wroclove.rb 2025 lightning talk following up on Mateusz Nowak's 'Might & Magic of Domain-Driven Design' talk (which used the decider pattern). Explains the decider pattern as a more concrete version of the DDD aggregate pattern built from pure functions: a decide(command, state) function that emits events (protecting invariants as a boundary), an evolve(state, event) function that returns new state, and an initial-state function. State must be passed explicitly because the functions are pure. The speaker implemented the pattern as a Ruby gem called decide_rb (couldn't use 'decider' — taken for 15 years), inspired by the aggregates repository from Arkency. Demonstrates an Issue example (open/resolved states) with both block-style and proc-style match branches and hints at evolution before running out of time.