[specification] Tagged Return Types

[mrodz]

RFC for v2.0.0

Depends On:

• [x] #120
• [ ] #132

Syntax

import Person, AmericanContent, OtherContent from lib
import Locale from std/locale

render_page = fn(input: Person) -> enum { american: AmericanContent, other: OtherContent } {
  if input.locale.is("en-us") {
    return @american(AmericanContent("I love paying taxes")
  } else {
    return @other(OtherContent("I love paying taxes and getting free school"))
  }
}

p = Person("John Doe", 17, "nl_NL")

if @other(x) ?= render_page(p) {
  print x
} else {
  print "mmmm Americans"
}

import LIST from cool_people
import process from std

type ClubResult enum {
  too_young,
  not_on_list,
  ok: Person,
}

can_access_club = fn(person: Person) -> ClubResult  {
  if person.age() < 21 {
    return @too_young
  }

  if LIST.contains(person.name()) {
    return @ok(person)
  } else {
    return @not_on_list
  }
}

myself = Person("Mateo", 16)

result: ClubResult = can_access_club(myself)

if @too_young ?= result {
  print "Error! You're too young"
  process.exit()
}

if @not_on_list ?= result {
  print "Go get cooler, you're not on the list"
  process.exit()
}

if @ok(person) ?= result {
  print "You can access the club, {person.name()}"
}

[mrodz]

Guaranteed to be stable for v2.0.0