An Erlang guard sequence which checks if a term is a list of length == n

Question

Is it possible to put a guard sequence that would check both term type and its length? I read that the first passing guard makes the whole sequence pass, so I suppose when I check for the term to be a list, it doesn't check the rest guards. Here is the code:

save_key(Key)
    when
      is_list(Key);
      length(Key) == 44 ->

    ok.

Answer 1

In order to combine guard expressions with and, you can separate them with a comma instead of semicolon as mentioned here:

-module(a).
-export([save_key/1]).

save_key(Key)
    when
      is_list(Key),
      length(Key) == 44 ->
    ok;
save_key(_) ->
    ko.

1> c(a).
{ok,a}
2> a:save_key([]).
ko
3> a:save_key(nil).
ko
4> a:save_key(lists:seq(1, 44)).
ok

Also note that since errors thrown in guards are ignored, you can just add the length(Key) == 44 and get the same behavior as a non-list will throw an error and will not match that clause:

save_key(Key) when length(Key) == 44 ->
    ok;
save_key(_) ->
    ko.