How to merge two Consumer into one in Haskell Pipes? -


i use haskell stream processing library pipes write command line tool. each command line actions may output result stdout , logs stderr pipes api.

i need consumer has type consumer (either string string) m r print chunk of data (left stderr, right stdout) single consumer.

code wrote (should improved)

this function consumeeither doesn't have flexibility want improve it.

consumeeither :: (monadio m) => consumer (either string string) m () consumeeither =   eithers <- await   case eithers of     (left l)  -> (yield l) (liftio . (io.hputstrln io.stderr))     (right r) -> (yiled r) (liftio . putstrln)

furthermore useful provide function takes 2 consumers , merge them 1 consumer.

question

does know example or implementation of following interface?

merge :: (monad m) => consumer m r -> consumer b m r -> consumer (either b) m r
  • 1st argument stderr
  • 2nd argument stdout

usage of function

import           pipes import qualified pipes.prelude p import qualified system.io io  stdoutorerr :: consumer (either string string) io () stdoutorerr = merge (p.tohandle io.stderr) p.stdoutln

thanks

(this @michael's answer, i'd write here can move question out of unanswered queue haskell tag.)

see (+++) in pipes-extras. keep in mind consumer pipe (to nowhere), p.tohandle io.stderr +++ p.stdoutln :: monadio m => pipe (either string string) (either b d) m ().

to consumer, have rid of lefts e.g >-> p.concat or >-> p.drain. there more robust , handsome ways of doing folds.


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