Javascript multimethods

multimethod-example.js

import multimethod from './multimethod.js'

const foo = multimethod(x => typeof x)
foo.string = x => 'called string method'
foo.number = x => 'called number method'

foo('x')
// 'called string method'

foo(1)
// 'called number method'

multimethod.js

// Create a multiple dispatch function.
// Methods can be registered by setting keys on function
// that correspond to key generated by `dispatch`.
// If no key is registered, will fall back to calling `multi._`,
// which throws an error by default.
// You can redefine `multi._` to set a default multimethod.
export default const multimethod = dispatch => {
  const multi = (...args) => {
    const key = dispatch(...args)
    const method = multi[key] || multi._
    return method(...args)
  }
  
  const _ = (...args) => {
    throw Error(`No multimethod defined for key "${dispatch(...args)}".`)
  }

  multi._ = _

  return multi
}