[Haskell-cafe] Maybe use advice

[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
<uzytkownik2 at 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
>
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