> import Parsing
>
> type Ide  =  String
>
> data Exp  =  Zero | One | TT | FF | Read | I Ide | Not Exp | Equal Exp Exp | Plus Exp Exp
>              deriving Show
>
> data Cmd  =  Assign Ide Exp | Output Exp |IfThenElse Exp Cmd Cmd | WhileDo Exp Cmd | Seq Cmd Cmd
>              deriving Show
>
>
>
> expr                          :: Parser Exp
> expr                          =  do e1 <- term
>                                     do symbol "+"
>                                        e2 <- expr
>                                        return (Plus e1 e2)
>                                  +++
>                                  do e1 <- term
>                                     do symbol "="
>                                        e2 <- expr
>                                        return (Equal e1 e2)
>                                  +++
>                                  term
>
> term                          :: Parser Exp
> term                          = do symbol "not"
>                                    do e1 <- term
>                                       return (Not e1)
>                                 +++
>                                 factor
>
> factor                        :: Parser Exp
> factor                        =  do symbol "read"
>                                     return Read
>                                  +++
>                                  do symbol "false"
>                                     return FF
>                                  +++
>                                  do symbol "true"
>                                     return TT
>                                  +++
>                                  do symbol "0"
>                                     return Zero
>                                  +++
>                                  do symbol "1"
>                                     return One
>                                  +++
>                                  do i1 <- identifier
>                                     return (I i1)
>                                  +++
>                                  do symbol "("
>                                     do e1 <- expr
>                                        do symbol ")"
>                                           return (e1)
>
> cmds                          :: Parser Cmd
> cmds                          =  do i1 <- identifier
>                                     do symbol ":="
>                                        do e1 <- expr
>                                           return (Assign i1 e1)
>                                  +++
>                                  do symbol "output"
>                                     do e1 <- expr
>                                        return (Output e1)
>                                  +++
>                                  do symbol "if"
>                                     do e1 <- expr
>                                        do symbol "then"
>                                           do c1 <- cmds
>                                              do symbol "else"
>                                                 do c2 <- cmds
>                                                    return (IfThenElse e1 c1 c2)
>                                  +++
>                                  do symbol "while"
>                                     do e1 <- expr
>                                        do symbol "do"
>                                           do c1 <- cmds
>                                              return (WhileDo e1 c1)
>                                  +++
>                                  do c1 <- cmds
>                                     do symbol ";"
>                                        do c2 <- cmds
>                                           return (Seq c1 c2)
>
>
> eeval                          :: String -> Exp
> eeval xs                       =  case (parse expr xs) of
>                                     [(n,[])]  -> n
>                                     [(_,out)] -> error ("unused input " ++ out)
>                                     []        -> error "invalid input"