Elixir - an atom that does not start with an uppercase

Question

In an elixir, atoms must begin with : so why even all literals that begin with an uppercase are also treated as atoms?

 IO.puts is_atom(Foo) # true, why???? IO.puts is_atom(foo) # error undefined function IO.puts is_atom(:foo) # true 

Answer 1

As you can see here , identifiers starting with capital letters are treated as atom aliases. In your case, Foo is an alias :'Elixir.Foo' , which is an atom.