The big picture

What is Objective Caml?

Objective Caml is an implementation of the Caml dialect of ML extended with class-based objects and a powerful module system in the style of SML's. It continues the Caml Light effort in many ways (separate compilation, low memory requirements, fast turnaround through bytecode interpretation), but also significantly departs from Caml Light on other points (objects, the module calculus, and a high-performance native-code compiler).

Objective Caml continues an earlier implementation of Caml plus modules, known as Caml Special Light. The only difference between Objective Caml and Caml Special Light is the addition of objects.

Should I switch from Caml Light to Objective Caml?

Caml Light is still maintained, though no longer actively developed. We make bug-fix releases from time to time and still provide support for it. It provides a more stable programming environment than Objective Caml, which is still evolving, though at a slower pace than immediately after its introduction.

If you're perfectly happy with Caml Light, value reliability more than performance, don't need the new module system, don't need the new object stuff, then stay with Caml Light. If you need objects, powerful modules, the native-code compiler, or just like being on the cutting edge, go for Objective Caml.

Objective Caml still does not conform to Standard ML, does it?

Of course not. We are supposed to do research in programming language design and implementation. Implementing a standard is not research. The ultimate aim of this whole effort is to make a language even better than Standard ML. For instance, Objective Caml is today the only ML dialect that supports object-oriented programming.

How do Objective Caml modules compare with Standard ML's?

From the user's point of view, both look pretty much the same and offer similar expressive power. The theoretical foundations are completely different, though. The main advantage of the Objective Caml system is that it blends gracefully with Modula-2 style separate compilation, while Standard ML modules provide rather poor support for separate compilation. (See the papers for more details.)

Compared with Standard ML proper, O'Caml's modules have in addition signatures and functors as structure components, and fully transparent higher-order functors in the style of New Jersey ML, though again relying on a completely different theoretical approach.

How hard is it to convert code from Objective Caml 2.0 to Objective Caml 3.0?

Very easy. The only change required is in stream patterns (change the ? operator to ??). With this exception, all new features of 3.0 (labels, optional arguments, polymorphic variants) introduce no incompatibilities with 2.0.

How hard is it to convert code from Objective Caml 1.0 to Objective Caml 2.0?

If your code doesn't use classes and objects, almost no changes are required. There are a couple of extra reserved keywords, and minor type changes in e.g. the Unix library.

Class definitions need to be reworked to conform to the new syntax. A semi-automatic translator is available and automates most of the translation work.

The O'Caml distribution contains a script (in tools/convert) that performs automatically the renaming of library functions and warns for usages of now-reserved keywords.

How hard is it to convert code from Caml Special Light to Objective Caml?

Conversion to Objective Caml is extremely easy, Objective Caml being almost 100% upwards compatible with Caml Special Light. The only difference is that Objective Caml has some additional reserved keywords such as new and class, which forced some library functions to be renamed.

The O'Caml distribution contains a script (in tools/convert) that performs automatically the renaming of library functions and warns for usages of now-reserved keywords.

How hard is it to convert code from Caml Light to Objective Caml?

Here is a list of the main differences:
  1. Capitalization of identifiers matters. Value names must be in lowercase, constructors and exceptions capitalized, record labels in lowercase, type constructors in lowercase.

  2. Types and exceptions defined manifestly in a module interface (.mli file) must also be defined identically in the implementation (.ml file).

  3. Various minor syntactic changes, e.g. use of . instead of __ to extract module components, suppression of ;; in files, characters literals between single quotes '...' instead of backquotes `...`, etc.

  4. The standard library has been restructured, a number of functions have changed names.

An automatic translator from Caml Light to Caml Special Light and Objective Caml, written by Bruno Barras, is available here.

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Author: Xavier Leroy -- Last modified: 2003/05/09