What is tail call optimization in Erlang and why does it matter for long-running loops?

A function call is a tail call when it's the very last operation in a clause — nothing happens with its result except returning it directly. Erlang recognizes this pattern and reuses the current stack frame instead of pushing a new one, so a tail-recursive function can loop indefinitely without growing the call stack.

%% tail-recursive: the recursive call is the last thing evaluated
loop(State) ->
    receive
        Msg -> loop(handle(Msg, State))   %% tail call
    end.

%% NOT tail-recursive: work happens AFTER the recursive call returns
sum([]) -> 0;
sum([H|T]) -> H + sum(T).                 %% addition happens after the call

This matters directly for OTP: every gen_server, supervisor, and hand-rolled process loop relies on tail call optimization to run forever inside a receive loop without ever running out of stack space. Without it, a long-lived Erlang process would eventually overflow its stack simply by staying alive and handling messages.

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