Am Samstag 05 Dezember 2009 21:09:01 schrieb Brent Yorgey:
If, on the other hand, you want the indentation to correspond to how "deep" within the expression the evaluation is taking place, then you can recursively pass along an extra parameter to your evaluation function, like so:
eval :: Dict -> Expression -> ([String], Double) eval d e = evalIndented 0 d e where evalIndented i d (Val x) = ([], x) evalIndented i d (Add x y) = let (tx, vx) = evalIndented (i+1) d x (ty, vy) = evalIndented (i+1) d y in (replicate i ' ' ++ "Add " ++ show vx ++ " and " ++ show vy, vx + vy)
...and so on.
I would even suggest eval :: Dict -> Expression -> ([(Int,String)],Double) eval d e = evalIndented 0 d e where evalIndented _ _ (Val x) = ([],x) evalIndented i d (Add x y) = let ... in ((i,"Add " ++ show vx ++ " and " ++ show vy):tx ++ ty, vx + vy) ... then you can easily extract the traces of subexpressions at a given depth and print those with adjusted indentation and change indentation schemes (indent two/four spaces further per level).