Stephen Tetley stephen.tetley at gmail.com
Wed Feb 10 15:26:34 EST 2010

```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 <john.moore54 at gmail.com> 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
>                | 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)
>    = 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
>
>
> John
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