module skeleton11
//Mart Lubbers s4109503
//Charlie Gerhardus s3050009
import StdEnv

:: Prog :== [Instr]
:: Instr = Write Expr | Atomic [Instr]
:: Expr = Int   Int
        | Plus  Expr Expr
        | Times Expr Expr
        | Read

possibleResults :: [Prog] -> [Int]
possibleResults x = produce x 0

produce :: [Prog] Int -> [Int]
produce x i
| all (isEmpty) x = [i]
= flatten [produce (updateAt idx ys x) (eval y i)\\(idx, [y:ys])<-zip2 [0..] x]

eval :: Instr Int -> Int
eval (Write (Int i)) s = i
eval (Write (Plus e1 e2)) s = eval (Write e1) s + eval (Write e2) s
eval (Write (Times e1 e2)) s = eval (Write e1) s * eval (Write e2) s
eval (Write Read) s = s
eval (Atomic x) s = hd (produce [x] s)

prog0 = [Write (Int 12), Write (Plus Read (Int 1))]
prog1 = [Write (Times Read (Int 2))]
test0 = [prog0]
test1 = [prog0, prog1]

Start = 0
/* Writing the semantics in clean is advantageous because you can execute the
 * code, have it type checked and it is very readable for a programmer.
 *
 * Disadvantages might be that it might be harder to prove stuff. Also
 * mathematic notation is generally more concise.
 * 
 * Some things are modelled very difficultly in a functional language. For
 * example file io, parallel processes or user interaction.
*/