RE: [Caml-list] Code-execution (evaluation) time

[David Allsopp <dra-news@metastack.com>]

Oliver Bandel wrote:
> Hello,
> 
> I want to refresh my OCaml knowledge after I (again) paused for a while.
> 
> I want to know, when certain parts of the code are executed,
> for example if I have more than one   let _ = ...
> statement in my code (for example in different comp0ilaion units each one
> of them).

Expressions (as in, things listed in http://caml.inria.fr/pub/docs/manual-ocaml/expr.html) are evaluated in the order in which they appear in a source file. If you have two files A.ml and B.ml which contain top-level statements, the order in which they execute will depend on the order in which you link the resulting .cmo or .cmx files (which should be left-to-right as passed to ocamlc/ocamlopt - don't know where that's specified).

As far as I can see, in pretty much all contexts within an expression where order of evaluation could matter including, but probably not limited to:

* Record initialisation (e.g. let i = ref 0 in {a = (incr i; !i); b = (incr i; !i)})
* Function application (similarly)
* let foo = _ and bar = _ in _

are intentionally left as unspecified (in practice, application and record initialisation are right-to-left and let x = _ and y = _ is left-to-right but the point is that you are never supposed to rely on that).

> I also found an old code snippet with the following parts:
> 
> ==============================
> let fnames = List.tl(Array.to_list Sys.argv)
> 
> (* some more code *)
> 
> let _ =
>    List.iter print_days_old fnames
> ==============================
> 
> So, the value fnames contains the args from the command line and can be
> used in the first statement of the code.

[let fnames = ..] is a complete expression so it's evaluated before the [let _] further down.


David