if condition then call_my_routine() else return end
if the resulting line is short, or:
if condition then call_my_routine() else return end
If it doesn't fit nicely in one line (usually because the body is more than one statement). However when the body is a single statement but doesn't fit on a line (because the condition is long), I sometimes do:
if condition then call_my_routine()
else return
end
especially if it's several elif with complex conditions, eg:
if complex_condition_1
then do_thing_1
elif complex_condition_2
then do_thing_2
else return
end
What's most important is that the code is readable and not hideous to look at, rather than sticking strictly to one style at all times.
