OCaml: Declaring a function before defining it

Question

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?

Answer 1

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:

let rec myFunctionA = 
(* some stuff here..... *) myFunctionB (*some stuff *)

and myFunctionB = 
(* some stuff here .... *) myFunctionA (* some stuff *)

Answer 2

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.