| Safe Haskell | Safe-Inferred |
|---|
Data.Concurrent.Deque.Class
Contents
Description
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)
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 NT = Nonthreadsafe
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 ConcQueue a = Deque Threadsafe Threadsafe SingleEnd SingleEnd Grow Safe a
A concurrent queue.
type ConcDeque a = Deque Threadsafe Threadsafe DoubleEnd DoubleEnd Grow Safe a
A concurrent deque.
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).
Class for basic Queue 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.
Methods
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.
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.
leftThreadSafe :: d elt -> Bool
Runtime indication of thread saftey for pushL (and popL).
(Argument unused.)
rightThreadSafe :: d elt -> Bool
Instances
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 where
Methods
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.
Instances
class DequeClass d => PushR d where
Methods
pushR :: d elt -> elt -> IO ()
Pushing is not the native operation for the right end, so it requires
that end be a DoubleEnd.
Instances
Operations that only make sense for bounded queues.
class DequeClass d => BoundedL d where
Methods
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.
Instances