IsString 类型类的实例，可以通过String文字来表示它:

The OverloadedStrings extension adds support for overloaded strings. You can find more detabils about it here. Short explanation of it: By definining your type to be an instance of IsString typeclass, you can denote it via String literal:

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

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