A module containing broadcast channels (one-to-many). Whereas a one-to-any
channel features one writer sending a single value to one (of many) readers, a
one-to-many channel features one writer sending the same value to many
readers. So a one-to-any channel involves claiming the channel-end to ensure
exclusivity, but a one-to-many channel involves enrolling on the channel-end
(subscribing) before it can engage in communication.
A communication on a one-to-many channel only takes place when the writer
and all readers currently enrolled agree to communicate. What happens when
the writer wants to communicate and no readers are enrolled is undefined
(the writer may block, or may communicate happily to no-one).
This module also contains reduce channels (added in version 1.1.1). Because
in CHP channels must have the same type at both ends, we use the Monoid
type-class. It is important to be aware that the order of mappends will be
non-deterministic, and thus you should either use an mappend that is commutative
or code around this restruction.
For example, a common thing to do would be to use lists as the type for
reduce channels, make each writer write a single item list (but more is
possible), then use the list afterwards, but be aware that it is unordered.
If it is important to have an ordered list, make each writer write a pair
containing a (unique) index value and the real data, then sort by the index
value and discard it.
Since reduce channels were added after the initial library design, there
is a slight complication: it is not possible to use newChannel (and all
similar functions) with reduce channels because it is impossible to express
the Monoid constraint for the Channel instance. Instead, you must use manyToOneChannel