模式匹配Haskell中的case语句中的变量 [英] Pattern matching variables in a case statement in Haskell

问题描述

如果我使用case语句将一个字符串文字与一个字符串文字进行比较，我会得到预期的行为：如果他们是相同的 - 它匹配，如果不是 - 它不。

If I compare a string literal to a string literal using the case statement, I get the expected behavior: if they are the same - it matches, if they are not - it does not.

但是，如果我把一个字符串字符串和一个字符串的常量比较，我得到模式匹配重叠警告和分支与常量总是匹配。

However, if I compare a string literal to a constant that is a string, I get "Pattern matches are overlapped" warning and the branch with the constant always matches.

这是一个会话示例：

Prelude> let var1 = "abc"
Prelude> let var2 = "def"
Prelude> case var1 of { var2 -> "Fail"; _ -> "Win" }

<interactive>:1:0:
    Warning: Pattern match(es) are overlapped
             In a case alternative: _ -> ...
"Fail"
Prelude> case "abc" of { var2 -> "Fail"; _ -> "Win" }

<interactive>:1:0:
    Warning: Pattern match(es) are overlapped
             In a case alternative: _ -> ...
"Fail"
Prelude> case "abc" of { "def" -> "Fail"; _ -> "Win" }
"Win"

>

Meanwhile, if behaves as expected:

> Prelude> if var1 == var2 then "Fail" else "Win" 
"Win"

在这里？

推荐答案

请参阅Don的回答为什么。执行你想要做的一个常见的做法是：

See Don's answer for why. A common idiom for doing what you are trying to do is this:

var1 = "abc"
var2 = "def"

foo x = case () of
    () | x == var1 -> "Fail"
       | x == var2 -> "Failzor"
       | otherwise -> "WIN"

当然在这种情况下，我们会失去 code>并直接写上保护程序的函数：

Of course in this case we would lose the case and just write the guards directly on the function:

foo x | x == var1 = "Fail"
      | ...

UPDATE

这些天， MultiWayIf 扩展会稍微减少语法噪音。

These days the MultiWayIf extension does this with slightly less syntactic noise.

{-# LANGUAGE MultiWayIf #-}

foo x = if | x == var1 -> "Fail"
           | x == var2 -> "Failzor"
           | otherwise -> "WIN"

这篇关于模式匹配Haskell中的case语句中的变量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IT屋！