OCaml：在定义函数之前声明它
有没有办法在 OCaml 中定义函数之前先声明它？我正在使用 OCaml 解释器。
我有两个函数：
let myFunctionA =
(* some stuff here..... *) myFunctionB (*some stuff *)
let myFunctionB =
(* some stuff here .... *) myFunctionA (* some stuff *)
但这不起作用，因为 myFunctionA 在创建之前无法调用 myFunctionB。
我已经进行了一些谷歌搜索，但似乎找不到任何东西。我怎样才能做到这一点？
Is there a way to declare a function before defining it in OCaml? I'm using an OCaml interpreter.
I have two functions:
let myFunctionA =
(* some stuff here..... *) myFunctionB (*some stuff *)
let myFunctionB =
(* some stuff here .... *) myFunctionA (* some stuff *)
This doesn't work though, since myFunctionA can't call myFunctionB before it's made.
I've done a few google searches but can't seem to find anything. How can I accomplish this?
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您想要的是使这两个函数相互递归。您必须使用“let ... and ...”，而不是使用“let ... let ...”，如下所示：
What you want is to make these two functions mutually recursive. Instead of using "let ... let ...", you have to use "let rec ... and ..." as follows:
实际上“let rec ..”有一个非常严重的限制：它只能在单个模块中工作。这迫使程序员在不需要的地方编写大模块......这个问题在低级 C 中不会发生！
有几种解决方法，但都不能令人满意。第一种方法是创建一个函数类型的变量，并最初在其中存储引发异常的函数，然后存储所需的值。
第二种是使用类类型和类（以及一种间接寻址）。如果您有很多相互递归的函数，这是最好的方法（因为您只需要向每个函数传递一个对象）。
最简单也是最丑陋的是将函数作为参数相互传递，这种解决方案很快就会失控。在遵循所有定义的模块中，您可以通过引入一组“let rec”包装器来简化调用代码。不幸的是，这无助于定义函数，并且通常大多数调用都会发生在此类定义中。
Actually "let rec .." has a very serious limitation: it only works within a single module. This forces the programmer to write big modules where it is not desired .. a problem which does not occur in lowly C!
There are several workarounds, all unsatisfactory. The first is to make a variable of the function type and initially store a function raising an exception in it, then later store the desired value.
The second is to use class types and classes (and one indirection). If you have a lot of mutually recursive functions this is the best way (because you only need to pass a single object to each of them).
The easiest and most ugly is to pass the functions to each other as arguments, a solution which rapidly gets out of control. In a module following all the definitions you can simplify the calling code by introducing a set of "let rec" wrappers. Unfortunately, this does not help defining the functions, and it is common that most of the calls will occur in such definitions.