A function can be declared without having it followed by its implementation, by having it followed by the forward procedure. The effective implementation of that function must follow later in the module. The function can be used after a forward declaration as if it had been implemented already. The following is an example of a forward declaration.
A function can be forward declared only once. Likewise, in units, it is not allowed to have a forward declared function of a function that has been declared in the interface part. The interface declaration counts as a forward declaration. The following unit will give an error when compiled:
Reversely, functions declared in the interface section cannot be declared forward in the implementation section. Logically, since they already have been declared.