#2309: containers: specialize functions that fail in a Monad to Maybe

[Ross Paterson]

To summarize:

Several container classes have types like

	lookup :: (Monad m, Ord k) => k -> Map k a -> m a

and use fail to signal exceptional conditions.  This allows them to be
used with a range of monads, e.g. [], IO and parser monads.  Some of
these use the string passed to fail.  However the strings are not very
useful, and probably shouldn't be exposed in production code:

	Data.Map.lookup: Key not found
	Data.Map.lookupIndex: Key not found
	Data.Map.minViewWithKey: empty map
	Data.Map.maxViewWithKey: empty map
	Data.Map.minView: empty map
	Data.Map.maxView: empty map

The problem with this device is that depending on the monad, fail can
be an ordinary value, an exception or a runtime error (the default).
It complicates checking whether a program is safe, because it hides
possible runtime errors among testable conditions.

The proposal was to revert these types to the simpler

	lookup :: (Ord k) => k -> Map k a -> Maybe a

No information would be lost, as each of these functions has only one
use of fail -- the Maybe type describes the situation precisely.  As the
initial "thing with zero and return", it can be lifted to any other.
Several people argued that the non-Maybe case is rare, and explicitly
marking use of a different monad is no bad thing.

Changing the monad classes is not a possibility at this stage: we need
to work with the classes as defined in Haskell 98.

No-one defended the status quo, but Twan van Laarhoven and Dan Doel
argued that if the constraint were changed from Monad to MonadPlus,
one should be able to assume a safe fail, while avoiding the the need
for lifting.  At present, fail in STM and Seq use the default error,
which could be changed.  Haskell 98 specifies the fail in IO as throwing
an exception (the MonadPlus instance is in the mtl package).

In favour of the proposal were: apfelmus, Conor McBride, David
Menendez, Don Stewart, Duncan Coutts, Iavor Diatchki, Isaac Dupree,
Josef Svenningsson, Krasimir Angelov, Lennart Augustsson, Neil Mitchell,
Ross Paterson.

That's not consensus, but it is a substantial majority, and I think
we've explored all the issues.  So I propose to make the change.