On Aug 7, 11:18 am, [EMAIL PROTECTED] (Fermin Galan) wrote:
> I'm trying to introduce conditional module loading in a Perl program,
> but I'm experiencing problems. For example:
>
> if ($some_condition) {
> use ModuleA;
> (doing something using ModuleA)}
>
> else {
> use ModuleB;
> (doing something using ModuleB)
>
> }
>
> That is, if $some_condition is true then load the ModuleA, if false load
> ModuleB. However, it doesn't seem to work this way and I've observed
> (counterintuitively :)
It's perfectly intuitive, if you happen to understand what "use"
actually does. ;-)
perldoc -f use
Imports some semantics into the current package from
the named module, generally by aliasing certain
subroutine or variable names into your package. It
is exactly equivalent to
BEGIN { require Module; import Module LIST; }
That is, use happens in a BEGIN block, which means it happens at
compile-time, before the "if-else" structure is evaluated.
> that both modules are always loaded (ModuleA and
> ModuleB), no matter the value of $some_condition.
Correct. Not only does $some_condition not have a value at the point
when the "use" happens, Perl doesn't know or care that you might want
to do something different based on that value.
> Is it possible to do such "conditional module loading" in Perl?
Yes. Either take the "use" out of the begin block:
require ModuleA;
import ModuleA;
or use the 'if' pragma (which I don't like, as it forces you to put
*other* code in a BEGIN{} block as well...)
perldoc if
Paul Lalli
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