You can use a guard for that

def foo(word) when word in [“a”, “b”] do
end

For a list starting with something, it’s trivial:

def foo([first | _rest]) when first in ["a", "b"] do
end

For a list ending with something, it’s more involved as the Erlang lists are single-linked lists (in the C lingo) and you have to traverse the entire list to find the ending:

def foo(list) when is_list(list) do
  if List.last(list) in ["a", "b"] do
    # all good, proceed
  else
    # report an error
  end
end

For this usually people make a public function that does these validations and handles returning errors and then define something like defp do_foo to handle the happy path only.

FWIW, the common convention name for bar should be do_foo as well.

To sum it all up:

def foo("a") do: do_foo("a")
def foo("b") do: do_foo("b")
def foo(word) do
  "not a or b!"
end

defp do_foo(word) do
  word
end