简单的 OCaml 练习

发布于 2024-12-09 20:00:29 字数 269 浏览 0 评论 0原文

我试图通过 Jason Hickey 笔记自学 OCaml，下面的练习让我难住了。问题：编写一个函数 sum，给定两个整数边界 m,n 和一个函数 f 计算总和。我正在尝试这个：

     let rec sum m n f=
     if m>n then 0
     else if m=n then f n
     else f m + sum n m+1 f

但它不起作用，产生类型错误。

原文

I'm trying to teach myself OCaml through Jason Hickey notes and the following exercise got me stumped.
Question: Write a function sum that given two integer bounds m,n and a function f computes a summation.
I'm trying this:

     let rec sum m n f=
     if m>n then 0
     else if m=n then f n
     else f m + sum n m+1 f

but it doesn't work, producing a type error.

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凉城 2024-12-16 20:00:29

你需要一些括号。

let rec sum m n f=
     if m>n then 0
     else if m=n then f n
     else f m + sum n (m+1) f

（尽管为了可读性，我通常会将最后一行括起来，如 else (fm) + (sum n (m+1) f)。）
没有括号时发生的情况是，它将其视为 (fm) + (sum nm) + (1 f) ，这会产生 sum n m 没有的错误类型 int，因为它是具有更复杂类型的偏函数应用程序。

作为一般规则，当表达式作为参数传递给函数时，它总是需要加上括号。与此相关的是，如果您实际上想要将 plus 函数作为参数传递，则可以将其放在括号中（例如： sum mn (+) （尽管这不会键入 check in在这种情况下，因为 + 需要两个数字））。

You need some parentheses.

let rec sum m n f=
     if m>n then 0
     else if m=n then f n
     else f m + sum n (m+1) f

(Although for readability, I would usually parenthesize the last line as else (f m) + (sum n (m+1) f). )
What's happening without the parentheses is that it's treating it as (f m) + (sum n m) + (1 f) which is producing the error that sum n m doesn't have type int, as it's a partial function application with a more complex type.

As a general rule, when an expression is being passed as an argument to a function, it always needs to be parenthesized. On a related note, if you ever actually wanted to pass the plus function as an argument, you would put it in parentheses (for example: sum m n (+) (although that wouldn't type check in this case, since + expects two numbers)).

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