Typed decoders for the event shapes AWS services send to Lambda.

Handlers receive the raw decoded JSON. These modules turn the common envelopes into structs with the fields you actually use, decode base64 and nested JSON, and convert DynamoDB attribute values into Elixir terms.

Return shapes follow one rule: decoders (Mayfly.Events.HTTP.decode/1, Mayfly.Events.SQS.decode/1, …) take a known envelope and return the struct directly, since they cannot fail on the shape they were given; decode/1 here dispatches on an unknown event and therefore returns {:ok, struct} or :unknown; response helpers (Mayfly.Events.HTTP.json/4, Mayfly.Events.SQS.process_batch/2, …) return exactly what a handler returns, so they can be the last expression of handle/3.

Use the specific module when you know the source, or decode/1 to dispatch:

def handle(event, _ctx, _state) do
  case Mayfly.Events.decode(event) do
    {:ok, %Mayfly.Events.HTTP.Request{} = req} -> serve(req)
    {:ok, %Mayfly.Events.SQS{records: records}} -> process(records)
    {:ok, other} -> {:ok, %{ignored: other.__struct__}}
    :unknown -> {:ok, %{raw: event}}
  end
end

Every decoder keeps the original map in :raw so nothing is lost.