[Haskell-beginners] let expression

[Daniel Fischer]

Am Mittwoch 10 Februar 2010 21:16:31 schrieb John Moore:
> 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.
>

> 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

Here?

Indentation. The patterns for the inner case-expression must be indented 
further than those for the outer (and all patterns in one case-expression 
must be indented to the same level).

>
>
> 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