[Hs-Generics] New scheme proposal: everythingWithContext

[José Pedro Magalhães]

Hi,

Sorry for taking so long to reply. I don't have a strong opinion about
this; if no one objects I'll add it.


Thanks,
Pedro

On Tue, May 15, 2012 at 2:51 AM, Andrew Miller <ak.miller at auckland.ac.nz>wrote:

> Hi,
>
> One thing that is hard to do with SYB (as well as with alternative
> generics packages in Haskell) at present is to query while keeping state
> that is carried down the tree but not to siblings, and use that in deciding
> what to return from the query.
>
> I propose a new scheme be added to Data.Generics.Schemes, called
> everythingWithContext (as defined below). An everywhereWithContext might
> also be useful, but I don't think you would be able to define the
> transformation using the existing combinators, so I have limited my
> proposal to everythingWithContext for now.
>
> Yours Sincerely,
> Andrew Miller
>
> {- | Summarise all nodes in top-down, left-to-right order, carrying some
> state down
>     the tree during the computation, but not left-to-right to siblings.
>     Example: Suppose you want to compute the maximum depth of adds in the
> below
>      simple co-recursive structure, ignoring all the other constructors.
> You could
>      write code like the following:
>
> data MyStructure = SomeConst Int | Add MyStructure MyStructure | Times
> MyStructure MyStructure | Wrapped Wrapper deriving (Data, Typeable)
> data Wrapper = Wrapper MyStructure deriving (Data, Typeable)
>
> myExample = Add (SomeConst 10) (Add (Wrapped . Wrapper $ (Add (Add (Add
> (Times (SomeConst 30) (SomeConst 90)) (SomeConst 70)) (SomeConst 40))
> (SomeConst 50))) (Add (SomeConst 20) (Add (SomeConst 60) (SomeConst 80))))
>
> computeDepth = everythingWithContext 0 max ((\s -> (0, s)) `mkQ`
> depthOfAdd)
>  where
>    depthOfAdd (Add _ _) s = (s, s + 1)
>    depthOfAdd _ s = (s, s)
>
> main = print $ computeDepth myExample
>  -}
> everythingWithContext :: s -> (r -> r -> r) -> GenericQ (s -> (r, s)) ->
> GenericQ r
> everythingWithContext s0 f q x =
>  foldl f r (gmapQ (everythingWithContext s' f q) x)
>    where (r, s') = q x s0
>
>
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