SML：带有常量的模式？

Question

我正在尝试在 SML 中编写模式匹配函数，但出现此错误

Error: non-constructor applied to argument in pattern: -

我正在尝试做这样的事情：

fun illegal (c:char) i j n m =
    let
        fun legal (#"A") 0 j = true
           |legal (#"B") i 0 = true
           |legal (#"C") i n = true
           |legal (#"D") m j = true
           | legal c  i j     = false
     in
        if (legal c i j = false) then 0
        else i
     end

**I suspect that the problem is that my n,m are two constants that have been valued right before. Any help appreciated
(I searched online and here I added parentheses in my chars, and tried some other stuff but my error persisted)** 

Answer 1

当您进行模式匹配时，像

i

这样的模式不会检查所绑定的表达式是否与现有

i

绑定具有相同的值，而是引入了一个新的

i

绑定到新表达式，这会遮蔽先前定义的绑定。

当你有：

legal (#"A") 0 j = true

您真正在寻找的是这样的东西：

legal (#"A") 0 j2 = j = j2