Haskell 中的常量和模式匹配

Question

如何在Haskell中定义宏常量？特别是，我希望以下代码段 运行 没有第二个模式匹配被重叠。

someconstant :: Int
someconstant = 3

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

Answer 1

你可以定义一个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"

但也要考虑匹配自定义变体类型而不是 Int 是否更好。