Hi John Provided the whitespace doesn't get mangled in the mail the last evalStep needs indenting correctly: evalStep d (Let n e1 e2) = case e1 of (Val a) -> case e2 of (Val b)-> Val (Let e1 e2) left -> Let e1 (evalStep d e2) right -> Let (evalStep d e1) e2 Also the "r" case in evaluate: "r" -> case (x:xs) of (x:xs) -> evaluate d xs x [] -> do { putStrLn "Empty" ;evaluate d(x:xs) e } You might want to consider using wildcards (i.e. underscores) _ rather than variables e.g 'left' and 'right' when you don't use the value of a pattern match at the right of the "->". Best wishes Stephen On 10 February 2010 20:16, John Moore wrote:

Hi All,          Trying to get this to work, keeps telling me there a parse error on right in the let expression. Can anyone see where the problem is and be able to explain it to me.

import Maybe data Expression = Val Double                | Add Expression Expression                | Subtract Expression Expression                | Multiply Expression Expression                | Divide Expression Expression         | Var String         | Let String Expression Expression         deriving Show demo1 = (Add(Multiply(Divide(Subtract(Val 25)(Val 5))(Val 10))(Val 7))(Val 30)) type Dict =[(String,Expression)] emptyDict :: Dict emptyDict = [] addEntry :: String->Expression ->Dict -> Dict addEntry n e d = (n,e): d lookupEntry :: String -> Dict -> Maybe Expression lookupEntry n [] = Nothing lookupEntry n (x:xs) = if (n == k)    then (Just v)                 else lookupEntry n xs    where (k,v) = x evalStep :: Dict -> Expression ->  Expression evalStep d(Val x)=   (Val x) evalStep d(Add x y)   = case x of       (Val a) -> case y of                    (Val b) ->  Val (a+b)                    left -> Add x (evalStep d y)       right -> Add (evalStep d x)y evalStep d(Subtract x y)   = case x of       (Val a) -> case y of                    (Val b) -> Val (a-b)                    left -> Subtract x (evalStep d y)       right -> Subtract (evalStep d x)y evalStep d(Multiply x y)   = case x of       (Val a) -> case y of                    (Val b) -> Val (a*b)                    left -> Multiply x (evalStep d y)       right -> Multiply (evalStep d x)y evalStep d (Divide x y)   = case x of       (Val a) -> case y of                    (Val b) -> Val (a/b)                    left -> Divide x (evalStep d y)       right -> Divide (evalStep d x)y evalStep d (Let n e1 e2)    = case e1 of        (Val a) -> case e2 of   (Val b)-> Val (Let e1 e2)          left -> Let e1 (evalStep d e2)       right -> Let (evalStep d e1) e2

evaluate :: Dict-> [Expression] -> Expression -> IO() evaluate d(x:xs) e = do      putStrLn (show e)      putStrLn "Do another step (y/n) or rollback (r)? :"      c <- getLine      case c of        "y" -> let e'= (evalStep d e)in evaluate d (e:x:xs) e'-- build up history

       "r" ->  case (x:xs) of   (x:xs)-> evaluate d xs x   []-> do { putStrLn "Empty"    ;evaluate d(x:xs) e   }        "n"  ->  putStrLn $ "Ok you said no :" ++ c

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