Proposal: Extensible exceptions

[Ian Lynagh]

Hi all,

This is a proposal to replace the current exception mechanism in the
base library with extensible exceptions.

It also reimplements the existing exceptions on top of extensible
exceptions, for legacy applications.

Proposed deadline: 25th July.
http://hackage.haskell.org/trac/ghc/ticket/2419

=== What are extensible exceptions?

Simon's extensible extensions paper is very easy to read, and describes
the problems and proposed solution very well:
    http://www.haskell.org/~simonmar/papers/ext-exceptions.pdf
I won't try to reproduce everything the paper says here, but here is the
list of what we want extracted from it:

* A hierarchy of exception types, such that a particular catch
  can choose to catch only exceptions that belong to a particular
  subclass and re-throw all others.
* A way to add new exception types at any point in the hierarchy
  from library or program code.
* The boilerplate code required to add a new type to the exception
  hierarchy should be minimal.
* Exceptions should be thrown and caught using the same primitives,
  regardless of the types involved.

I heartily recommend having a read through of the paper.

=== Patches and examples

The patches are here:
    http://darcs.haskell.org/ext-excep/
I've attached Examples.hs, which gives some examples of using it.

The patches aren't polished; if this proposal is accepted then there is
some more work to do, moving things around inside the base package to
simplify the dependencies, and to maximise the amount of code that can
be shared between all the impls. There's also some GHC-specific fiddling
to be done, to make GHC.TopHandler use the new exceptions. This can all
be done without further library proposals, though.

Also, currently it derives Data.Typeable, which is unportable, but we
can easily work around that. The only extensions that I don't think that
we can do without are ExistentialQuantification and Rank2Types.
DeriveDataTypeable makes the implementation easier, and
DeriveDataTypeable and PatternSignatures make using it easier.

=== Library function differences

As far as the library functions are concerned, here are the main
differences:

The old and new types for catch are:
    Old: catch ::                IO a -> (Exception -> IO a) -> IO a
    New: catch :: Exception e => IO a -> (e         -> IO a) -> IO a
i.e. catch can now catch any type of exception; we don't have to force
all the different types of extension into one fixed datatype.

All the other exception functions are similarly changed to handle any
type of extension, e.g. we now have
    try :: Exception e => IO a -> IO (Either e a)

Now that you can write handlers for different exception types, you might
want to catch multiple different types at the same point. You can use
catches for this. For example, the OldException module needs to catch
all the new exception types and put them into the old Exception type, so
that the legacy handler can be run on them. It looks like this:
    catch :: IO a -> (Exception -> IO a) -> IO a
    catch io handler =
        io `catches`
            [Handler (\e -> handler e),
             Handler (\exc -> handler (ArithException exc)),
             Handler (\exc -> handler (ArrayException exc)),
             ...]
where the first Handler deals with exceptions of type Exception, the
second those of type ArithException, and so on.

If you want to catch all exceptions, e.g. if you want to cleanup and
rethrow the exception, or just print the exception at the top-level, you
can use the new function catchAny:
    catchAny :: IO a -> (forall e . Exception e => e -> IO a) -> IO a
You can happily write
    `catchAny` \e -> print e
where
    `catch` \e -> print e
would give you an ambiguous type variable error.

There's also
    ignoreExceptions :: IO () -> IO ()
which can be used instead of try for things like
    ignoreExceptions (hClose h)
(where we don't look at the result, so the exception type would be
ambiguous if we used try). (I'm not sure if this is the best name for
this function).

All the build failures I've seen with the new exceptions library have
been cases where you need to change a "catch" to "catchAny", "try" to
"ignoreExceptions", or occassionally a different function, e.g.
"bracket" or "handle", is used to handle any extension, so adding a type
signature involving the SomeException type solves the problem.

The old interface is available in Control.OldException. Currently it
doesn't catch exceptions that don't fit into the old Exception type; we
could catch them, show them and treat them as user errors, but then the
exception has changed if it gets rethrown.


Thanks
Ian

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