variables - 如何通过文字进行模式匹配并同时为其分配变量？

Question

我如何将这两者结合起来，以便能够进行模式匹配，同时将匹配的文字放入变量中？

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(bind)a，然后对其进行模式匹配，使其在所有分支的范围内。

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

或者使用 as-patterns。

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

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