I thought this was a pretty great introduction. Part 2, in particular, finally helped me understand function type signatures in ML languages: the fact that, under the hood, functions only accept a single argument, and the way the type signatures are presented are merely an aesthetic cleanup of what it actually looks like (which would include more parentheses to indicate associativity).

One of the examples did refuse to compile for me, and I felt Part 1 was still too light on basic syntax. I tried to do the exercise at the end of Part 1, about prompting for, squaring, and printing an integer. This works:

  let this_integer = read_int ()
  let _ = print_int (this_integer * this_integer); print_newline ()

But this doesn't:

  let this_integer = read_int () in
    let _ = print_int (this_integer * this_integer); print_newline ()

And nothing in the tutorial explains why that should be so -- at least not that I could find.

I've also yet to see a really good explanation of when you need to use parentheses for grouping and when you don't -- this is the only part of the language that really feels like a "syntax error" to me. Like, "Look, ocaml is clean and doesn't need to use all this extraneous syntax! Except when it does."

Anyway, good introduction.