[Haskell-cafe] Maybe use advice
Maciej Marcin Piechotka
uzytkownik2 at gmail.com
Mon Jun 6 21:11:44 CEST 2011
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
> 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?
f (Rotate x : Rotate x' : l)
= Just $ f (Rotate (x+x') : fromMaybe l (f l))
f l = Nothing -- As far as I understend
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