> import STFM

> data Com = Assign Var Exp
>          | Read Var
>          | Write Exp
>          | While Exp [Com]

> type Var = String

> data Exp = Ref Var
>          | Num Integer
>          | Bin BinOp Exp Exp

> data BinOp = Plus
>            | Minus
>            | Times
>            | Div

> interpret :: [Com] -> [Integer] -> [Integer]
> interpret cs input = runSTFM (fmap snd $ evalComs cs input)

> evalExp :: Exp -> STFM Integer
> evalExp ...

> evalCom :: Com -> [Integer] -> STFM ([Integer],[Integer])
> evalCom ...

> evalComs :: [Com] -> [Integer] -> STFM ([Integer],[Integer])
> evalComs ...