    {-# LANGUAGE TypeFamilies, CPP, TypeSynonymInstances, MultiParamTypeClasses,
        FlexibleInstances, EmptyDataDecls  #-}

    #ifdef DEFAULT_SIGNATURES
    {-# LANGUAGE DefaultSignatures #-}
    #endif

    {- |
       An abstract, parameterizable interface for queues.

       This interface includes a non-associated type family for Deques
       plus separate type classes encapsulating the Deque operations.
       That is, we separate type selection (type family) from function overloading
       (vanilla type classes).

       This design strives to hide the extra phantom-type parameters from
       the Class constraints and therefore from the type signatures of
       client code.

    -}
    module Data.Concurrent.Deque.Class
     (
      -- * Highly parameterized Deque type(s)
       Deque
      -- ** The choices that select a queue-variant.
      -- *** Choice #1 -- thread safety.
     , Threadsafe, Nonthreadsafe
      -- *** Choice #2 -- double or single functionality on an end.
     , SingleEnd, DoubleEnd
      -- *** Choice #3 -- bounded or growing queues:
     , Bound, Grow
      -- *** Choice #4 -- duplication of elements.
     , Safe, Dup
      -- ** Aliases enabling more concise Deque types:
     , S, D, NT, T

      -- ** Aliases for commonly used Deque configurations:
     , Queue, ConcQueue, ConcDeque, WSDeque

      -- * Class for basic Queue operations
     , DequeClass(..)

      -- * Extra capabilities: type classes

       -- | These classes provide a more programmer-friendly constraints than directly
       -- using the phantom type parameters to constrain queues in user code.  Also note
       -- that instances can be provided for types outside the type `Deque` type family.
       --
       -- We still make a distinction between the different capabilities
       -- (e.g. single-ended / double ended), and thus we need the below type classes for
       -- the additional operations unsupported by the minimal "DequeClass".

      -- ** The \"unnatural\" double ended cases: pop left, push right.
     , PopL(..), PushR(..)

      -- ** Operations that only make sense for bounded queues.
     , BoundedL(..), BoundedR(..)
    )

       where

    import Prelude hiding (Bounded)

    {- |

     A family of Deques implementations.  A concrete Deque implementation
     is selected based on the (phantom) type parameters, which encode
     several choices.

     For example, a work stealing deque is threadsafe only on one end and
     supports push/pop on one end (and pop-only) on the other:

      >> (Deque NT T  D S Grow elt)

     Note, however, that the above example is overconstraining in many
     situations.  It demands an implementation which is NOT threadsafe on
     one end and does NOT support push on one end, whereas both these
     features would not hurt, if present.

     Thus when accepting a queue as input to a function you probably never
     want to overconstrain by demanding a less-featureful option.

       For example, rather than @(Deque NT D T S Grow elt)@
       You would probably want: @(Deque nt D T s Grow elt)@

     -}
    -- data family Deque lThreaded rThreaded lDbl rDbl bnd safe elt
    type family Deque lThreaded rThreaded lDbl rDbl bnd safe elt

    -- | Haskell IO threads ("Control.Concurrent") may concurrently access
    --   this end of the queue.  Note that this attribute is set
    --   separately for the left and right ends.
    data Threadsafe
    -- | Only one thread at a time may access this end of the queue.
    data Nonthreadsafe

    -- | This end of the queue provides push-only (left) or pop-only
    --   (right) functionality. Thus a 'SingleEnd' / 'SingleEnd' combination
    --   is what is commonly referred to as a /single ended queue/, whereas
    --   'DoubleEnd' / 'DoubleEnd' is
    --   a /double ended queue/.  Heterogeneous combinations are sometimes
    --   colloquially referred to as \"1.5 ended queues\".
    data SingleEnd
    -- | This end of the queue supports both push and pop.
    data DoubleEnd

    -- | The queue has bounded capacity.
    data Bound
    -- | The queue can grow as elements are added.
    data Grow

    -- | The queue will not duplicate elements.
    data Safe
    -- | Pop operations may possibly duplicate elements.  Hopefully with low probability!
    data Dup

    -- Possible #5:
    -- data Lossy -- I know of no algorithm which would motivate having a Lossy mode.

    ----------------------------------------

    type T  = Threadsafe
    type NT = Nonthreadsafe
    type S  = SingleEnd
    type D  = DoubleEnd

    -- | A traditional single-threaded, single-ended queue.
    type Queue a = Deque Nonthreadsafe Nonthreadsafe SingleEnd SingleEnd Grow Safe a
    -- | A concurrent queue.
    type ConcQueue a = Deque Threadsafe Threadsafe SingleEnd SingleEnd Grow Safe a
    -- | A concurrent deque.
    type ConcDeque a = Deque Threadsafe Threadsafe DoubleEnd DoubleEnd Grow Safe a
    -- | Work-stealing deques (1.5 ended).  Typically the worker pushes
    --   and pops its own queue (left) whereas thieves only pop (right).
    type WSDeque a = Deque Nonthreadsafe Threadsafe DoubleEnd SingleEnd Grow Safe a

    --------------------------------------------------------------------------------

    -- | Class encompassing the basic queue operations that hold for all
    --   single, 1.5, and double ended modes.  We arbitrarily call the
    --   ends \"left\" and \"right\" and choose the natural operations to be
    --   pushing on the left and popping on the right.
    class DequeClass d where
       -- | Create a new deque.  Most appropriate for unbounded deques.
       --   If bounded, the size is unspecified.
       newQ  :: IO (d elt)

    #ifdef DEFAULT_SIGNATURES
    #warning "Providing default binding and signature for newQ..."
       default newQ :: BoundedL d => IO (d elt)
       newQ = newBoundedQ 256
    #endif

       -- | Is the queue currently empty?  Beware that this can be a highly transient state.
       nullQ :: d elt -> IO Bool

       -- | Natural push: push onto the left end of the deque.
       pushL :: d elt -> elt -> IO ()
       -- | Natural pop: pop from the right end of the deque.
       tryPopR  :: d elt -> IO (Maybe elt)

       -- TODO: Consider adding a peek operation?

       -- TODO: It would also be possible to include blocking/spinning pops.
       -- But maybe those should go in separate type classes...

       -- | Runtime indication of thread saftey for `pushL` (and `popL`).
       -- (Argument unused.)
       leftThreadSafe  :: d elt -> Bool

       -- | Runtime indication of thread saftey for `tryPopR` (and `pushR`).
       -- (Argument unused.)
       rightThreadSafe :: d elt -> Bool

    class DequeClass d => PopL d where
       -- | PopL is not the native operation for the left end, so it requires
       --   that the left end be a 'DoubleEnd', but places no other requirements
       --   on the input queue.
       --
       tryPopL  :: d elt -> IO (Maybe elt)

    class DequeClass d => PushR d where
       -- | Pushing is not the native operation for the right end, so it requires
       --   that end be a 'DoubleEnd'.
       pushR :: d elt -> elt -> IO ()

    class DequeClass d => BoundedL d where
       -- | Create a new, bounded deque with a specified capacity.
       newBoundedQ :: Int -> IO (d elt)
       -- | For a bounded deque, pushing may fail if the deque is full.
       tryPushL :: d elt -> elt -> IO Bool

    class PushR d => BoundedR d where
       -- | For a bounded deque, pushing may fail if the deque is full.
       tryPushR :: d elt -> elt -> IO Bool