Poor man's type checking

examples.js

const ts = require('./type-signatures')

// Without using module
// number -> string -> number
const addNums_ = (n, str) => n + parseInt(str)

// With using module
const addNums = ts('number', 'string')(
  (n, str) => n + parseInt(str)
)

// Works because the types match
// console.log(addNums(2, '3'))

// Produces a type error
// console.log(addNums('2', '3'))

// Supports currying

const add = ts('number')(
  x => ts('number')(
    y => x + y
  )
)

// console.log(add(2)(3))

const add3 = add(3)

// console.log(add3('5'))

function test() {
  addNums('2', '3')
}

type-signatures.js

Object.defineProperty(global, '__stack', {
  get: function() {
    const orig = Error.prepareStackTrace
    Error.prepareStackTrace = (_, stack) => stack
    const err = new Error
    Error.captureStackTrace(err, arguments.callee)
    const stack = err.stack
    Error.prepareStackTrace = orig
    return stack
  }
})

const ts = (...types) => f => (...args) => {
  if (types.length !== args.length) {
    const err = new TypeError('Number of arguments does not match type signature')
    return console.error(err.stack)
  }

  const errs = args
        .map((arg, i) => {
          const type = types[i]
          if (typeof arg !== type) {
            const fileName = __stack[3].getFileName()
            const lineNumber = __stack[3].getLineNumber()
            const columnNumber = __stack[3].getColumnNumber()
            const err = new TypeError(`Couldn't match expected type '${type}' with actual type '${typeof arg}'`)
            console.error(`at ${fileName}:${lineNumber}:${columnNumber}`)
            return err
          } else {
            return null
          }
        })
        .filter(Boolean)
  
  if (errs.length > 0) {
    return errs.forEach(err => console.error(err.stack))
  }

  return f(...args)
}

module.exports = ts