When do you put double semicolons in F#?

Question

This is a stupid question. I\'ve been reading a couple books on F# and can\'t find anything that explains when you put ;; after a statement, nor can I find a pattern in the

Answer 1

I suspect that you have seen F# code written when #light syntax wasn't enabled by default (#light syntax is on by default for the May 2009 CTP and later ones as well as for Visual Studio 2010) and then ;; means the end of a function declaration.

So what is #light syntax? It comes with the #light declaration:

The #light declaration makes whitespace significant. Allowing the developer to omit certain keywords such as in, ;, ;;, begin, and end.

Here's a code written without #light syntax:

let halfWay a b =
  let dif =  b - a in
  let mid = dif / 2 in
  mid + a;;

and becomes with light syntax:

#light
let halfWay a b =
  let dif =  b - a
  let mid = dif / 2
  mid + a

As said you can omit the #light declaration now (which should be the case if you're on a recent CTP or Visual Studio 2010).

See also this thread if you want know more on the #light syntax: F# - Should I learn with or without #light?