实现二进制搜索时类型错误

Question

我有以下代码：

open Array

let binSearch a x =
    let l = ref 0 in
    let r = ref (length a) in
    let m = ref 0 in
    while l < r do
        !m := (!l + !r) / 2; (*line 8*)
        if (a.(!m) = x) then
        begin
            !r := !m;
            !l := !r
        end
        else if (a.(!m) > x) then
            !p := !m
        else
            !l := !m + 1
    done;
    a.(!l) = x
;;

当我试图编译它时，我会得到以下错误：

File "binSearch.ml", line 8, characters 2-4:
Error: This expression has type int but an expression was expected of type
     'a ref

为什么编译器期望'a ref

Answer 1

您不需要将!放在:=操作符的左侧。操作符:=有'a ref -> 'a -> unit类型，换句话说，在左侧应该有对'a的引用，在右边应该有对'a的引用。因此，与其：

 !r := !m (* WRONG *)

你需要

 r := !m