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Modern Heredocs - 2017-12-04

The Wise Old Elf was doing code review for Sloeberry Snoozyflakes, one of the newest elves on his team.

Slowberry was writing a simple routine to count the number of nice children there were this year, in order to get an estimate of the number of presents that Santa would need to deliver.

sub number_of_children {
    my $self            = shift;
    my $naughty_or_nice = shift;

    my $sql = "SELECT count(*)\n" .
              "  FROM children\n" .
              "  WHER naughty_or_nice = \$1";

    my $dbh = $self->database_handle;
    return $dbh->selectall_arrayref($sql,{},$naughty_or_nice)->[0];
}

"That multi-line sql assignment is a little hard to read.", the Wise Old Elf commented. "Could we change it to make it cleaner?"

Slowberry changed his code to use a heredoc:

sub number_of_children {
    my $self            = shift;
    my $naughty_or_nice = shift;

    my $sql = <<END
       SELECT count(*)
         FROM children
         WHER naughty_or_nice = \$1
END

    my $dbh = $self->database_handle;
    return $dbh->selectall_arrayref($sql,{},$naughty_or_nice)->[0];
}

"That's better," the elf continued, "but if you'd used SQL as the heredoc deliminator rather than than TEXT, then your editor would probably enable syntax highlighting, and you'd probably have noticed that you typo-ed WHERE."


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sub number_of_children {
    my $self = shift;
    my $naughty_or_nice = shift;

    my $sql = <<SQL
SELECT count(*)
   FROM children
  WHERE naughty_or_nice = $1
SQL

    my $dbh = $self->database_handle;
    return $dbh->selectall_arrayref($sql,{},$naughty_or_nice)->[0];
}
 

"Not bad, but you probably should put single quotes around the first heredoc deliminator so you don't have escape that $. In fact you should always put " or ' around the heredoc deliminator just to make it clear to whoever is reading your code what you actually meant."


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sub number_of_children {
    my $self = shift;
    my $naughty_or_nice = shift;

    my $sql = <<'SQL'
SELECT count(*)
   FROM children
  WHERE naughty_or_nice = $1
SQL

    my $dbh = $self->database_handle;
    return $dbh->selectall_arrayref($sql,{},$naughty_or_nice)->[0];
}
 

"Pretty good. The terminating heredoc deliminator looks weird all over there on the left hand side, not indented like the rest of the subroutine. We're using Perl 5.26, so we can add a tilde in the heredoc syntax to allow us to indent."


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sub number_of_children {
    my $self = shift;
    my $naughty_or_nice = shift;

    my $sql = <<~'SQL'
SELECT count(*)
   FROM children
  WHERE naughty_or_nice = $1
    SQL

    my $dbh = $self->database_handle;
    return $dbh->selectall_arrayref($sql,{},$naughty_or_nice)->[0];
}
 

"That's awesome. Not that it matters here, but the way this works is that the same amount of whitespace that's before the final heredoc terminator is stripped from the rest of the heredoc. Maybe now we could remove some of the variables?"


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sub number_of_children {
    my $self = shift;
    my $naughty_or_nice = shift;

    return $self->database_handle
                ->selectall_arrayref(<<~'SQL',{},$naughty_or_nice)->[0];
SELECT count(*)
   FROM children
  WHERE naughty_or_nice = $1
    SQL
}
 

"There we go. Now you just need to add some more tests - that broken WHERE from earlier can't have possibly been executed, otherwise we'd have seen a test failure - and we're ready to ship..."

Gravatar Image This article contributed by: Mark Fowler <mark@twoshortplanks.com>