How to catch handler errors

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When a handler crashes (error(badmatch), throw(_), division by zero, etc.) you want to send a response you chose instead of letting the exit spread. Wrap the rest of the stack with livery_middleware:wrap/1 and turn each class of failure into the response shape you want.

Wrap the stack

Stack = [
    {livery_request_id, undefined},
    livery_middleware:wrap(fun errors_to_resp/3),
    %% ... handler
].

errors_to_resp(throw, {validation, Why}, _Stack) ->
    livery_resp:text(400, Why);
errors_to_resp(error, {badkey, K}, _Stack) ->
    livery_resp:text(422,
        iolist_to_binary([<<"missing field: ">>, K]));
errors_to_resp(_Class, _Reason, _Stack) ->
    livery_resp:text(500, <<"internal error">>).

The wrapper catches throw, error, and exit from anything downstream. Class is throw | error | exit.

Rely on the default without a wrapper

When a handler runs inside livery_req_proc (the worker the H1/H2/H3 adapters spawn), a crash becomes livery_resp:text(500, <<"internal server error">>) on its own. Use the wrapper when you want a different response (validation errors as 400, business errors as 422, etc.).

Catch errors in tests

livery_test_adapter:run/3 runs in the calling process. Without a wrapper, it turns a crash into the same 500 internal server error response, so your test sees the status instead of the exception. To test your own error responses, add a wrapper in the stack you are testing.

Use domain exceptions as control flow

Throw to short-circuit from deep inside the call tree:

authenticate(Req) ->
    case verify(Req) of
        {ok, U} -> U;
        error   -> throw({validation, <<"bad credentials">>})
    end.

show(Req) ->
    _User = authenticate(Req),
    livery_resp:json(200, payload()).

The wrap middleware turns the throw({validation, ...}) into a 400, with no error handling in the handler itself.

See also