a11/mart/skeleton11.icl

   1 module skeleton11
   2
   3 import StdEnv
   4
   5 :: Prog :== [Instr]
   6 :: Instr = Write Expr | Atomic [Instr]
   7 :: Expr = Int   Int
   8         | Plus  Expr Expr
   9         | Times Expr Expr
  10         | Read
  11
  12 possibleResults :: [Prog] -> [Int]
  13 possibleResults x = produce x 0
  14
  15 produce :: [Prog] Int -> [Int]
  16 produce x i
  17 | all (isEmpty) x = [i]
  18 = flatten [produce (updateAt idx ys x) (eval y i)\\(idx, [y:ys])<-zip2 [0..] x]
  19
  20 eval :: Instr Int -> Int
  21 eval (Write (Int i)) s = i
  22 eval (Write (Plus e1 e2)) s = eval (Write e1) s + eval (Write e2) s
  23 eval (Write (Times e1 e2)) s = eval (Write e1) s * eval (Write e2) s
  24 eval (Write Read) s = s
  25 eval (Atomic x) s = hd (produce [x] s)
  26
  27 prog0 = [Write (Int 12), Write (Plus Read (Int 1))]
  28 prog1 = [Write (Times Read (Int 2))]
  29 test0 = [prog0]
  30 test1 = [prog0, prog1]
  31
  32 Start = 0
  33 /* Writing the semantics in clean is advantageous because you can execute the
  34  * code, have it type checked and it is very readable for a programmer.
  35  *
  36  * Disadvantages might be that it might be harder to prove stuff. Also
  37  * mathematic notation is generally more concise.
  38  *
  39  * Some things are modelled very difficultly in a functional language. For
  40  * example file io, parallel processes or user interaction.
  41 */