I've released the first version of monad-unlift. From its README:

Typeclasses for providing for unlifting of monad transformers

and stacks, and concrete implementations of common transformers

implementing this type classes.

This package is a companion to monad-control, providing

simplified functions to a common subset of transformers and

functionality. I've copied in the content of the README below to

give some examples and explanations.

I'd like to thank Erik Hesselink and Hiromi Ishii for providing some brilliant solutions to make this package more useful, based on the constraints package.

Synopsis

Overview

A common pattern is to have some kind of a monad transformer,

and want to pass an action into a function that requires actions in

a base monad. That sounds a bit abstract, so let's give a concrete

example:

Doing this manually is possible, but a bit tedious:

This also doesn't generalize at all; you'll be stuck writing concurrently variants for every monad transformer stack. Fortunately, the monad-control package

generalizes this to a large number of transformer stacks. Let's

implement our generalized concurrently:

Notice how, in the signature for concurrentlyG, we no longer return (a, b), but (StM m a, StM m b). This is because there may be additional monadic context

for each thread of execution, and we have no way of merging these

together in general. Some examples of context are:

• With WriterT, it's the values that you called tell on

• With EitherT, the returned value may not exist at all

In addition to this difficulty, many people find the types in monad-control difficult to navigate, due to their

extreme generality (which is in fact the power of that

package!).

There is a subset of these transformer stacks that are in fact monad morphisms. Simply stated, these are transformer stacks that are isomorphic to ReaderT. For these monads, there is not

context in the returned value. Therefore, there's no need to

combine returned states or deal with possibly missing values.

This concept is represented by the monad-unlift package, which

provides a pair of typeclasses for these kinds of transformer

stacks. Before we dive in, let's see how we solve our

concurrentlyG problem with it:

Notice how we get (a, b) in the return type as desired. There's no need to unwrap values or deal with context.

MonadTransUnlift

MonadTransUnlift is a class for any monad transformer which is isomorphic to ReaderT, in the

sense that the environment can be captured and applied later. Some

interesting cases in this space are:

• IdentityT and things isomorphic to it; in this

  case, you can think of the environment as being

  ()

• Transformers which contain a mutable reference in their

  environment. This allows them to behave like stateful transformers

  (e.g., StateT or WriterT), but still obey the monad morphism laws. (See below for more details.)

Due to weaknesses in GHC's ImpredicativeTypes, we have a helper

datatype to allow for getting polymorphic unlift functions,

appropriately named Unlift. For many common cases, you can get away with using askRun instead, e.g.:

Using Unlift, this would instead be:

or equivalently:

MonadBaseUnlift

MonadBaseUnlift extends this concept to entire

transformer stacks. This is typically the typeclass that people end

up using. You can think of these two typeclasses in exactly the

same way as MonadTrans and MonadIO, or more precisely MonadTrans and MonadBase.

For the same ImpredicativeTypes reason, there's a helper type UnliftBase. Everything we just discussed should transfer directly to MonadBaseUnlift, so learning

something new isn't necessary. For example, you can rewrite the

last snippet as:

Reference transformers

When playing transformer stack games with a transformer like StateT, it's common to accidentally discard state

modifications. Additionally, in the case of runtime exceptions,

it's usually impossible to retain the state. (Similar statements

apply to WriterT and RWST, both in strict and lazy variants.)

Another approach is to use a ReaderT and hold onto

a mutable reference. This is problematic since there's no built in

support for operations like get, put, or tell. What we want is to have a MonadState and/or MonadWriter instance.

To address this case, this package includes variants of those

transformers that use mutable references. These reference are

generic using the mutable-containers

package, which allows you to have highly efficient references like

PRef instead of always using boxed references like IORef.

Note that, for generality, the reference transformers take type

parameters indicating which mutable reference type to use. Some

examples you may use are:

• IORef for boxed references in IO

• STRef s for boxed references in ST

• PRef RealWorld for an unboxed reference in IO

See the synopsis for a complete example.

conduit

The transPipe function in conduit has caused

confusion in the past due to its requirement of provided functions

to obey monad morphism laws. This package makes a good companion to

conduit to simplify that function's usage.

Other notable instances

Both the HandlerT transformer from yesod-core and LoggingT/NoLoggingT are valid monad morphisms. HandlerT is in fact my first example of

using the "enviornment holding a mutable reference" technique to

overcome exceptions destroying state.