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Weird List Comprensions in Erlang

A photo of Brujo Benavides wrote this on January 13, 2015 under erlang, inaka .


So, today I found myself writing a typical recursive function…

my_fun([], Acc) -> Acc;
my_fun([Thing|Things], Acc) ->
  NewAcc =
    case check_thing(Thing) of
      {good, ParsedThing} ->
        [ParsedThing || Acc];
      bad -> Acc
  my_fun(Things, NewAcc).

If you spotted the issue with that code, that's awesome! You can skip this paragrah. For other readers: That function always returns [], even if check_thing always returns {good, _}. Why? Because I made a typo and instead of a list constructor I used a List Comprehension here:

{good, ParsedThing} -> [ParsedThing || Acc];

But then I wondered: Why did that even compile in the first place? Instead of reading the documentation (and loosing all the fun in the process) I decided to try things out on a console…


My first assumption was Acc must act as a generator there. And I was amused, since more than once I saw myself writing things like:

1> [ random_thing() || _ <- lists:seq(1, 100)].

And I would love not to have to discard that variable. Let's try it out!

1> [x || []].

Looks good so far…

2> [x || [a]].
3> [x || [a, b]].

Eeeeehm… now it doesn't look right at all. What's going on here?

4> [x || lists:seq(1,3)].
** exception error: bad filter [1,2,3]

Ooooh!!! It's a filter, not a generator!


But what's the meaning of a LC that has no generator? Let's try it out…

5> [x || true].
6> [x || false].

So, If I create a LC with just filters it can either have one element (if the filters are met) or zero elements (otherwise). Interesting! And it works pretty much like andalso in the sense that once a filter returned false no subsequent filter is evaluated… right?

if_long_enough(X) ->
  [X || is_list(X), length(X) > 0].
1> my_mod:if_long_enough(a).
2> my_mod:if_long_enough("x").
3> my_mod:if_long_enough("xx").
4> my_mod:if_long_enough("").

The Docs

Now that we had enough fun, I wanted to see if this feature was documented somewhere. I found nothing about LCs without generators neither on the documentation nor the best Erlang online book ever, where it only states that You can have more than one!


To be fair, it doesn't seem like a really useful feature and I would be skeptical to let a code like this pass a code review, but it's probably a funny thing to play with :) Enjoy it!!


I shouldn't be writing this, but… If you are one of those devs that uses andalso to implement elseless-ifs, like this one:

is_awake(User) andalso say_hello(User).

Then you should know that tools like dialyzer won't be happy with your code unless say_hello returns boolean(). But you can avoid that problem (while keeping your code dirty) by doing:

[say_hello(User) || is_awake(User)].

Which is, in fact, even uglier than the original version, ¯\(ツ)