[Haskell-cafe] Maybe use advice

[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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