Re: [Haskell-cafe] Maybe use advice

On Tue, 2011-06-07 at 04:09 +0800, Lyndon Maydwell wrote:

(missed including cafe)

f :: [Modification] -> Maybe [Modification] and f _ = Just $ f ... are incompatible

My bad: f ... = let cs' = (Rotate (x+x') : fromMaybe cs (f cs)) in fromMaybe cs (f cs) Or refactoring it: g l = fromMaybe l (f l) f (Rotate x : Rotate x' : cs) = g (Rotate (x+x') : g cs) Regards

I managed to get the behaviour I'm after with the use of Either, but this really is messy:

-- Sets of changes o (Modifier (Changes []) i) = Just $ i o (Modifier (Changes [c]) i) = Just $ Modifier c i o (Modifier (Changes l) i) = g (f (Left l)) where g (Right l) = Just $ Modifier (Changes l) i g (Left l) = Nothing

f (Left (Scale x y : Scale x' y' : l)) = f $ Right $ Scale (x*x') (y*y') : h (f $ Left l) f (Left (Translate x y : Translate x' y' : l)) = f $ Right $ Translate (x+x') (y+y') : h (f $ Left l) f (Left (Rotate x : Rotate x' : l)) = f $ Right $ Rotate (x+x') : h (f $ Left l) f x = x

h (Left l) = l h (Right l) = l

On Tue, Jun 7, 2011 at 3:11 AM, Maciej Marcin Piechotka wrote:

...

On Mon, 2011-06-06 at 23:38 +0800, Lyndon Maydwell wrote:

...

I'm writing an optimisation routine using Uniplate. Unfortunately, a sub-function I'm writing is getting caught in an infinite loop because it doesn't return Nothing when there are no optimisations left.

I'd like a way to move the last Just into f, but this makes recursion very messy. I was wondering if there was a nice way to use something like the Monad or Applicative instance to help here.

-- Sets of changes o (Modifier (Changes []) i) = Just $ i o (Modifier (Changes [c]) i) = Just $ Modifier c i o (Modifier (Changes l) i) = Just $ Modifier (Changes (f l)) i where f (Scale x y : Scale x' y' : l) = f $ Scale (x*x') (y*y') : f l f (Translate x y : Translate x' y' : l) = f $ Translate (x+x') (y+y') : f l f (Rotate x : Rotate x' : l) = f $ Rotate (x+x') : f l f l = l

Any ideas?

Something like:

... f (Rotate x : Rotate x' : l) = Just $ f (Rotate (x+x') : fromMaybe l (f l)) f l = Nothing -- As far as I understend

Regards

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe