(missed including cafe)
f :: [Modification] -> Maybe [Modification]
and
f _ = Just $ f ...
are incompatible
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
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
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