You can come up w/ an algebra for a specific problem domain by thinking about the data models and repurposing some simple abstract algebra. E.g.: new(stat

You can come up w/ an algebra for a specific problem domain by thinking about the data models and repurposing some simple abstract algebra. E.g.: new(state, &args) => mutation apply(state, mutation) => state' Applied to finance: newPayment(balanceSheet, amount, date) => payment apply(balanceSheet, payment) => balanceSheet' newDiscount(balanceSheet', amount, date) => discount apply(balanceSheet', discount) => balanceSheet'' Applied to chess: newMovement(board, K, E2) => movement apply(board, movement) => board' newMovement(board', b, D6) => movement' apply(board', movement') => board'' As a bonus, it's clear where to enforce invariants and fail early: newMovement(board', K, E3) => Illegal Movement (Can't move to E3) newMovement(board'', K, E2) => Illegal Movement (White King not available on board) apply(board''', randomMovement) => Illegal Play