John Moore john.moore54 at gmail.com
Sun Feb 7 15:14:17 EST 2010

```Hi,
This gives me the explaination that it expecting a Expression but it been
giving a IO b can some explain whats going on please.

import Maybe
data Expression = Val Double
| 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)

= 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) = do
v <- evalStep d (addEntry n e1 d) e2
putStrLn ("Removing definition for "++n)
return v

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

John
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