Enroll.hs

-- Communicating Haskell Processes.
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-- | A module with support for things that are enrollable (barriers and broadcast
-- channels).

module Control.Concurrent.CHP.Enroll (Enrolled, Enrollable(..), furtherEnroll,
  enrollPair, enrollList) where

import Control.Concurrent.CHP.Base

class Enrollable b z where
  -- | Enrolls on the given barrier, then executes the given function (passing
  -- it in the enrolled barrier) and when that function body finishes, resigns
  -- from the barrier and returns the result.  If a poison or IO exception is thrown
  -- in the inside block, the barrier will be correctly resigned from, and
  -- the exception rethrown.
  --
  -- Do not attempt to return the
  -- enrolled barrier out of the inner function and use it afterwards.
  enroll :: b z -> (Enrolled b z -> CHP a) -> CHP a

  -- | Resigns from a barrier, then executes the given action, and when the
  -- action finishes, re-enrolls on the barrier and continues.  This function
  -- is designed for use from /inside/ the body of the 'enroll' function, to
  -- temporarily resign from a barrier, do some things, then re-enroll.  Do
  -- not use the enrolled barrier inside the resign block.  However, you may
  -- enroll on the barrier inside this, nesting enroll and resign blocks as
  -- much as you like
  resign :: Enrolled b z -> CHP a -> CHP a

-- | Just like enroll, but starts with an already enrolled item.  During the
-- body, you are enrolled twice -- once from the original enrolled item (which
-- is still valid in the body) and once from the new enrolled item passed to
-- the inner function.  Afterwards, you are enrolled once, on the original item
-- you had.
--
-- This function was added in version 1.3.2.
furtherEnroll :: Enrollable b z => Enrolled b z -> (Enrolled b z -> CHP a) -> CHP a
furtherEnroll (Enrolled x) = enroll x

-- | Like 'enroll' but enrolls on the given pair of barriers
enrollPair :: (Enrollable b p, Enrollable b' p') => (b p, b' p') -> ((Enrolled
  b p, Enrolled b' p') -> CHP a) -> CHP a
enrollPair (b0,b1) f = enroll b0 $ \eb0 -> enroll b1 $ \eb1 -> f (eb0, eb1)

-- | Like 'enroll' but enrolls on the given list of barriers
enrollList :: Enrollable b p => [b p] -> ([Enrolled b p] -> CHP a) -> CHP a
enrollList [] f = f []
enrollList (b:bs) f = enroll b $ \eb -> enrollList bs $ \ebs -> f (eb:ebs)