Practical Swift Programs

Config Selection

Programs often choose one configuration for local runs and another for production.

Pick a configuration

useProduction
config_selection.swift
Replay: real traced execution (multi-file project)
let useProduction = false
let localHost = "localhost"
let productionHost = "api.example.com"
let host = useProduction ? productionHost : localHost
let port = useProduction ? 443 : 8080
let endpoint = "\(host):\(port)"

print(endpoint)

let useProduction = true
let localHost = "localhost"
let productionHost = "api.example.com"
let host = useProduction ? productionHost : localHost
let port = useProduction ? 443 : 8080
let endpoint = "\(host):\(port)"

print(endpoint)

  1. useProduction ← false, localHost ← localhost, productionHost ← api.example.com

    1let useProduction→ false = false  //@useProduction=true2let localHost→ localhost = "localhost"3let productionHost→ api.example.com = "api.example.com"4let host→ localhost = useProductionfalse ? productionHostapi.example.com : localHostlocalhost5let port→ 8080 = useProductionfalse ? 443 : 80806let endpoint→ localhost:8080 = "\(hostlocalhost):\(port8080)"78print(endpointlocalhost:8080)
    outputlocalhost:8080

  1. useProduction ← true, localHost ← localhost, productionHost ← api.example.com

    1let useProduction→ true = true2let localHost→ localhost = "localhost"3let productionHost→ api.example.com = "api.example.com"4let host→ api.example.com = useProductiontrue ? productionHostapi.example.com : localHostlocalhost5let port→ 443 = useProductiontrue ? 443 : 80806let endpoint→ api.example.com:443 = "\(hostapi.example.com):\(port443)"78print(endpointapi.example.com:443)
    outputapi.example.com:443

Follow the Choice

  1. useProduction starts as false.
  2. The host choice uses localHost, so host becomes localhost.
  3. The port choice uses the local port, so port becomes 8080.
  4. endpoint joins them as localhost:8080.
  5. The program prints localhost:8080. | useProduction | chosen host | chosen port | endpoint | | --- | --- | ---: | --- | | false | localhost | 8080 | localhost:8080 | | true | api.example.com | 443 | api.example.com:443 |
configuration A configuration value can select host names, ports, and other settings before the rest of a program starts.

Exercise: config_selection.swift

Reproduce localhost:8080, then set useProduction to true and predict api.example.com:443.