On Sun, Feb 14, 2010 at 3:20 PM, Daniel Fischer
Am Sonntag 14 Februar 2010 20:54:54 schrieb John Moore:
Hi all, I must be doing something wrong with my tabs. Because this keeps telling me parse error. Can some one see if it is indentation. I'm sending a file so as you'll be able to see what going on. If I copy it here it can appear wrong the difference is with the Var expression.
John
First of all, *don't mix tabs and spaces for indentation* (best, don't use tabs at all).
I find the latter a bit too restrictive, especially since I use a text editor that is a glorified version of ed. Hitting the spacebar 19 times is too much to ask of me. And as such, I am occasionally forced to do something when a tab doesn't get me past the word `where' or `let', but isn't that the majority of what Layout specifies? Is there a good answer for caveman programmers who like ed and tabs?
It's
evalStep d (Var x) = case x of (Val a) ->case x of (Val b) ->(lookup x d) if (isJust m_e)then evalStep(lookup x d) else fail "Error in expression -- no definition for variable!"
The "if ... " is aligned with (Val b), so GHC tries to parse it as a pattern. But if-expressions aren't patterns, so you get a parse error.
But the code doesn't really make sense (and isn't well typed) anyway. You probably want something like
evalStep d (Var x) = case lookup x d of Just e -> e Nothing -> error "Error in expression -- no definition for variable!"
for that. _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners