F# and tuple output

Question

Over at http://diditwith.net, I see that, in F#, it isn't strictly necessary to pass out parameters to a function that otherwise requires them. The language will auto-magically stuff the result and the output parameter into a tuple. (!)

Is this some kind of side effect (pardon the pun) of the general mechanics of the language, or a feature that was specifically articulated in the F# specification and deliberately programmed into the language?

It's an awesome feature, and if it was expressly put into F#, then I'm wondering what other nuggets of gold like this are lurking within the language, because I've pored over dozens of web pages and read through three books (by D. Syme, T. Petricek, and C. Smith) and I hadn't seen this particular trick mentioned at all.

EDIT: As Mr. Petricek has responded, below, he does mention the feature in at least two places in his book, Real-World Functional Programming. My bad.

Answer 1

This is not a side-effect of some other, more general, mechanism in the F# language.

It has been added specifically for this purpose. .NET libraries often return multiple values by adding out (or ref) parameters at the end of the method signature. In F#, returning multiple values is done by returning tuple, so it makes sense to turn the .NET style into the typical F# pattern.

I don't think F# does many similar tricks, especially when it comes to interoperability, but you can browse through some of the handy snippets here and here.

(I quickly checked and Real-World Functional Programming mentions the trick briefly on pages 88 and 111.)