Postfix conditionals and language design

[dougo]

So I'm reading some Perl code. I usually stay away from Perl as much as I can, but I'm trying to figure out some undocumented behavior of the MusicBrainz RDF server so I have to UTSL. Anyway, I get to this line:

      return ("No artist name or artist id given.", $data, 0, undef) 

and I get confused, because I saw the artist ID check but not the artist name check. So I start reading backwards to see if I missed something; maybe the artist name check happened in some non-obvious function call? I go skimming through various other files to see if some other code is filling in the artist ID or something. Eventually, I give up and come back to that line, and I notice that the next line is:

          if ($data->{artist} eq '');

and I smack my head. I'm willing to chalk this up to bad style (and I hope people would agree this is bad style), but now I'm wondering: when is it ever good style to put the conditional guard after the code it's guarding? Or, more generally, when is it ever good style for the order of code (in the same basic block) to be the opposite of the order of execution?

Comments

[petdance.livejournal.com]

Sure, you can also say

die ..... unless $something

but I really don't like that in most cases.

Basically, I put the most important part first in any long construct. In the case of the open(), that's what's really key. Think of it in English. Are you saying "Unless it's raining, we'll have a picnic", or do you say "We'll have a picnic, unless it's raining." The picnic is usually the most important part.

[dougo.livejournal.com]

Right, but I wouldn't say "We'll have a picnic, so I need you to make some potato salad, and leave out the onions... unless it's raining."