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Re: Re: (tye)Re: When to use Prototypes?

by danger (Priest)
on Nov 09, 2001 at 23:15 UTC ( #124443=note: print w/replies, xml ) Need Help??

in reply to Re: (tye)Re: When to use Prototypes?
in thread When to use Prototypes?

When you use an empty prototype (no args) for constants, the parser then knows not to interpret anything that follows as arguments for to that sub:

sub LOW_PRE_PI {3.141} sub HI_POST_PI () {3.142} my @n_proto = (1, LOW_PRE_PI + 5, 11, 15); my @y_proto = (1, HI_POST_PI + 5, 11, 15); print "@n_proto\n@y_proto"; __END__ Output: 1 3.141 1 8.142 11 15

In the first case, everything following the unprototyped version is taken as an argument list for the subroutine.

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(tye)Re4: When to use Prototypes?
by tye (Sage) on Nov 09, 2001 at 23:59 UTC

    Yes, that is one difference, but I find code that makes use of that difference to be a bit of a problem. I'd probably write HI_POST_PI()+5 if I noticed.

    The difference that I'm talking about is that subroutines with a () prototype that also do nothing but return a value become compile-time constants. Perl can replace instances of calls to these with the constant returned at compile time, allowing lots of interesting tricks:

    > Perl -MO=Deparse -w use strict; sub DEBUG() { 0 } sub SymRef() { "My::Package::hash" } sub PI() { 3.141592 } if( DEBUG ) { warn "Okay, we are debugging...\n"; do_lots_of_debugging_stuff(); } $My::Package::hash{key}= "value"; print SymRef->{key},$/; print PI()+5,$/; __END__ sub DEBUG () { 0; } sub SymRef () { 'My::Package::hash'; } sub PI () { 3.141592; } '???'; $My::Package::hash{'key'} = 'value'; print $My::Package::hash{'key'}, $/; print 8.141592, $/; - syntax OK
    Without the () prototypes we'd get:
    sub DEBUG { 0; } sub SymRef { 'My::Package::hash'; } sub PI { 3.141592; } if (DEBUG ) { warn "Okay, we are debugging...\n"; do_lots_of_debugging_stuff ; } $My::Package::hash{'key'} = 'value'; print SymRef()->{'key'}, $/; print PI() + 5, $/; - syntax OK
    and, if we ran the code, we'd get:
    Can't use string ("My::Package::hash") as a HASH ref while "strict refs" in use

            - tye (but my friends call me "Tye")

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