比较OCaml中的两个整数列表

Question

我想比较两个整数列表。我从模式匹配开始，但是嵌套匹配有问题，所以尝试了另一种方法。

我收到警告说模式匹配并不是详尽无遗的，它说列表可以是空的。这很奇怪，因为我一开始就检查过了。

let rec cmp3 l1 l2 = 
    if l1 = [] && l2 = [] then 0 
    else if l1 = [] then -1
    else if l2 = [] then 1 else 
    let (h::t) = l1 and (hh::tt) = l2 in
    if h > hh then 1 
    else if hh > h then -1 
    else cmp3 t tt;;
              Characters 125-131:
      let (h::t) = l1 and (hh::tt) = l2 in
          ^^^^^^
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
[]
Characters 141-149:
      let (h::t) = l1 and (hh::tt) = l2 in
                          ^^^^^^^^
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
[]
val cmp3 : 'a list -> 'a list -> int = <fun>

Answer 1

编译器不能假设这两个列表都有相同的长度--这就是它发出警告的原因。如果您确信列表的长度总是相同的，那么就由您来放弃此警告--但这并不是编写程序的安全方法。

而且你有很多如果，它更好地使用匹配，它是更可读的。例如：

let rec cmp l ll = 
match (l,ll) with
| [], [] -> 0
| [],_ -> -1
| _,[] -> 1
| (h::t), (hh::tt) -> if h > hh then 1
                      else if h < hh then -1 
                      else cmp t tt;;