- Highly parameterized Deque type(s)
- Class for basic Queue operations
- Extra capabilities: type classes
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.
- type family Deque lThreaded rThreaded lDbl rDbl bnd safe elt
- data Threadsafe
- data Nonthreadsafe
- data SingleEnd
- data DoubleEnd
- data Bound
- data Grow
- data Safe
- data Dup
- type S = SingleEnd
- type D = DoubleEnd
- type NT = Nonthreadsafe
- type T = Threadsafe
- type Queue a = Deque Nonthreadsafe Nonthreadsafe SingleEnd SingleEnd Grow Safe a
- type ConcQueue a = Deque Threadsafe Threadsafe SingleEnd SingleEnd Grow Safe a
- type ConcDeque a = Deque Threadsafe Threadsafe DoubleEnd DoubleEnd Grow Safe a
- type WSDeque a = Deque Nonthreadsafe Threadsafe DoubleEnd SingleEnd Grow Safe a
- class DequeClass d where
- class DequeClass d => PopL d where
- class DequeClass d => PushR d where
- class DequeClass d => BoundedL d where
- class PushR d => BoundedR d where
Highly parameterized Deque type(s)
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.
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.
Choice #2 -- double or single functionality on an end.
This end of the queue provides push-only (left) or pop-only
(right) functionality. Thus a
is what is commonly referred to as a single ended queue, whereas
a double ended queue. Heterogeneous combinations are sometimes
colloquially referred to as "1.5 ended queues".
Choice #3 -- bounded or growing queues:
Choice #4 -- duplication of elements.
Aliases enabling more concise Deque types:
Aliases for commonly used Deque configurations:
A traditional single-threaded, single-ended queue.
A concurrent queue.
A concurrent deque.
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 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.
Create a new deque. Most appropriate for unbounded deques. If bounded, the size is unspecified.
Is the queue currently empty? Beware that this can be a highly transient state.
Natural push: push onto the left end of the deque.
Natural pop: pop from the right end of the deque.
Runtime indication of thread saftey for
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 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.
Pushing is not the native operation for the right end, so it requires
that end be a
Operations that only make sense for bounded queues.
Create a new, bounded deque with a specified capacity.
For a bounded deque, pushing may fail if the deque is full.