Deno Args

import args from 'https://deno.land/x/args@1.0.0/wrapper.ts'
import { HelpFlag, Option } from 'https://deno.land/x/args@1.0.0/argument-types.ts'
import { FiniteNumber, Choice } from 'https://deno.land/x/args@1.0.0/value-types.ts'

const parser = args
  .with(EarlyExitFlag('help', {
    describe: 'Show help',
    exit () {
      console.log(parser.help())
      return Deno.exit()
    }
  }))
  .with(Option('a', {
    type: FiniteNumber,
    describe: 'Value of a'
  }))
  .with(Option('b', {
    type: FiniteNumber,
    describe: 'Value of b'
  }))
  .with(Option('operator', {
    type: Choice<'add' | 'sub'>(
      {
        value: 'add',
        describe: 'Add two numbers'
      },
      {
        value: 'sub',
        describe: 'Subtract two numbers'
      }
    ),
    alias: ['o'],
    describe: 'Operator to use'
  }))

const res = parser.parse(Deno.args)

if (res.error) {
  console.error('Failed to parse CLI arguments')
  for (const e of res.error) {
    console.error(e.toString)
  }
  Deno.exit(1)
} else {
  const { a, b, operator } = res.value
  switch (operator) {
    case 'add':
      console.log(a + b)
    case 'sub':
      console.log(a - b)
  }
}