Haskell中的常量和模式匹配

Question

如何在Haskell中定义宏常量？特别是,我想在没有第二个模式匹配的情况下运行以下代码片段重叠.

someconstant :: Int
someconstant = 3

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

Answer 1

您可以定义模式同义词:

{-# LANGUAGE PatternSynonyms #-}

pattern SomeConstant :: Int
pattern SomeConstant = 3

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

但也要考虑在自定义变体类型上匹配是否更好Int.