Q:

Checking for membership in an Erlang guard

What is the simplest way to write an if statement in Erlang, where a part of the guard is member(E, L), i.e., testing if E is a member of the list L? The naive approach is:
if 
  ... andalso member(E,L) -> ...
end

But is does not work becuase, if I understand correctly, member is not a guard expression.  Which way will work?

A:

Member functionality is, as you say, not a valid guard. Instead you might consider using a case pattern? It's possibly to include your other if-clauses in the case expression.
case {member(E,L),Expr} of
  {true,true} -> do(), is_member;
  {true,false} -> is_member;
  {false,_} -> no_member
end

A:

It is not possible to test list membership in a guard in Erlang. You have to do this:
f(E, L) ->
    case lists:member(E, L) of
        true  -> ...;
        false -> ...
    end.

