pattern-matching-creator.js

const equal = (a, b) => a === b

const create = (...args) => {
  const rules = []
  const creator = {
    when(arg, valid, value, definition) {
      const key = args.indexOf(arg)
      rules[key] = rules[key] || []
      rules[key].push([ (a) => valid(a, value), definition ])
      return creator
    },
    done() {
      return (...args) => 
        args.reduce((fn, arg, key) => 
          fn || rules[key] && rules[key].reduce((fn, [valid, definition]) => 
            fn || valid(arg) && definition
          , 0)
        , 0)(...args)
    }
  }
  return creator;
}

const math = create('a', 'b', 'c')
 .when('a', equal, '*', (a, b, c) => b*c)
 .when('a', equal, '+', (a, b, c) => b+c)
 .when('a', equal, '-', (a, b, c) => b-c)
 .when('a', equal, '/', (a, b, c) => b/c)
 .done()

console.log('Matches first parameter, first rule:', math('*', 2, 3) === 6)
console.log('Matches first parameter, second rule:', math('+', 2, 3) === 5)