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Perl: the Markov chain saw

Re^8: Unhappy returns

by Aristotle (Chancellor)
on Oct 10, 2005 at 19:22 UTC ( #498915=note: print w/replies, xml ) Need Help??

in reply to Re^7: Unhappy returns
in thread Unhappy returns

It isn't completely consistent.

#!/usr/bin/perl use strict; use warnings; sub x { local$,=" "; print "$_[ 0 ]:", map( "[$_]", @_[ 1 .. $#_ ] ), +"\n" } x "for", sub { for( 5 .. 7 ) { 1 } }->(); x "while(5)", sub { while( 5 ) { 1; last } }->(); x "while(0)", sub { while( 0 ) { 1 } }->(); x "if(5)", sub { if( 5 ) { 1 } }->(); x "if(0)", sub { if( 0 ) { 1 } }->(); x "do{}while(0)", sub { do { 1 } while 0 }->(); x "do{}until(5)", sub { do { 0 } until 5 }->(); __END__ for: [] while(5): while(0): if(5): [1] if(0): [0] do{}while(0): [1] do{}until(5): [0]

I admit the value returned from the function for surprised me. It returns an honest-to-goodness false (ie. an empty string) -- not even undef. (I checked.) Interesting.

Also somewhat surprising, but not nearly as much, is that the functions with while return nothing at all.

Because I knew about the behaviour of if – when the condition is the last thing evaluated, it is returned, while when the body runs, its last expression's value is returned.

Which is exactly as expected by the model I described. Which is exactly what perlsub describes:

The return value of a subroutine is the value of the last expression evaluated by that sub, or the empty list in the case of an empty sub.

I guess Perl is simply inconsistent here.

Update: per Re: "last expression" quiz, I added do{} pseudo-loop tests. They return a code reference. It’s probably safe to say that Perl simply has no rule, and that what you get back from a sub depends on implementation details of perl.

Update: gack, I forgot to invoke the do subs using ->. Fixed, thanks to robin in Re^4: "last expression" quiz. No wonder I got back coderefs. The actual behaviour does not seem very consistent either, though.

Makeshifts last the longest.

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