[erlang-questions] List comprehension filter expressions not throwing exceptions
Mon Oct 4 23:52:52 CEST 2010
On 4/10/2010, at 10:23 PM, Anthony Shipman wrote:
> On Mon, 4 Oct 2010 03:24:00 pm Richard O'Keefe wrote:
>> It would be better to be consistent about this.
>> If only bindings Pattern <- Expr and guard tests were allowed,
>> general filters could still be hacked using
>> true <- [Boolean_Expression]
>> and everyone would know exactly where they stood.
> I think it would seriously reduce the usefulness if I couldn't have function
> calls in the filter tests.
The text you quoted (repeated above) *proved* that it would not
by showing a systematic way to eliminate such ugliness.
I've just checked a bunch of other people's Erlang ocde.
Functions that I saw used in list comprehensions include
- whereis/1 -- could be allowed in guards
- member/2 -- could be allowed in guards
- keymember/3 -- could be allowed in guards
where I'm happy to see them in list comprehensions because I'd be
happy to see them in guards (yes, member/2 takes O(N) time, but
so after all does length/1), plus
- two that should have been eliminated by better data structure design
- another that should have been a better algorithm
- a case where the code would have been half as long had the function
been inlined, and more efficient as well
- a case of the form [X || X <- L, X /= f(L), ...]
which takes quadratic time in its present form, but would have taken
linear time had it been written FL = f(L), [X || X <- L, X /= FL, ...]
and is arguably wrong even then
- several things that could be handled easily by allowing (_;_) in
list comprehension guards and certainly can be handled with an
- two examples that at first I thought were good ones, but on closer
inspection could have been handled by better data structure design
This experience has forced me to two conclusions:
(1) plain function calls in list comprehension filters are generally
a bad code smell
(2) nested functions in Erlang are also a bad code smell more often
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