caml-prépa¶

A compiler for the OCaml you learn in preparatory class, that you can look inside.

What is this?¶

It is a real compiler for the subset of OCaml taught in French preparatory classes (CPGE, the two-year courses leading to the engineering-school entrance exams): let rec, pattern matching, lists, arrays, records, sum types, references, loops, exceptions.

You can write a programme in your browser and run it. That part is unremarkable. Plenty of sites do that.

It also shows you its working. Type a programme and it will tell you, for the same code, at the same time:

what it prints
what type it worked out for every definition, without being told
which definition each name refers to, laid out as a tree of scopes
what Python it would compile to
how it read what you wrote, printed back out

Most compilers are a black box: source goes in, a result comes out, and if you are lucky an error message. This one lets you watch.

What is in it for me?¶

If you are learning OCaml, the two things that trip everybody up are types and scope, and this shows you both directly rather than through an error message.

You wrote a function and OCaml says it has some type you did not expect? Ask what type it inferred, and for which sub-expression. You are not sure which x the x on line 12 refers to? The scope tree says so:

module    top              {sum}
  binding                    {l}
    case                       {}
    case                       {rest, t}

That says: the file defines sum, l exists inside it, and rest and t exist inside the second match arm and nowhere else.

If you are curious how a compiler works, this one is small enough to read in an afternoon: about 3,500 lines of Python, laid out in the three parts a compiler course names. A front end turns text into a tree, a middle end works out what the tree means, and a back end runs it. The second tutorial walks through all three by following one small programme.

If you can read Python, you can read this compiler.

If you just want to try OCaml without installing anything, the playground needs no account and no install, and works offline once it has loaded. Nothing you type leaves your browser.

What it is not¶

It is not OCaml. It is a teaching compiler for a teaching subset, and it says so where it differs: int is unbounded here and 63-bit in the real thing, there is no Printf yet, and a very deep recursion will run out of stack where OCaml would not. The reference card lists the lot.

It is not an editor. It will not complete your code, colour it, or underline your mistakes: it is a text box and five answers.

If you want the real thing for coursework, install OCaml. This is for seeing how it works.

Where to go¶

Getting started Run the playground, install it locally, use it from the command line.
A tour of the language What you can write, from let to pattern matching, for someone who knows a little Python.
Using the playground What each of the five views is telling you, and what the error messages mean.
How the compiler works Three parts, one small programme, followed all the way through.

caml-prépa is a worked example built on astero, a library for deriving a compiler's handling of names from a declaration. It is open source under Apache-2.0.