add Cap like in Bus

[safareli]

I have a module which looks like this:

module Control.Monad.Aff.AVar.RW
  ( module Control.Monad.Eff.AVar
  , AVar
  , Cap
  , AVarR
  , AVarR'
  , AVarW
  , AVarW'
  , AVarRW
  , split
  , makeVar
  , makeEmptyVar
  , status
  , isEmptyVar
  , isFilledVar
  , isKilledVar
  , takeVar
  , tryTakeVar
  , putVar
  , tryPutVar
  , readVar
  , tryReadVar
  , killVar
  ) where

import Prelude

import Control.Monad.Aff.AVar (takeVar, putVar, readVar) as AffAvar
import Control.Monad.Aff.Class (class MonadAff, liftAff)
import Control.Monad.Eff.AVar (AVAR, AVarStatus(..), isEmpty, isFilled, isKilled)
import Control.Monad.Eff.AVar as AVar
import Control.Monad.Eff.Class (class MonadEff, liftEff)
import Control.Monad.Eff.Exception (Error)
import Data.Maybe (Maybe)
import Data.Tuple (Tuple(..))

data Cap

data AVar (r ∷ # Type) a = AVar (AVar.AVar a)
type AVarR = AVarR' ()
type AVarR' r = AVar (read ∷ Cap | r)
type AVarW = AVarW' ()
type AVarW' r = AVar (write ∷ Cap | r)
type AVarRW = AVar (read ∷ Cap, write ∷ Cap)

split ∷ forall a. AVarRW a -> Tuple (AVarR a) (AVarW a)
makeVar ∷ forall m eff a. MonadEff (avar ∷ AVAR | eff) m => a -> m (AVarRW a)
makeEmptyVar ∷ forall m eff a. MonadEff (avar ∷ AVAR | eff) m => m (AVarRW a)
status ∷ forall m eff a r. MonadEff (avar ∷ AVAR | eff) m => AVarR' r a -> m (AVar.AVarStatus a)
isEmptyVar ∷ forall m eff a r. MonadEff (avar ∷ AVAR | eff) m => AVarR' r a -> m Boolean
isFilledVar ∷ forall m eff a r. MonadEff (avar ∷ AVAR | eff) m => AVarR' r a -> m Boolean
isKilledVar ∷ forall m eff a r. MonadEff (avar ∷ AVAR | eff) m => AVarR' r a -> m Boolean
takeVar ∷ forall m eff a. MonadAff (avar ∷ AVAR | eff) m => AVarRW a -> m a
tryTakeVar ∷ forall m eff a. MonadEff (avar ∷ AVAR | eff) m => AVarRW a -> m (Maybe a)
putVar ∷ forall m eff a r. MonadAff (avar ∷ AVAR | eff) m => a -> AVarW' r a -> m Unit
tryPutVar ∷ forall m eff a r. MonadEff (avar ∷ AVAR | eff) m => a -> AVarW' r a -> m Boolean
readVar ∷ forall m eff a r. MonadAff (avar ∷ AVAR | eff) m => AVarR' r a -> m a
tryReadVar ∷ forall m eff a r. MonadEff (avar ∷ AVAR | eff) m => AVarR' r a -> m (Maybe a)
killVar ∷ forall m eff a r. MonadEff (avar ∷ AVAR | eff) m => Error -> AVarW' r a -> m Unit

The Cap is exact same thing as in Bus and i find it very useful. for example if are passing AVar to someone they can do many staff with it but if you instead pass AVarW you know they can only write to it and not read.

if you like I can make similar changes (except Monad{Aff,Eff} i guess) and then in ps-bus we can reuse this Cap.

[safareli]

We can add this as new module or just update current Avar type. if we want backwards compatibility then we can rename Avar to Avar' and add type Avar = AvarRW.

[safareli]

One interesting observation I came up to while Avar with RW staff is that for example AVarW Void can be used as canceler of some computation as only operation one can perform on it is to kill with Error.