OCaml:另一个函数中的调用函数 [英] OCaml: Call function within another function

问题描述

在一个模块中，我有一个用户定义的类型和一个返回字符串的递归函数.然后，我想创建一个函数，该函数将创建该类型的对象并将其传递给该函数.这是我拥有的代码的一个简单示例:

In one module I have a user defined type and a recursive function that returns a string. Then I want to create a function that will create an object of that type and pass it to the function. Here is a simple example of the code I have:

type species =
    Animal of string
    | Mammal of species
    | Fish of species

let rec species_to_string = function
    | Animal (x) -> x
    | Mammal (x) -> "Mammal (" ^ (species_to_string x) ^ ")"
    | Fish (x) -> "Fish (" ^ (species_to_string x) ^ ")"

let process () =    
    let dog = Mammal(Animal("Dog"))
    let dogstring = species_to_string dog
    print_string dogstring

但是，当我尝试对此进行编译时，会收到错误消息:

However when I try to compile this, I receive the error:

File "test.ml", line 13, characters 1-4:
Error: Syntax error

在上面的示例中，第13行是倒数第二行.

where line 13 is the second last line in my example above.

我的代码似乎不是问题.当我将代码更改为此:

My code doesn't seem to be the issue. When I change the code to this:

type species =
    Animal of string
    | Mammal of species
    | Fish of species;;

let rec species_to_string = function
    | Animal (x) -> x
    | Mammal (x) -> "Mammal (" ^ (species_to_string x) ^ ")"
    | Fish (x) -> "Fish (" ^ (species_to_string x) ^ ")";;

let dog = Mammal(Animal("Dog"));;
let dogstring = species_to_string dog;;
print_string dogstring;;

它可以编译并正确运行.但是我需要将最后三行放在一个函数中，以便可以被另一个模块调用.我在做什么错了?

it compiles and runs correctly. But I need to put the last 3 lines in a function so it can be called by another module. What am I doing wrong?

推荐答案

您需要说

let dog = Mammal(Animal("Dog")) in
let dogstring = species_to_string dog in
print_string dogstring

也就是说，您需要使用关键字in.

That is, you need to use the keyword in.

更长的解释:OCaml中let有两种不同的用法.在模块的顶层，它定义了模块的内容.您对species_to_string和process的定义就是这种情况.在这些情况下，它似乎没有in.

Longer explanation: there are two different uses of let in OCaml. At the top level of a module it defines the contents of the module. This is the case for your definitions of species_to_string and process. In these cases it appears without in.

在所有其他情况下(在最外面的定义内)，唯一允许的形式是let var = expr in expr.也就是说，in关键字是必需的.

In all other cases (inside the outermost definitions) the only allowed form is let var = expr in expr. I.e., the in keyword is required.

let有两种不同用途，这令人困惑，这是毫无疑问的.但是一旦习惯了就可以了.

Having two different uses for let is confusing, there's no question. But once you get used to it it's OK.

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