# monad-control

Lift control operations, like exception catching, through monad transformers https://github.com/basvandijk/monad-control

Version on this page: | 1.0.0.4 |

Stackage Nightly 2017-08-20: | 1.0.2.2 |

LTS Haskell 9.1: | 1.0.2.2 |

Stackage Nightly 2017-08-20: | 1.0.2.2 |

Latest on Hackage: | 1.0.2.2 |

**Bas van Dijk, Anders Kaseorg**

**Bas van Dijk**

#### Module documentation for 1.0.0.4

- Control
- Control.Monad
- Control.Monad.Trans

- Control.Monad

This package defines the type class `MonadControlIO`

, a subset of
`MonadIO`

into which generic control operations such as `catch`

can be
lifted from `IO`

. Instances are based on monad transformers in
`MonadTransControl`

, which includes all standard monad transformers in
the `transformers`

library except `ContT`

.

Note that this package is a rewrite of Anders Kaseorg's `monad-peel`

library. The main difference is that this package provides CPS style
operators and exploits the `RankNTypes`

language extension to simplify
most definitions.

This `criterion`

based benchmark shows that `monad-control`

is on average about 2.5
times faster than `monad-peel`

.

## Changes

* Added some good documentation. Courtesy of Franz Thoma.

1.0.2.1

* Refer to Michael Snoyman's excellent tutorial on monad-control.

1.0.2.0

* Improve documentation by including type equalities in the Haddock documentation.

* Add helpers to define MonadTransControl for stack of two:

RunDefault2, defaultLiftWith2, defaultRestoreT2

1.0.1.0

* Added the functions:

liftThrough

:: (MonadTransControl t, Monad (t m), Monad m)

=> (m (StT t a) -> m (StT t b)) -- ^

-> t m a -> t m b

captureT :: (MonadTransControl t, Monad (t m), Monad m) => t m (StT t ())

captureM :: MonadBaseControl b m => m (StM m ())

* Added Travis-CI integration

1.0.0.5

* Support transformers-0.5 & ransformers-compat-0.5.*.

1.0.0.4

* Support transformers-compat-0.4.*.

1.0.0.3

* Unconditionally add ExceptT instances using transformers-compat.

Courtesy of Adam Bergmark.

1.0.0.2

* Add a base >= 4.5 constraint because monad-control only builds on GHC >= 7.4.

1.0.0.1

* Use Safe instead of Trustworthy.

This requires a dependency on stm.

1.0.0.0

* Switch the associated data types StT and StM to associated type synonyms.

This is an API breaking change. To fix your MonadTransControl or

MonadBaseControl instances simply remove the StT or StM constructors

and deconstructors for your monad transformers or monad.

* Add the embed, embed_ and liftBaseOpDiscard functions.

0.3.3.1

* Unconditionally add ExceptT instances using transformers-compat.

Courtesy of Adam Bergmark.

0.3.3.0

* Support transformers-0.4.0.0

* Drop unicode syntax and symbols

0.3.2.3

* Fix haddock documentation error

0.3.2.2

* Fix preprocessor directive for GHC 7.6.3

0.3.2.1

* Resolve #14. Bump upper version bound of base to 5

0.3.2

* Added defaultLiftWith and defaultRestoreT to simplify defining

MonadTransControl for newtypes.

0.3.1.4

* Compatibility with ghc head

0.3.1.3

* Added a Trustworthy flag

0.3.1.2

* Fix issue #9. Replace all Unicode in type variables.

0.3.1.1

* Add MonadBaseControl instances for ST and STM.

0.3

(Released on: Fri Dec 2 09:52:16 UTC 2011)

* Major new API which IMHO is easier to understand than the old one.

* On average about 60 times faster than the previous release!

* New package lifted-base providing lifted versions of functions from the base

library. It exports the following modules:

- Control.Exception.Lifted

- Control.Concurrent.Lifted

- Control.Concurrent.MVar.Lifted

- System.Timeout.Lifted

Not all modules from base are converted yet. If you need a lifted version of

some function from base, just ask me to add it or send me a patch.

0.2.0.3

(Released on: Sat Aug 27 21:18:22 UTC 2011)

* Fixed issue #2

https://github.com/basvandijk/monad-control/issues/2

0.2.0.2

(Released on: Mon Aug 8 09:16:08 UTC 2011)

* Switched to git on github.

* Tested with base-4.4 and ghc-7.2.1.

* Use the new cabal test-suite feature.

0.2.0.1

(Released on: Wed Mar 16 15:53:50 UTC 2011)

* Added laws for MonadTransControl and MonadControlIO

* Bug fix: Add proper laziness to the MonadTransControl instances

of the lazy StateT, WriteT and RWST

These all failed the law: control $ \run -> run t = t

where t = return undefined

* Add INLINABLE pragmas for most public functions

A simple benchmark showed some functions

(bracket and mask) improving by 30%.

0.2

(Released on: Wed Feb 9 12:05:26 UTC 2011)

* Use RunInBase in the type of idLiftControl.

* Added this NEWS file.

* Only parameterize Run with t and use RankNTypes to quantify n and o

-liftControl :: (Monad m, Monad n, Monad o) => (Run t n o -> m a) -> t m a

+liftControl :: Monad m => (Run t -> m a) -> t m a

-type Run t n o = forall b. t n b -> n (t o b)

+type Run t = forall n o b. (Monad n, Monad o, Monad (t o)) => t n b -> n (t o b)

Bumped version from 0.1 to 0.2 to indicate this breaking change in API.

* Added example of a derivation of liftControlIO.

Really enlightening!

0.1

(Released on: Sat Feb 5 23:36:21 UTC 2011)

* Initial release

This is the announcement message sent to the Haskell mailinglists:

http://www.mail-archive.com/haskell@haskell.org/msg23278.html

Dear all,

Several attempts have been made to lift control operations (functions

that use monadic actions as input instead of just output) through

monad transformers:

MonadCatchIO-transformers[1] provided a type class that allowed to

overload some often used control operations (catch, block and

unblock). Unfortunately that library was limited to those operations.

It was not possible to use, say, alloca in a monad transformer. More

importantly however, the library was broken as was explained[2] by

Michael Snoyman. In response Michael created the MonadInvertIO type

class which solved the problems. Then Anders Kaseorg created the

monad-peel library which provided an even nicer implementation.

monad-control is a rewrite of monad-peel that uses CPS style

operations and exploits the RankNTypes language extension to simplify

and speedup most functions. A very preliminary and not yet fully

representative, benchmark shows that monad-control is on average about

2.6 times faster than monad-peel:

bracket: 2.4 x faster

bracket_: 3.1 x faster

catch: 1.8 x faster

try: 4.0 x faster

mask: 2.0 x faster

Note that, although the package comes with a test suite that passes, I

still consider it highly experimental.

API DOCS:

http://hackage.haskell.org/package/monad-control

INSTALLING:

$ cabal update

$ cabal install monad-control

TESTING:

The package contains a copy of the monad-peel test suite written by

Anders. You can perform the tests using:

$ cabal unpack monad-control

$ cd monad-control

$ cabal configure -ftest

$ cabal test

BENCHMARKING:

$ darcs get http://bifunctor.homelinux.net/~bas/bench-monad-peel-control/

$ cd bench-monad-peel-control

$ cabal configure

$ cabal build

$ dist/build/bench-monad-peel-control/bench-monad-peel-control

DEVELOPING:

The darcs repository will be hosted on code.haskell.org ones that

server is back online. For the time being you can get the repository

from:

$ darcs get http://bifunctor.homelinux.net/~bas/monad-control/

TUTORIAL:

This short unpolished tutorial will explain how to lift control

operations through monad transformers. Our goal is to lift a control

operation like:

foo ∷ M a → M a

where M is some monad, into a transformed monad like 'StateT M':

foo' ∷ StateT M a → StateT M a

The first thing we need to do is write an instance for the

MonadTransControl type class:

class MonadTrans t ⇒ MonadTransControl t where

liftControl ∷ (Monad m, Monad n, Monad o)

⇒ (Run t n o → m a) → t m a

If you ignore the Run argument for now, you'll see that liftControl is

identical to the 'lift' method of the MonadTrans type class:

class MonadTrans t where

lift ∷ Monad m ⇒ m a → t m a

So the instance for MonadTransControl will probably look very much

like the instance for MonadTrans. Let's see:

instance MonadTransControl (StateT s) where

liftControl f = StateT $ \s → liftM (\x → (x, s)) (f run)

So what is this run function? Let's look at its type:

type Run t n o = ∀ b. t n b → n (t o b)

The run function executes a transformed monadic action 't n b' in the

non-transformed monad 'n'. In our case the 't' will be a StateT

computation. The only way to run a StateT computation is to give it

some state and the only state we have lying around is the one from the

outer computation: 's'. So let's run it on 's':

instance MonadTransControl (StateT s) where

liftControl f =

StateT $ \s →

let run t = ... runStateT t s ...

in liftM (\x → (x, s)) (f run)

Now that we are able to run a transformed monadic action, we're almost

done. Look at the type of Run again. The function should leave the

result 't o b' in the monad 'n'. This 't o b' computation should

contain the final state after running the supplied 't n b'

computation. In case of our StateT it should contain the final state

s':

instance MonadTransControl (StateT s) where

liftControl f =

StateT $ \s →

let run t = liftM (\(x, s') → StateT $ \_ → return (x, s'))

(runStateT t s)

in liftM (\x → (x, s)) (f run)

This final computation, "StateT $ \_ → return (x, s')", can later be

used to restore the final state. Now that we have our

MonadTransControl instance we can start using it. Recall that our goal

was to lift "foo ∷ M a → M a" into our StateT transformer yielding the

function "foo' ∷ StateT M a → StateT M a".

To define foo', the first thing we need to do is call liftControl:

foo' t = liftControl $ \run → ...

This captures the current state of the StateT computation and provides

us with the run function that allows us to run a StateT computation on

this captured state.

Now recall the type of liftControl ∷ (Run t n o → m a) → t m a. You

can see that in place of the ... we must fill in a value of type 'm

a'. In our case this will be a value of type 'M a'. We can construct

such a value by calling foo. However, foo expects an argument of type

'M a'. Fortunately we can provide one if we convert the supplied 't'

computation of type 'StateT M a' to 'M a' using our run function of

type ∀ b. StateT M b → M (StateT o b):

foo' t = ... liftControl $ \run → foo $ run t

However, note that the run function returns the final StateT

computation inside M. So the type of the right hand side is now

'StateT M (StateT o b)'. We would like to restore this final state. We

can do that using join:

foo' t = join $ liftControl $ \run → foo $ run t

That's it! Note that because it's so common to join after a

liftControl I provide an abstraction for it:

control = join ∘ liftControl

Allowing you to simplify foo' to:

foo' t = control $ \run → foo $ run t

Probably the most common control operations that you want to lift

through your transformers are IO operations. Think about: bracket,

alloca, mask, etc.. For this reason I provide the MonadControlIO type

class:

class MonadIO m ⇒ MonadControlIO m where

liftControlIO ∷ (RunInBase m IO → IO a) → m a

Again, if you ignore the RunInBase argument, you will see that

liftControlIO is identical to the liftIO method of the MonadIO type

class:

class Monad m ⇒ MonadIO m where

liftIO ∷ IO a → m a

Just like Run, RunInBase allows you to run your monadic computation

inside your base monad, which in case of liftControlIO is IO:

type RunInBase m base = ∀ b. m b → base (m b)

The instance for the base monad is trivial:

instance MonadControlIO IO where

liftControlIO = idLiftControl

idLiftControl directly executes f and passes it a run function which

executes the given action and lifts the result r into the trivial

'return r' action:

idLiftControl ∷ Monad m ⇒ ((∀ b. m b → m (m b)) → m a) → m a

idLiftControl f = f $ liftM $ \r -> return r

The instances for the transformers are all identical. Let's look at

StateT and ReaderT:

instance MonadControlIO m ⇒ MonadControlIO (StateT s m) where

liftControlIO = liftLiftControlBase liftControlIO

instance MonadControlIO m ⇒ MonadControlIO (ReaderT r m) where

liftControlIO = liftLiftControlBase liftControlIO

The magic function is liftLiftControlBase. This function is used to

compose two liftControl operations, the outer provided by a

MonadTransControl instance and the inner provided as the argument:

liftLiftControlBase ∷ (MonadTransControl t, Monad base, Monad m, Monad (t m))

⇒ ((RunInBase m base → base a) → m a)

→ ((RunInBase (t m) base → base a) → t m a)

liftLiftControlBase lftCtrlBase =

\f → liftControl $ \run →

lftCtrlBase $ \runInBase →

f $ liftM (join ∘ lift) ∘ runInBase ∘ run

Basically it captures the state of the outer monad transformer using

liftControl. Then it captures the state of the inner monad using the

supplied lftCtrlBase function. If you recall the identical definitions

of the liftControlIO methods: 'liftLiftControlBase liftControlIO' you

will see that this lftCtrlBase function is the recursive step of

liftLiftControlBase. If you use 'liftLiftControlBase liftControlIO' in

a stack of monad transformers a chain of liftControl operations is

created:

liftControl $ \run1 -> liftControl $ \run2 -> liftControl $ \run3 -> ...

This will recurse until we hit the base monad. Then

liftLiftControlBase will finally run f in the base monad supplying it

with a run function that is able to run a 't m a' computation in the

base monad. It does this by composing the run and runInBase functions.

Note that runInBase is basically the composition: '... ∘ run3 ∘ run2'.

However, just composing the run and runInBase functions is not enough.

Namely: runInBase ∘ run ∷ ∀ b. t m b → base (m (t m b)) while we need

to have ∀ b. t m b → base (t m b). So we need to lift the 'm (t m b)'

computation inside t yielding: 't m (t m b)' and then join that to get

't m b'.

Now that we have our MonadControlIO instances we can start using them.

Let's look at how to lift 'bracket' into a monad supporting

MonadControlIO. Before we do that I define a little convenience

function similar to 'control':

controlIO = join ∘ liftControlIO

Bracket just calls controlIO which captures the state of m and

provides us with a runInIO function which allows us to run an m

computation in IO:

bracket ∷ MonadControlIO m

⇒ m a → (a → m b) → (a → m c) → m c

bracket before after thing =

controlIO $ \runInIO →

E.bracket (runInIO before)

(\m → runInIO $ m >>= after)

(\m → runInIO $ m >>= thing)

I welcome any comments, questions or patches.

Regards,

Bas

[1] http://hackage.haskell.org/package/MonadCatchIO-transformers

[2] http://docs.yesodweb.com/blog/invertible-monads-exceptions-allocations/

[3] http://hackage.haskell.org/package/monad-peel