使用只有一个元组值的变体类型构造函数 [英] Using a variant type constructor with just one tuple value

问题描述

# type foo = Foo of int * int
# let t = (1, 2)
# Foo t
Error: The constructor Foo expects 2 argument(s),
   but is applied here to 1 argument(s)

我必须做 Foo（1，2）才能避免这个错误，即使 t 有合适的类型吗？

How is it that I must do Foo (1, 2) to avoid that error even t has the appropriate type?

推荐答案

在我看来，这是OCaml语法的一个麻烦的部分。尽管它的外观，构造函数Foo不需要2元组作为其参数。它在语法上需要括号中的两个值 - 但它们不是元组。因此， t 的类型错误。这个工作的方式是：

This is one of the troubling parts of OCaml syntax, in my opinion. Despite the way it looks, the constructor Foo doesn't require a 2-tuple as its argument. It requires, syntactically, two values in parentheses--but they aren't a tuple. So it's simply the case that t has the wrong type. The way to make this work is to say:

let (a, b) = t in Foo (a, b)

问题真的是括号用于两个不同的东西一旦你习惯了这一点，它并不难处理。

The problem really is that parentheses are being used for two different things (or so I claim). Once you get used to this it's not so difficult to deal with.

编辑：如果你 Foo取单个元组，而不是两个单独的值，你可以这样定义：

Edit: if you want the constructor Foo to take a single tuple, rather than two separate values, you can define it like this:

type foo = Foo of (int * int)

然后，其余的原始代码将工作。

Then the rest of your original code will work.

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