Haskell中的非穷尽模式匹配 [英] Non-exhaustive pattern matching in Haskell

问题描述

我正在学习Haskell，想编写一个简单的程序，将每个字符串中的每个字母重复两次.

I am learning Haskell and wanted to write a simple program which just repeats each letter in a string twice.

我想到了这个

repl :: String -> String
repl " " = " "
repl (x:xs) = x:x:repl xs

编译时，我没有收到任何警告，但是当我执行repl "abcd"时发生了运行时错误:

When compiling, I didnt get any warning but a run-time error occurred when I did repl "abcd":

"abcd ***:例外:repl.hs:(2,1)-(3,23):函数repl中的非穷举模式

"abcd*** Exception: repl.hs:(2,1)-(3,23): Non-exhaustive patterns in function repl

为什么编译器从不报告此问题，为什么当有很多语言(如OCaml)在编译时明确报告此问题时，为什么它在Haskell中被忽略?

Why did the compiler never report this and why is it ignored in Haskell when there are many languages like OCaml which clearly report this at compile time?

推荐答案

模式匹配警告默认情况下处于关闭状态.您可以使用-fwarn-incomplete-patterns启用它，也可以使用-W和-Wall将其作为更大的警告包的一部分.

The pattern match warning is turned off by default. You can turn it on with the -fwarn-incomplete-patterns or as part of a larger bundle of warnings with -W and -Wall.

您可以从ghci执行此操作:

Prelude> :set -W

您还可以在编译或将其作为编译指示包含在模块顶部时，将标志传递给ghc:

You can also pass the flag to ghc when you compile or include it as a pragma on top of your module:

{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}

对于您的特定程序，它应发出以下警告:

For your specific program, it should give the following warning:

/home/tjelvis/Documents/so/incomplete-patterns.hs:2:1: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘repl’: Patterns not matched: []

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