Yesterday I wrote an SQL statement:

   select sum(whatever)
   from
      -- 10 tables, omitted --
   where
      -- whole pile of stuff omitted --
   group by
      -- another pile of things

[download]

The idea being that I would plug in various placeholder values and get back the value I was interested in. This worked very well, and as I was pressed for time I simply derefenced the result of fetchrow_arrayref to get my value. The code looked something like:

  $sth->execute($foo, $bar, $rat) or die $sth->errstr, $/;
  my $sum = $sth->fetchrow_arrayref->[0];
  $sth->finish;

[download]

This worked perfectly well for quite some time until I hit a particular set of inputs which produced an empty result set. At that point, fetchrow_arrayref returned undef, and perl rightly got upset when I asked it to dereference it to obtain the first element.

So I sighed, cursed my haste and realised I had to copy the result into an intermediary to check whether it was defined:

  $sth->execute($foo, $bar, $rat) or die $sth->errstr, $/;
  my $r = $sth->fetchrow_arrayref;
  my $sum = $r ? $r->[0] : 0;
  $sth->finish;

[download]

but before I did so, I had an insight that allowed me to zen the code:

  $sth->execute($foo, $bar, $rat) or die $sth->errstr, $/;
  my $sum = ($sth->fetchrow_arrayref || [0])->[0];
  $sth->finish;

[download]

That is, call the fetchrow_arrayref. If that evaluates to false, the expression lazily collapses to a single element array ref containing zero. And then dereference whatever happened, and take the first element.

And then I finished the program. I did leave a comment above the statement explaining what was going on. I like the idiom, but I worry that it's too clever for its own good.

Comments? Opinions?

• another intruder with the mooring in the heart of the Perl

In reply to Is this too clever? (or, stupid DBI tricks) by grinder

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