Thanks Maciej! An additional Just was required in the refactored version of f:
f (Rotate x : Rotate x' : cs) = Just $ g (Rotate (x+x') : g cs) This is much cleaner than what I was doing. On Tue, Jun 7, 2011 at 4:14 AM, Maciej Piechotka <uzytkown...@gmail.com> wrote: > 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 >> <uzytkown...@gmail.com> 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 >> > >> > > > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
