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Perl's comma operator , performs several duties. The most widely - known duty is to serve as a list separator. These are referred to as "fat commas" [Conway 05].:

my @list = (2, 3, 5, 7);

Outside of list context, the comma can also be used to combine multiple expressions into one statement. Each expression is evaluated, and its result is discarded. The last expression's result is returned as the result of the comma operator. These are referred to as " Comma operators are called thin commas" [Conway 052005]. This behavior was adopted from C.

The potential for confusing thin commas with fat commas in list context is large enough to forbid use of the thin commas. Commas shall only must be used to only to separate items in list context.

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This code example validates a file , and indicates if it exists.

sub validate_file {
  my $file = shift(@_);
  if (-e $file) {
    return 1; # file exists
  }
  die "$file does not exist";
}

my $file = $ARGV[0];
validate_file($file), print "hi!\n";

This code behaves as expected. The comma operator is used to separate out the call to validate_file and subsequent call to print in the same statement. ThusConsequently, the return value of validate_file is discarded before print is called.

This line of code looks like it would behave the same , but instead behaves quite differently:

print validate_file($file), "hi!\n";

The print statement takes a list of items to print, and, in list context, the comma operator is assumed to separate list items. Consequently, if the file is valid, this program prints 1 before its friendly greeting.

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validate_file($file);
print "hi!\n";

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If multiple functions must be invoked within one statement, a do block can be used to evaluate a list of expressions , without using list context.

print do { validate_file($file); "hi!\n"};

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Bibliography

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