Thanks for the tips Tran and John, this is what I came up with:
foldValues :: (Monad m, Eq k) => (v -> v -> v) -> Producer (k, v) m r ->
Producer (k, v) m r
foldValues append xs =
P.concat <-< folds step Nothing id (view (groupsBy keyEq) xs)
where
keyEq (k, _) (k', _) = k == k'
step (Nothing) (k, v) = Just (k, v)
step (Just (_, v0)) (k, v) = Just (k, v0 `append` v)
Is there any way I can get this to have the type `Pipe (k, v) (k, v) m r`?
Ideally I would like to be able to apply this operation to a consumer as
well.
On Thursday, June 11, 2015 at 5:05:45 PM UTC+10, Tran Ma wrote:
>
> At some point I had to do something similar (summarise values by keys,
> assuming duplicate keys are in a continuous range) and used pipes-group
> https://hackage.haskell.org/package/pipes-group
>
> On Thursday, 11 June 2015 10:53:19 UTC+10, Jacob Stanley wrote:
>>
>> I have a situation which I found easy to solve with `conduit` but could
>> not figure out with `pipes`.
>>
>> -- | Given a stream of key/value pairs, fold all the values associated
>> with a
>> -- key in to a single value. Assumes that the stream has already been
>> -- sorted/grouped by key.
>> foldValues :: forall m k v. (Monad m, Eq k) => (v -> v -> v) -> Conduit
>> (k, v) m (k, v)
>> foldValues step = goM =$= C.catMaybes
>> where
>> goM :: Conduit (k, v) m (Maybe (k, v))
>> goM = do
>> s <- C.mapAccum go Nothing
>> case s of
>> Nothing -> yield Nothing
>> Just (k, x) -> yield (Just (k, x))
>>
>> go :: (k, v) -> Maybe (k, v) -> (Maybe (k, v), Maybe (k, v))
>> go (k, v) (Nothing) = (Just (k, v), Nothing)
>> go (k, v) (Just (k0, x)) | k == k0 = (Just (k, step x v), Nothing)
>> | otherwise = (Just (k, v), Just (k0,
>> x))
>>
>>
>> It seems to be related to the fact that with `pipes` I cannot observe
>> that the stream has stopped emitting values. I had a quick look at
>> `pipes-parse` and it seems like maybe that would help?
>>
>
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