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