Using Swift 4.2's @dynamicMemberLookup to expose environment variables

dynamic_member_lookup_environment.swift

import Darwin

@dynamicMemberLookup
struct Environment {
  subscript(dynamicMember name: String) -> String? {
    get {
      guard let value = getenv(name) else { return nil }

      return String(validatingUTF8: value)
    }

    nonmutating set {
      if let value = newValue {
        setenv(name, value, /*overwrite:*/ 1)
      } else {
        unsetenv(name)
      }
    }
  }
}

let environment = Environment()

print("Username = \(environment.USER ?? "<unknown user>")")
print("Changing username...")
environment.USER = "admin"
print("Username = \(environment.USER ?? "<unknown user>")")
print("Deleting username...")
environment.USER = nil
print("Username = \(environment.USER ?? "<unknown user>")")