experimental higher-order contract function

contract.js

signatureToTypes = sig => sig
  .split(' -> ')
  .map(type => ({
    Number: {
      ctor: Number,
      type: 'number',
    },
    Object: {
      ctor: Object,
      type: 'object',
    },
    String: {
      ctor: String,
      type: 'string',
    },  
  })[type])

throwError = msg => {
  throw new Error(msg)
}

contract = (sig, f) => {
  const types = signatureToTypes(sig)
  
  return (...args) =>
    types.every(({ ctor, type }, i) => args[i] instanceof ctor || typeof args[i] === type) 
      ? f(...args)
      : throwError(`Contract broke, expected types were: ${types.map(t => t.ctor.name)}`)
}

add = contract('Number -> Number',
  (a, b) => a + b
)

/*
  @contract('Number -> Number')
  add = (a, b) => a + b
*/