Determining if an item is a string or a list in Erlang

Question

I am writing a program that can have either a list or a string as an argument. How can I tell the difference between a string and a list programmatically in Erlang. Something like:

print(List) -> list; 
print(String) -> string.

Answer 1

io_lib:printable_list might be what you are looking for. However it doesn't handle unicode only latin-1 encodings. If you need to detect unicode strings I think you might be out of luck. The best bet is pseudo typing your lists like so: {string, [$a, $b, $c]}. Kind of a build your types.

use a constructor like so string(L) when is_list(L) -> {string, L}.

and just use that typing construct all through your app.

On the other hand you could just treat all strings as just lists and not make the distinction.

Answer 2

Best thing what you can do is tagging your structures as Jeremy Wall suggested. Anyway you can decide check input to your module/subsystem/application/...

is_string([]) -> true;
is_string([X|T]) -> is_integer(X) andalso X>=0 andalso is_string(T);
is_string(_) -> false.

Unfortunately it is expensive operation and you can't use it in guards.

Answer 3

Why would you need to separate these? Strings are lists in erlang (most of the time).