Objective Caml - Questions and Answers

Programming problems and weird error messages

Can I have two mutually recursive compilation units / structures / signatures / functors?

No, unfortunately. This would raise serious typing and compilation problems. Some workarounds are explained in the user manual.

How do I express sharing constraints?

Use manifest type specifications in the arguments of the functor. For instance, assume defined the following signatures:

module type S1 = sig ... type t ... end
module type S2 = sig ... type u ... end

To define a functor F that takes two arguments X:S1 and Y:S2 such that X.t and Y.u share, write:

module F (X: S1) (Y: S2 with type u = X.t) =
  struct ... end

Internally, this expands to

module F (X: S1) (Y: sig ... type u = X.t ... end) =
  struct ... end

On this form, it is clear that F can only be applied to structures A and B that meet the specifications, i.e. such that B.u is provably equivalent to A.t. Moreover, given the declaration of the functor argument Y, the body of the functor can assume that B.u = A.t. Therefore, this works exactly like a sharing constraint.

Compilation units are forced to be structure. What if I want to make a unit with a functor or a signature instead?

In Objective Caml, functors and signatures (module types) can be components of structures. So, just make the functor or signature be a component of a compilation unit. A good example is the module Set from the standard library.

It says `this expression has type t but is here used with type t`. I'm confused. Isn't this the same type?

It is the same type name, but not necessarily the same type. Two type constructors can have the same name, but actually represent different types. See the user manual.

What is this business about type variables that cannot be generalized?

The simplest solution to the long-standing problem of polymorphic references. See the user manual. A more detailed explanation (in the context of Caml Light 0.7) can be found in the Caml FAQ, and in the Caml mailing list.