[erlang-questions] On the usage of tests in guards
Tue Mar 3 03:39:09 CET 2009
Recently I was involved in an e-mail argument about whether
it was true that 'when' was a design mistake in Erlang (I
say NO) and whether it was true that most uses of 'when'
could be replaced by type tests inside patterns (I say no,
and even if they could be, they shouldn't be).
It occurred to me to get some numbers to see what other
people were doing, and I ran some fairly crude tools over
the R12B-5 distribution, finding that about 50% of the tests
in guards were _not_ type tests, and that this also seemed
to be true of 'receive' guards.
Wondering whether this would generalise, I just took a look
at the MochiWeb sources. This looked at all 'when' guards
(not at 'if', sadly, but then, we hadn't been arguing about
'if'). Here's the summary.
119 Total type tests
58 =:= (really "member")
212 Total comparisons
So comparisons form about 2/3 of the guard tests in MochiWeb,
an even higher proportion than OTP.
A lot of the >= and =< tests were character ranges,
and many of the =:= tests were in guards like
X =:= c1 ; X =:= c2 ; X =:= c3
checking for specific characters (or occasionally other literals).
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