It's not a dumb question at all!
You will find the `(+++)`/`left`/`right` functions from this lpaste useful:
http://lpaste.net/101606
They are all more efficient than `tee`. You can use them any time you
need to operate on just the `Left` or `Right` values of a stream.
An even cleaner solution (when possible) is to model your consumers as
write handles, as described in this post:
http://www.haskellforall.com/2014/03/how-to-model-handles-with-pipes.html
Specifically, I mean this:
write1 :: a -> Effect' m ()
write2 :: b -> Effect' m ()
Then it's really easy to merge them into a single write handle:
writeTotal :: Either a b -> Effect m ()
writeTotal = either write1 write2
However, that trick only works if your handles do the exact same action
for every incoming value. In other words, it only works if your two
consumers are of the form:
consumer1 :: Consumer' a m r
consumer1 = for cat write1
consumer2 :: Consumer' b m r
consumer2 = for cat write2
-- Merge them into a single consumer like this:
consumerTotal :: Consumer' (Either a b) m r
consumerTotal = for cat (either write1 write2)
That's the only case that permits this simpler trick. Otherwise use
something like the `(+++)`/`left`/`right` operations I linked to.
On 03/21/2014 02:38 AM, Karl-Oskar Rikås wrote:
Never mind, this was a stupid question. I can just make a consumer
that ignores Left values and then write another pipe that ignores all
Right values.
Regards, klrr
On Thursday, March 20, 2014 5:19:56 PM UTC+1, Karl-Oskar Rikås wrote:
Okey, writing some pipes code I ended up writing a function that
takes two producers and turn them into one.
```
merge :: Producer a m r -> Producer a m r -> Producer (Either a a)
m ()
merge pL pR = do
(output, input) <- lift $ spawn Unbounded
lift . forkIO $ do runEffect $ pL >-> P.map Left >-> toOutput output
performGC
lift . forkIO $ do runEffect $ pR >-> P.map Right >-> toOutput
output
performGC
fromInput input
```
Now I also need something like Pipes.Prelude.tee but that only
consumes Left values and forward Right values without consuming
them, looking at the source of tee I got quite confused. Has
anybody written such function or could help me implement it?
Regards, klrr
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