One of Haskell’s strengths is immutable data structures. These structures make it easier to reason about code, simplify concurrency and parallelism, and in some cases can improve performance by allowing sharing. However, there are still classes of problems where mutable data structures can both be more convenient, and provide a performance boost. This library is meant to provide such structures in a performant, well tested way. It also provides a simple abstraction over such data structures via typeclasses.
Before anything else, let me provide the caveats of this package:
- Don’t use this package unless you have a good reason to! Immutable data structures are a better approach most of the time!
- This code is intentionally not multithread safe. If you need something like a concurrent queue, there are many options on Hackage, from
TChan, to chaselev-deque.
We’ll first talk about the general approach to APIs in this package. Next, there are two main sets of abstractions provided, which we’ll cover in the following two sections, along with their concrete implementations. Finally, we’ll cover benchmarks.
The API takes heavy advantage of the
PrimMonad typeclass from the primitive
package. This allows our data structures to work in both
Each data structure has an associated type,
MCState, which gives the
primitive state for that structure. For example, in the case of
RealWorld, whereas for
STRef s, it would be
s. This associated
type is quite similar to the
PrimState associated type from primitive, and in
many type signatures you’ll see an equality constraint along the lines of:
PrimState m ~ MCState c
For those who are wondering,
MCState stands for “mutable container state.”
All actions are part of a typeclass, which allows for generic access to
different types of structures quite easily. In addition, we provide type hint
functions, such as
asIORef, which can help specify types when using such
generic functions. For example, a common idiom might be:
ioref <- fmap asIORef $ newRef someVal
Wherever possible, we stick to well accepted naming and type signature
standards. For example, note how closely
Single cell references
The base package provides both
STRef as boxed mutable references,
for storing a single value. The primitive package also provides
generalizes over both of those and works for any
PrimMonad instance. The
MutableRef typeclass in this package abstracts over all three of those. It
has two associated types:
MCState for the primitive state, and
to specify what is contained by the reference.
You may be wondering: why not just take the reference as a type parameter? That
wouldn’t allow us to have monomorphic reference types, which may be useful
under some circumstances. This is a similar motivation to how the
mono-traversable package works.
In addition to providing an abstraction over
this package provides four additional single-cell mutable references.
BRef all contain a 1-length mutable vector under the surface,
which is unboxed, storable, and boxed, respectively. The advantage of the first
two over boxed standard boxed references is that it can avoid a significant
amount of allocation overhead. See the relevant Stack Overflow
and the benchmarks below.
BRef doesn’t give this same advantage (since the values are still
boxed), it was trivial to include it along with the other two, and does
actually demonstrate a performance advantage. Unlike
is no restriction on the type of value it can store.
The final reference type is
PRef. Unlike the other three mentioned, it
doesn’t use vectors at all, but instead drops down directly to a mutable
bytearray to store values. This means it has slightly less overhead (no need to
store the size of the vector), but also restricts the types of things that can
be stored (only instances of
You should benchmark your program to determine the most efficient reference
type, but generally speaking
PRef will be most performant, followed by
SRef, and finally
Collections allow you to push and pop values to the beginning and end of
themselves. Since different data structures allow different operations, each
operation goes into its own typeclass, appropriately named
MutablePopBack. There is also a
MutableCollection which provides:
CollElementassociated type to indicate what kinds of values are in the collection.
newCollfunction to create a new, empty collection.
mono-traversable package provides a typeclass
abstracts over sequence-like things. In particular, it provides operations for
unsnoc. Using this abstraction, we can provide
an instance for all of the typeclasses listed above for any mutable reference
containing an instance of
IORef [Int] or
BRef s (Seq Double).
Note that the performance of some of these combinations is terrible. In
popBack on a list requires traversing the entire
list, and any push operations on a
Vector requires copying the entire
contents of the vector. Caveat emptor! If you must use one of these
structures, it’s highly recommended to use
Seq, which gives the best overall
However, in addition to these instances, this package also provides two
additional data structures: double-ended queues and doubly-linked lists. The
former is based around mutable vectors, and therefore as unboxed (
SDeque), and boxed (
BDeque) variants. Doubly-linked lists have no
such variety, and are simply
For general purpose queue-like structures,
SDeque is likely to
give you best performance. As usual, benchmark your own program to be certain,
and see the benchmark results below.
The following benchmarks were performed on January 7, 2015, against version 0.2.0.
benchmarking IORef time 4.322 μs (4.322 μs .. 4.323 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 4.322 μs (4.322 μs .. 4.323 μs) std dev 1.401 ns (1.114 ns .. 1.802 ns) benchmarking STRef time 4.484 μs (4.484 μs .. 4.485 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 4.484 μs (4.484 μs .. 4.484 μs) std dev 941.0 ps (748.5 ps .. 1.164 ns) benchmarking MutVar time 4.482 μs (4.482 μs .. 4.483 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 4.482 μs (4.482 μs .. 4.483 μs) std dev 843.2 ps (707.9 ps .. 1.003 ns) benchmarking URef time 2.020 μs (2.019 μs .. 2.020 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 2.020 μs (2.019 μs .. 2.020 μs) std dev 955.2 ps (592.2 ps .. 1.421 ns) benchmarking PRef time 2.015 μs (2.014 μs .. 2.015 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 2.014 μs (2.014 μs .. 2.015 μs) std dev 901.3 ps (562.8 ps .. 1.238 ns) benchmarking SRef time 2.231 μs (2.230 μs .. 2.232 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 2.231 μs (2.230 μs .. 2.231 μs) std dev 1.938 ns (1.589 ns .. 2.395 ns) benchmarking BRef time 4.279 μs (4.279 μs .. 4.279 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 4.279 μs (4.279 μs .. 4.279 μs) std dev 1.281 ns (1.016 ns .. 1.653 ns)
time 8.371 ms (8.362 ms .. 8.382 ms) 1.000 R² (1.000 R² .. 1.000 R²) mean 8.386 ms (8.378 ms .. 8.398 ms) std dev 29.25 μs (20.73 μs .. 42.47 μs) benchmarking IORef (Seq Int) time 142.9 μs (142.7 μs .. 143.1 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 142.7 μs (142.6 μs .. 142.9 μs) std dev 542.8 ns (426.5 ns .. 697.0 ns) benchmarking UDeque time 107.5 μs (107.4 μs .. 107.6 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 107.5 μs (107.4 μs .. 107.6 μs) std dev 227.4 ns (171.8 ns .. 297.8 ns) benchmarking SDeque time 97.82 μs (97.76 μs .. 97.89 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 97.82 μs (97.78 μs .. 97.89 μs) std dev 169.5 ns (110.6 ns .. 274.5 ns) benchmarking BDeque time 113.5 μs (113.4 μs .. 113.6 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 113.6 μs (113.5 μs .. 113.7 μs) std dev 300.4 ns (221.8 ns .. 424.1 ns) benchmarking DList time 156.5 μs (156.3 μs .. 156.6 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 156.4 μs (156.3 μs .. 156.6 μs) std dev 389.5 ns (318.3 ns .. 502.8 ns)
As of version 0.2.0, this package has 100% test coverage. If you look at the
report yourself, you’ll see some uncovered code; it’s just the automatically
Show instance needed for QuickCheck inside the test suite itself.
DLListto avoid conflict with difference lists
Dequeoptimizations by avoiding modulus operations completely.
- Fix a bug in
Deque’s new vector allocation/copy code.
ForeignPtrdirectly (slightly more efficient)
- Restructure under the Data.Mutable module.
- Added PRef
- Added reference benchmark.
- Added boxed deque and references.