How to generate a random alphanumeric string with Erlang?

It might depend on the randomness you need. Erlang's crypto module produces stronger random data than the random module (see also [erlang-questions] Yaws security alert - Yaws 1.93 and this question). If you want to use strong_rand_bytes to generate an ID maybe getting the base64 of it might be enough:

> base64:encode(crypto:strong_rand_bytes(Bytes)).

You could turn this into a list if needed.

I have prepared small module do to this
Also it uses crypto:rand_uniform/2 but not obsolete random:uniform

module(cloud_rnd).

-export([rnd_chars/1, rnd_numbers/1, rnd_chars_numbers/1]).

rnd_chars(L)         -> get_rnd(L, chars).
rnd_numbers(L)       -> get_rnd(L, numbers).
rnd_chars_numbers(L) -> get_rnd(L, chars_numbers).

get_rnd(L, chars)         -> gen_rnd(L, "abcdefghijklmnopqrstuvwxyz");
get_rnd(L, numbers)       -> gen_rnd(L, "1234567890");
get_rnd(L, chars_numbers) -> gen_rnd(L, "abcdefghijklmnopqrstuvwxyz1234567890").

gen_rnd(Length, AllowedChars) ->
  MaxLength = length(AllowedChars),
  lists:foldl(
    fun(_, Acc) -> [lists:nth(crypto:rand_uniform(1, MaxLength), AllowedChars)] ++ Acc end,
    [], lists:seq(1, Length)
  ).

According to Generating random strings in Erlang it only takes a few lines of Erlang to generate a string of a specified length from a certain set of characters.

get_random_string(Length, AllowedChars) ->
    lists:foldl(fun(_, Acc) ->
                        [lists:nth(random:uniform(length(AllowedChars)),
                                   AllowedChars)]
                            ++ Acc
                end, [], lists:seq(1, Length)).

The blog post has a line-by-line explanation of the code. Look to the comments for a couple of optimization tips.