Ruby gem created by the speaker of a wroclove.rb 2025 lightning talk on the decider pattern. Named decide_rb because 'decider' was taken on RubyGems for 15 years. Demonstrates the decider pattern with an initial state, a decide function combining commands and state into events (supporting both pattern-matching-style blocks and proc-returns-true branches), and an evolve function producing new state. Implementation style is inspired by the aggregates repository from Arkency, simplified for demonstration. Presented as a competition to the array-sample approach.