abstract-deque-0.1.5: Abstract, parameterized interface to mutable Deques.

Safe HaskellSafe-Infered




An abstract, parameterizable interface for queues.

This interface includes a non-associated type family for Deques plus separate type classes encapsulating the Deque operations. This design strives to hide the extra phantom-type parameters from the Class constraints and therefore from the type signatures of client code.


Highly parameterized Deque type(s)

type family Deque lThreaded rThreaded lDbl rDbl bnd safe elt

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)

The choices that select a queue-variant.

Choice #1 -- thread safety.

data Threadsafe

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 Nonthreadsafe

Only one thread at a time may access this end of the queue.

Choice #2 -- double or single functionality on an end.

data SingleEnd

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 DoubleEnd

This end of the queue supports both push and pop.

Choice #3 -- bounded or growing queues:

data Bound

The queue has bounded capacity.

data Grow

The queue can grow as elements are added.

Choice #4 -- duplication of elements.

data Safe

The queue will not duplicate elements.

data Dup

Pop operations may possibly duplicate elements. Hopefully with low probability!

Aliases enabling more concise Deque types:

type S = SingleEnd

type D = DoubleEnd

type T = Threadsafe

Aliases for commonly used Deque configurations:

type Queue a = Deque Nonthreadsafe Nonthreadsafe SingleEnd SingleEnd Grow Safe a

A traditional single-threaded, single-ended queue.

type WSDeque a = Deque Nonthreadsafe Threadsafe DoubleEnd SingleEnd Grow Safe a

Work-stealing deques (1.5 ended). Typically the worker pushes and pops its own queue (left) whereas thieves only pop (right).

Classes containing Deque operations

class DequeClass d where

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.


newQ :: IO (d elt)

Create a new deque. Most appropriate for unbounded deques. If bounded, the size is unspecified.

nullQ :: d elt -> IO Bool

Is the queue currently empty? Beware that this can be a highly transient state.

pushL :: d elt -> elt -> IO ()

Natural push: push onto the left end of the deque.

tryPopR :: d elt -> IO (Maybe elt)

Natural pop: pop from the right end of the deque.

Auxilary type classes.

In spite of hiding the extra phantom type parameters in the DequeClass, we wish to retain the ability for clients to constrain the set of implementations they work with **statically**.

The "unnatural" double ended cases: pop left, push right.

class DequeClass d => PopL d where


tryPopL :: d elt -> IO (Maybe elt)

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.


class DequeClass d => PushR d where


pushR :: d elt -> elt -> IO ()

Pushing is not the native operation for the right end, so it requires that end be a DoubleEnd.


Operations that only make sense for bounded queues.

class DequeClass d => BoundedL d where


newBoundedQ :: Int -> IO (d elt)

Create a new, bounded deque with a specified capacity.

tryPushL :: d elt -> elt -> IO Bool

For a bounded deque, pushing may fail if the deque is full.

class PushR d => BoundedR d where


tryPushR :: d elt -> elt -> IO Bool

For a bounded deque, pushing may fail if the deque is full.