effect
Attach an effect-key lambda to the current step.
The lambda receives the current context C and the pre-step saga state S and must return a non-blank string that uniquely identifies the step's side effect within a run. Blank keys are rejected at Intent-append time with IllegalStateException.
Must be called after step.
Return
This builder for method chaining.
Parameters
block
Lambda that computes the effect-key from context and state.
Throws
if called before step.