Pudu Playground

Program source

// Borrowing and changing
// A signature says whether a function reads a value or changes it: & borrows, &mut lends it to be changed.
module Main

import Std.Io as Io

type Account = { owner: Str, mut balance: Int }

fn describe(account: &Account) -> Str {
  "{account.owner} has {account.balance}"
}

fn deposit(account: &mut Account, amount: Int) -> () {
  account.balance = account.balance + amount
}

fn withdraw(account: &mut Account, amount: Int) -> Result[(), Str] {
  if amount > account.balance {
    return Err("{account.owner} cannot withdraw {amount}")
  }
  account.balance = account.balance - amount
  Ok(())
}

fn main() -> Int {
  var account = Account{owner: "Ada", balance: 10}
  deposit(&mut account, 25)
  let _after = Io.writeLine(describe(&account))
  match withdraw(&mut account, 100) {
    case Ok(_) => { let _said = Io.writeLine("withdrew 100") }
    case Err(reason) => { let _said = Io.writeLine(reason) }
  }
  match withdraw(&mut account, 30) {
    case Ok(_) => { let _said = Io.writeLine("withdrew 30: " + describe(&account)) }
    case Err(reason) => { let _said = Io.writeLine(reason) }
  }
  0
}

Output

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Output