[erlang-questions] How expensive are guard clauses?

[Roger Lipscombe]

I'm considering adding more-expressive guard clauses to some of my
functions, in order to prevent garbage data getting past certain
points in my application.

If I have a macro like something the following...

    -define(is_valid_id(Id),
        (is_binary(Id) andalso byte_size(Id) =:= 16 andalso Id =/=
<<"0000000000000000">>)).

...and I use it in the following way...

    frobnicate(StudentId, CourseId) when ?is_valid_id(StudentId),
?is_valid_id(CourseId) ->
        ...

...just how expensive, in terms of timing, is that? Is it something I
need to worry about? Can I scatter these guards everywhere? Should I
restrict them to a few places? Should I make it so that they can be
turned off in production (through a compiler define)?

Obviously, I'll be looking to actually measure it, but I'm wondering
if anyone's tried this and got anything to suggest.