Occurs to me that a common async pattern is going to be to put an
unconditional infinite loop in a go block, so we might want a macro for
such loops if they'll become much more common:
(defmacro endlessly
"Executes the body over and over again."
[& body]
`(loop [] ~@(concat body '(recur))))
(go
(endlessly
(let [x (<! c1)]
(!> c2 x)
(!> c3 x))))
Or even a go-forever:
(defmacro go-forever
"Like 'go', but repeats its body forever."
[& body]
`(go (loop [] ~@(concat body '(recur)))))
Also, if the go is only useful as long as there's a value from some source:
(defmacro while-let
"Like when-let, but repeats its body if it succeeded the last time"
[[s expr] & body]
`(loop []
(when-let [~s ~expr]
~@body
(recur))))
(go
(while-let [x (<! c1)]
(!> c2 x)
(!> c3 x)))
That version should halt if c1 is closed by its producer. (It won't close
c2 and c3; on the other hand a go consuming from either will park forever
once those stop receiving values. The original splitter would park forever
if c1 stopped getting values, but would load c2 and c3 with nils for as
long as something was consuming from each channel if c1 was closed.)
One can also envision (go (while-let [x (alt! ...)] ...)), which would run
until all of the alternatives were closed.
If one wants to close c2 and c3 if c1 is closed, though, it's more
annoying, as one wants a multiline body if c1 has a value and a different
multiline body to execute a single time when c1 is closed. At that point
there's no nice macrology that can make things much more concise than, but
as readable as, a loop with an if-let and two do blocks inside:
(go
(loop []
(if-let [x (<! c1)]
(do (>! c2 x) (>! c3 x) (recur))
(do (close! c2) (close! c3)))))
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