abstract-deque-0.2: Abstract, parameterized interface to mutable Deques.

Safe HaskellSafe-Inferred




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.


Highly parameterized Deque type(s)

type family Deque lThreaded rThreaded lDbl rDbl bnd safe elt Source

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 Source

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 Source

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

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

data SingleEnd Source

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 Source

This end of the queue supports both push and pop.

Choice #3 -- bounded or growing queues:

data Bound Source

The queue has bounded capacity.

data Grow Source

The queue can grow as elements are added.

Choice #4 -- duplication of elements.

data Safe Source

The queue will not duplicate elements.

data Dup Source

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

Aliases enabling more concise Deque types:

Aliases for commonly used Deque configurations:

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

A traditional single-threaded, single-ended queue.

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

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

Class for basic Queue operations

class DequeClass d whereSource

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)Source

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

nullQ :: d elt -> IO BoolSource

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

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

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

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

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

leftThreadSafe :: d elt -> BoolSource

Runtime indication of thread saftey for pushL (and popL). (Argument unused.)

rightThreadSafe :: d elt -> BoolSource

Runtime indication of thread saftey for tryPopR (and pushR). (Argument unused.)

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.

class DequeClass d => PopL d whereSource


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

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 whereSource


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

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 whereSource


newBoundedQ :: Int -> IO (d elt)Source

Create a new, bounded deque with a specified capacity.

tryPushL :: d elt -> elt -> IO BoolSource

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

class PushR d => BoundedR d whereSource


tryPushR :: d elt -> elt -> IO BoolSource

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