MiniML is a simple expression evaluator done in Caml Light for programming language courses back in university.
Since the original version was done as a university project, everything is in Portuguese.
Sometime ago I used it as a way to learn F# and in the process converted the comments and corresponding documentation to English.
Although everything is located on my web site, the source code of the F# version is also available on GitHub.
For more information, have a look at the manual.