Context-sensitive interfaces

The idea

Perl evaluates expressions in a context supplied by their caller. List context requests a list, scalar context requests one scalar value, and void context ignores the result. A subroutine can inspect that context with wantarray, which is true for list context, false for scalar context, and undefined for void context. Context is a semantic input even though it is not written in the argument list.

Context-sensitive interfaces can be convenient, but they can also make a harmless-looking assignment change behavior. Document what each context means and avoid guessing that scalar context always selects the final item; the expression or subroutine defines its scalar behavior. Returning a reference gives callers one stable container when a result should preserve the same structure everywhere.

use strict; use warnings;
sub pair { return wantarray ? (2,5) : 7; }
my @items=pair();
my $total=pair();
print join(",",@items),"\n",$total,"\n";

2,5
7

The array assignment requests list context, while the scalar assignment selects the explicitly defined scalar result.