Question

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?

Answer 1

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