trace -package:matrix package:streamly

trace :: (IsStream t, MonadAsync m) => (a -> m b) -> t m a -> t m a
streamly Streamly.Internal.Data.Stream.IsStream.Transform Streamly.Prelude
Apply a monadic function to each element flowing through the stream and discard the results. 
>>> Stream.drain $ Stream.trace print (Stream.enumerateFromTo 1 2)
1
2
Compare with tap.
trace_ :: (IsStream t, Monad m) => m b -> t m a -> t m a
streamly Streamly.Internal.Data.Stream.IsStream.Transform
Perform a side effect before yielding each element of the stream and discard the results. 
>>> Stream.drain $ Stream.trace_ (print "got here") (Stream.enumerateFromTo 1 2)
"got here"
"got here"
Same as intersperseMPrefix_ but always serial. See also: trace Pre-release