问题描述
这里有Haskell新手.
Haskell newbie here.
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.1
当尝试调试第三方Haskell程序中与语言环境相关的怪异错误时，我正在尝试打印默认编码:
While trying to debug weird locale-related bug in third-party Haskell program, I'm trying to print default encoding:
import System.IO
main = do
print localeEncoding
但是失败了:
$ ghc -o printlocale main.hs
main.hs:4:2:
No instance for (Show TextEncoding)
arising from a use of `print' at main.hs:4:2-21
Possible fix: add an instance declaration for (Show TextEncoding)
In the expression: print localeEncoding
In the expression: do { print localeEncoding }
In the definition of `main': main = do { print localeEncoding }
我的google-fu让我失望.我想念什么?
My google-fu is failing me. What am I missing?
推荐答案
要在Haskell中打印某种类型的值，该类型必须是Show类的实例.
To print a value of some type in Haskell, the type must be an instance of the Show class.
localeEncoding :: TextEncoding
并且TextEncoding不是Show的实例.
and TextEncoding is not an instance of Show.
TextEncoding类型实际上是一种存储类型，用于存储编码和解码的方法:
The TextEncoding type is actually an existential type storing the methods for encoding and decoding:
data TextEncoding
= forall dstate estate . TextEncoding {
mkTextDecoder :: IO (TextDecoder dstate),
mkTextEncoder :: IO (TextEncoder estate)
}
由于这些是函数，因此没有明智的方式来显示它们.当前的localeEncoding是通过iconv通过C函数nl_langinfo确定的.
Since these are functions, there's no sensible way to show them. The current localeEncoding is determined using iconv, via the C function nl_langinfo.
因此，TextEncoding本身不是可显示的类型，因此您无法打印它.但是，您可以通过mkTextEncoding构造此类型的新值.例如.创建一个utf8环境:
So, TextEncoding as such is not a showable type, so you cannot print it. However, you can construct new values of this type, via mkTextEncoding. E.g. to create a utf8 environment:
mkTextEncoding "UTF-8"
我们可能会考虑一个功能请求，以使用TextEncoding存储语言环境的表示形式，因此可以打印此标签.但是，目前这是不可能的.
We might consider a feature request to store the representation of the locale with the TextEncoding, so this label could be printed. However, that's currently not possible.
这篇关于Haskell:打印TextEncoding的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！