-- Identifiers are synonyms for strings
type Id = String

-- Binary operators
data Binop = Plus | Minus | Times | Div
 deriving (Eq,Show)

-- Statements
data Stm = CompoundStm Stm Stm
         | AssignStm Id Exp
	 | PrintStm [Exp]
 deriving (Eq,Show)

-- Expressions
data Exp = IdExp Id
	 | NumExp Int
         | OpExp Exp Binop Exp
         | EseqExp Stm Exp
 deriving (Eq,Show)

-- Representation of the program
--  "a := 5+3 ; b := (print(a,a-1),10*a); print(b)"
-- as a derivation tree:
prog = 
 CompoundStm (AssignStm "a" (OpExp (NumExp 5) Plus (NumExp 3)))
  (CompoundStm
     (AssignStm "b"
                (EseqExp (PrintStm [IdExp "a",
                                    OpExp (IdExp "a") Minus (NumExp 1)])
                         (OpExp (NumExp 10) Times (IdExp "a"))))
     (PrintStm [IdExp "b"]))