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Think about Loose Coupling

Re: do {$i++} until vs. $i++ until

by Anonymous Monk
on Mar 25, 2008 at 04:13 UTC ( #676042=note: print w/replies, xml ) Need Help??

in reply to do {$i++} until vs. $i++ until

This is documented in perlsyn:

The while and until modifiers have the usual "while loop" semantics (conditional evaluated first), except when applied to a do-BLOCK (or to the deprecated do-SUBROUTINE statement), in which case the block executes once before the conditional is evaluated. This is so that you can write loops like:
do { $line = <STDIN>; ... } until $line eq ".\n";

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Re^2: do {$i++} until vs. $i++ until
by hipowls (Curate) on Mar 25, 2008 at 07:32 UTC

    The disadvantage of that idiom is that the scope of $line must extend outside the block where is it used. Declaring $line inside the block results in a syntax error

    use strict; do { my $line = <STDIN>; ... } until $line eq ".\n"; # Syntax error __END__ Global symbol "$line" requires explicit package name
    unlike a while(condition) {} the condition is not in the same scope as the block. It can be written as
    use strict; INPUT: while ( my $line = <STDIN> ) { last INPUT if $line eq ".\n"; ... }
    which will restrict $line to the while block

    Update: Code updated

      Those two snippets aren't equivalent. Aside from the fact that the latter also exits if <STDIN> returned undef, you reversed the order of the loop body (...) and the exit condition (last INPUT if $line eq ".\n";)

        True they are not functionally equivalent but I didn't care to replicate the bug of looping on a closed file descriptor;-) It works properly when used in a command pipeline.

        As far as last INPUT if $line eq ".\n" goes it is probably correct to put it at the top of the loop since ".\n" is being used as an equivalent to EOF. Whatever is done with ".\n" most likely requires special handling of some sort, for example a sendmail equivalent wouldn't include it in the message and would treat it the same as EOF.

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