Extensible exceptions are a good solution to the exception problem in
Haskell. However, there is one problem: they are not extensible enough!
The problem is that the functions defined in `Control.Exception`

for dealing
with exceptions can only be used with the `IO`

monad. A lot of Haskell code
uses a stack of monads, at the bottom of which is `IO`

, but the `IO`

monad
is not used directly.

There have been many attempts to solve this problem, but the stumbling block
has been the presence of short-circuiting monad transformers: sometimes,
these prevented the cleanup actions from being run, making it effectively
impossible to catch exceptions in such monads. The `monad-control`

package
has been developed as a solution to this problem: it defines a way to turn
a monad transformer stack "inside-out", which ensures that cleanup actions
are run even when the original action short-circuits. The `lifted-base`

package, built on top of `monad-control`

, exports the
`Control.Exception.Lifted`

module, which contains versions of the
`Control.Exception`

functions that work on any monad stack with `IO`

at its
base.

This has pretty much solved the above problems. However, one thing that the
solutions that came before `monad-control`

did was provide a type class
encapsulating exception functionality that could be implemented by pure
monads, allowing you to use the same interface to throw and catch exceptions
in both pure and `IO`

-based code. This also makes it possible to express
which can throw an exception, but which don't necessarily do any IO and
which are polymorphic in their exception throwing (i.e., you could run the
function in `IO`

and it would use `throwIO`

, or you could run it as an
`Either`

and it would use `Left`

).

That's what this package does. It provides a `MonadException`

type class (in
the `Control.Monad.Exception.Class`

module), which has instances for `IO`

and `IO`

-like monads (for which `monad-control`

is used to provide the
correct instances as described above), as well as for some pure monads.
Several overlapping instances (in the spirit of `mtl-evil-instances`

) are
provided, so it is not necessary to provide a pass-through instance for
`MonadException`

for every monad transformer you write.

This package also defines an `ExceptionT`

monad transformer (in
`Control.Monad.Trans.Exception`

) that can be used to add `MonadException`

functionality to otherwise pure monad stacks. `mtl-evil-instances`

is used
to automatically provide pass-through instances for the `mtl`

type classes
for this transformer.

Finally, this package includes the module `Control.Exception.Monadic`

, which
is a full replacement for `Control.Exception`

, whose functions work on
any instance of `MonadException`

and not just `IO`

. The functions for
dealing with asynchronous exceptions require `IO`

however, so these are only
polymorphic for any `IO`

-like monadic (as determined by `monad-control`

).