如何通过文字进行模式匹配，同时为其分配变量

Question

我如何将这两者结合起来，以便能够进行模式匹配，同时在变量中具有匹配的文字？

fun1 :: Int -> String
fun1 1 = -- ..... how to bind 1 to a variable in the function declaration?
fun1 55 = -- ..... how to bind 55 to a variable in the function declaration?
fun1 123 = -- ..... how to bind 123 to a variable in the function declaration?
fun1 a = -- ...........   all is OK

Answer 1

首先绑定a，然后对其进行模式匹配，因此它在所有分支的范围内。

fun a = case a of
1 -> ...
55 -> ...
123 -> ...
_ -> ...

或者用作图案。

fun a@1 = ...
fun a@55 = ...
fun a@123 = ...
fun a = ...

另请参见示例https://www.haskell.org/tutorial/patterns.html