[erlang-questions] List comprehension question

amindfv <>
Thu Jan 16 17:55:12 CET 2014


This is intentional. I use a pattern like this a lot:

-type foo :: {'a', string()} | {'b', string()}.

Foos() -> [{a, "hello"}, {b, "goodbye"}].

As() -> [ S || {a, S} <- Foos() ]. % ["hello"]

Tom


El Jan 16, 2014, a las 11:39, Rich Neswold <> escribió:

> Hello,
> 
> This morning I became aware of a (powerful) feature of list
> comprehensions. Take the following example:
> 
>    [ X + Y || {X, Y} <- L].
> 
> If we set L to [{1,2}, ok, {1,3}], we get the result [3,4] instead of
> a pattern match exception (as I was expecting.) This means that list
> comprehensions give you a "free" lists:filter/2 in the generator
> expressions!
> 
> I've looked through the OTP documentation, quite a few Stack Overflow
> questions, and several list comprehension tutorials and none of them
> explicitly state that generator expressions filter elements that don't
> match their pattern. The web pages emphasize generators and guards are
> like combinations of lists:map and lists:filter, which isn't exactly
> correct. For instance, the above example is not the same as:
> 
>    lists:map(fun ({X, Y}) -> X + Y end, L).
> 
> but is more equivalent to:
> 
>    lists:foldr(fun ({X, Y}, Acc) -> [X + Y | Acc];
>                    (_, Acc) -> Acc
>                end, [], L).
> 
> Is this an expected, but undocumented, feature of list comprehensions?
> Or am I in "undefined behavior" territory?
> 
> --
> Rich
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> 
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