haskell - Elegant egress from `Async` actions -


is there simple way of cancelling async a value, such won't interrupted in middle of critical action? suppose use semaphore inside loop condition.

async $ whilem readsemaphore runloopbody

but i'd know whether async or other related library supports out-of-the-box.

my code, requested.

-- | -- todo | - rename (?) --        - time-out awaitresult :: string -> io -> io awaitresult s act =   putstr s   sem <- newmvar true   <- async $ ellipsis sem   r <- act   swapmvar sem false   return r       ellipsis :: mvar bool -> io ()     ellipsis sem = void $       whilem (readmvar sem) $ form [".  ", ".. ", "...", "   "] $ \dots ->         putstr dots         cursorbackward 3         threaddelay (floor $ 0.4 * second)       -- todo | - if windows console wasn't shit, i'd use checkmark       putstr " (" >> withpretty fggreen "done" >> putstrln ")"

the phrasing of question made me think of more antagonistic relationship, in case using control.concurrent.mask limit when task can interrupted should reasonable.

based on updated code seems threads have tight coupling , first alternative can think of spawning action instead of indicator , using poll, think less noisy mvar route:

import control.monad (form_) import control.exception (throw) import control.concurrent (threaddelay) import control.concurrent.async (async,async,poll)  second :: (num a) => second = 1000000  awaitresult :: string -> io -> io awaitresult s act =   putstrln s   <- async $ act   ellipsis       ellipsis :: async -> io     ellipsis =       result <- poll       case result of         nothing ->           form_ [".  ",".. ","...","   "] $ \dots ->             putstr dots             putstr "\r"             threaddelay $ floor $ 0.4 * second           ellipsis         (left e) -> throw e         (right x) -> return x  main = awaitresult "testing" (threaddelay (5 * second) >> return 5)

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