Re: [Haskell-cafe] Maybe use advice

Is it necessary (helpful) to use 'rewrite'? Nearly every time I've tried it, in the end 'transform' has been a better choice. Then you wouldn't need the 'Just's at all, and it should work fine. John

From: Lyndon Maydwell

(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 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

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