IO Monad Class Hierarchy: io-classes
package
This package provides a monad class hierarchy which is an interface for both the io-sim and IO monads. It was developed with the following constraints in mind:
- be a drop-in replacement for
IO
monad; IO
instances do not alter its original semantics, providing a shallow bindings to async, base, stm, and exceptions packages as well as timer API;- provide zero-cost abstractions.
We provide also non-standard extensions of this API in sublibraries:
- io-classes:strict-stm
strict
TVar
's, and other mutableSTM
variables, with support of the nothunks library; - io-classes:strict-mvar:
strict
MVar
s io-classes:si-timers: timers api:
- 32-bit safe API using
DiffTime
measured in seconds (rather than time in microseconds represented asInt
as inbase
) - cancellable timeouts.
- 32-bit safe API using
- io-classes:mtl: MTL instances.
io-classes:strict-stm and nothunks were successfully used in a large code base to eliminate space leaks and keep that property over long development cycles.
Documentation
Hackage doesn't yet support public sublibraries, thus Haddocks are published here.
Support material
Exception Class Hierarchy
This package provides an alternative class hierarchy giving access to
exceptions API. The [exception
] package class hierarchy is also
supported by io-sim, so you
can also use either one.
The MonadThrow
defined in this package allows working with exceptions
without having explicit access to catch
or mask
. It only provides
access to throwIO
, bracket
, bracket_
, and finally
. MonadCatch
class provides API which allows working with exceptions, e.g. catch
or
bracketOnError
, and MonadMask
gives access to low-level mask
and
friends. This division makes code review process somewhat easier. Using
only MonadThrow
constraint, the reviewer can be sure that no low-level
exception API is used, which usually requires more care. Still
MonadThrow
is general enough to do resource handling right.
Time and Timer APIs
The time and timer APIs of this package follows closely the API exposed
by base and
time packages. We separately
packaged a more convenient API in
si-timers
(ref SI),
which provides a monoidal action of DiffTime
on monotonic time as well
as exposes 32-bit safe timer API (on 32-bit systems time in microseconds
represented as an Int
can only hold timeouts of ~35 minutes).
si-timers sublibrary provides cancellable timers API, see registerDelayCancellable.
Control.Monad.Class.MonadTimer.NonStandard.MonadTimeout
provides a
low-level timeout abstraction. On systems that support a native timer
manager, it's used to implement its API, which is very efficient even
for low-latency timeouts. On other platforms (e.g. Windows
), it's
good enough for subsecond timeouts but it's not good enough for
fine-grained timeouts (e.g. sub milliseconds) as it relays on the GHC
thread scheduler. We support MonadTimeout
on Linux
, MacOS
,
Windows
, and IOSim
(and unofficially on GHCJS
).
MonadDelay
and MonadTimer
classes provide a well-established
interface to delays & timers.
Software Transactional Memory API
We provide two interfaces to stm
API: lazy, included in
io-classes;
and strict one provided by
io-classes:strict-stm.
Threads API
We draw a line between base
API and async
API. The former is
provided by
MonadFork
the latter by
MonadAsync
Both are shallow abstractions around APIs exposed by the base
and
async
packages.
MVar
API
We also provide lazy and strict MVar
s:
- lazy MVar API
- strict MVar API is provided in the io-classes:strict-mvar sublibrary.
Some other APIs
- MonadEventlog: provides an API to the Debug.Trace event log interface.
- MonadST:
provides a way to lift
ST
-computations. - MonadSay: dummy debugging interface
Monad Transformers
We provide support for monad transformers, see io-classes:mtl sublibrary. Although at this stage it might have its limitations and so there might be some rough edges. PRs are welcomed, contributing.
Differences from base
, async
, or exceptions
packages
Major differences
getMonotonicTime
returnsTime
(a newtype wrapper aroundDiffTime
)Deadlock
exceptions are not thrown to the main thread (see ref), so they cannot be caught. This was a design decision, which allows to catch all deadlocks which otherwise could be captured by acatch
.
Minor differences
Some of the types have more general kind signatures, e.g.
type Async :: (Type -> Type) -> Type -> Type
The first type of kind Type -> Type
describes the monad which could be
instantiated to IO
, IOSim
or some other monad stacks built with
monad transformers. The same applies to many other types, e.g. TVar
,
TMVar
.
The following types although similar to the originals are not the same
as the ones that come from base
, async
, or exceptions
packages:
Handler
(origin:base
)MaskingState
(origin:base
)Concurrently
(origin:async
)ExceptionInLinkedThread
(origin:async
):io-class
es version does not storeAsync
ExitCase
(origin:exceptions
)
Debuging & Insepction
We provide quite extended debugging & inspection API. This proved to be extremely helpful when analysing complex deadlocks or livelocks or writing complex quickcheck properties of a highly concurrent system. Some of this is only possible because we can control the execution environment of io-sim.
labelThread
as part ofMonadThread
(IO, io-sim, which is also part ofGHC
API, ref labelThread);MonadLabelledSTM
which allows to label of variousSTM
mutable variables, e.g.TVar
,MVar
, etc. (io-sim, not provided byGHC
);MonadInspectSTM
which allows inspecting values ofSTM
mutable variables when they are committed. (io-sim, not provided byGHC
).
IO Simulator Monad: io-sim
package
A pure simulator monad built on top of the lazy ST
monad which
supports:
- optional dynamic race discovery and schedule exploration (see IOSimPOR)
- synchronous and asynchronous exceptions; including: throwing, catching and masking synchronous and asynchronous exceptions;
- concurrency (using simulated threads), with interfaces shaped by the
base
andasync
libraries; - software transactional memory (
STM
); - simulated time;
- timeouts;
- dynamically typed traces and event log tracing;
- lifting any
ST
computations; - inspection of
STM
mutable data structures; - deadlock detection;
MonadFix
instances for both IOSim and its correspondingSTM
monad.
io-sim together with
io-classes
is a drop-in replacement for the IO
monad (with some ramifications).
It was designed to write easily testable Haskell code (including
simulating socket programming or disk IO). Using
io-classes
and
si-timers
libraries one can write code that can run in both: the real IO
and the
IOSim
monad provided by this package. One of the design goals was to keep the
API as close as possible to base
, exceptions
, async
, and stm
packages.
io-sim package also provides two interpreters, a standard one and IOSimPOR which supports dynamic discovery of race conditions and schedule exploration with partial order reduction.
io-sim provides API to
explore traces produced by a simulation. It can contain arbitrary
Haskell terms, a feature that is very useful to build property-based
tests using QuickCheck
.
The package contains thorough tests, including tests of STM
against
the original specification (as described in
Composable Memory Transactions
and its GHC
implementation. This can be seen in both ways: as a check
that our implementation matches the specification and the GHC
implementation, but also the other way around: that GHC
s STM
implementation meets the specification.
- Control
- Concurrent
- Monad
- Concurrent
- Control
- Monad
- Class
- MonadEventlog
- MonadST
- MonadSTM
- MonadSay
- MonadThrow
- MonadTime
- MonadTimer
- Control.Monad.Class.Trans
- Class
- Monad
- Control
- Monad
- Class
- MonadTime
- MonadTimer
- Class
- Monad
- Control
- Concurrent
- Class
- Concurrent
- Control
- Concurrent
- Class
- Class
- Concurrent
- Test
- Control
- Concurrent
- Class
- MonadMVar
- Class
- Concurrent
- Control
- Control
- Monad
- Data
- List