A demo spell program

999_program.spell

account (balance) = { balance: balance }

account#deposit (account amount) = { balance: account.balance + amount }

account#withdraw (account amount) = 
  ? account.balance < amount -> error#signal "Insufficient funds!"
  ?                        _ -> { balance: account.balance - amount }
  
main () = 
  do
    account#deposit account 10
    account#deposit account 200
    acount#withdraw account 30
  with  
    account <- account 10

fib#recursive (value) =
  ? value <= 1 -> value
  ?          _ -> (fib#recursive (value - 1)) + (fib#recursive (value - 2))
  
fib#iterative (value) = round (phi ** value / sq5)
  with 
    sq5 <- sqrt 5
    phi <- (1 + sq5) / 2

filter (predicate list) = 
  ? null list             -> list
  ? predicate (head list) -> (head list) : filter predicate (tail list)
  ?                     _ -> filter predicate (tail list)
  
array#partition (array predicate) = (filter predicate array) , (filter (invert predicate) array)
  
quicksort (array) =
  ? array#len array <= 1 -> array
  ?                    _ -> quicksort' array
  with
    quicksort' (array) = quicksort (first partition) ++ quicksort (last partition)
      with
        pivot <- array#midpoint array
        partition <- array#partition array { element | element < pivot }