Haskell中的常量和模式匹配 [英] Constants in Haskell and pattern matching

问题描述

在Haskell中如何定义一个宏常量？尤其是，我想让下面的代码片段在没有第二个模式匹配的情况下运行重叠。

How is it possible to define a macro constant in Haskell? Especially, I would like the following snippet to run without the second pattern match to be overlapped.

someconstant :: Int
someconstant = 3

f :: Int -> IO ()
f someconstant = putStrLn "Arg is 3"
f _            = putStrLn "Arg is not 3"

推荐答案

您可以定义模式同义词：

You can define a pattern synonym:

{-# LANGUAGE PatternSynonyms #-}

pattern SomeConstant :: Int
pattern SomeConstant = 3

f :: Int -> IO ()
f SomeConstant = putStrLn "Arg is 3"
f _            = putStrLn "Arg is not 3"

但是也要考虑在自定义变体类型上进行匹配而不是 Int 。

But also consider whether it's not better to match on a custom variant type instead of an Int.

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