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

[Don Stewart]

+1 in favour.

This is Haskell, and as such, program crashable effects should not be
the default, whenever possible.

Regarding support for older programs relying on IO effects here,
deprecated versions with the old type would be a good concession to
backwards compatibility, or adding a lifter of, 

    ... => Maybe a -> m a

to Maybe might also be wise.

-- Don

ross:
> Several functions on containers used to have types like
> 
> 	lookup :: (Ord k) => k -> Map k a -> Maybe a
> 
> but these were generalized to
> 
> 	lookup :: (Monad m, Ord k) => k -> Map k a -> m a
> 
> The only methods of Monad used are return and fail. The problem is that,
> depending on the monad, fail can be an ordinary value or a runtime error:
> this device makes it harder to check whether a program is safe, because
> it hides possible runtime errors among testable conditions.
> 
> The proposal is to change these signatures back, specializing them
> to Maybe.
> 
> The functions involved are:
> 
> 	lookup :: Ord k => k -> Map k a -> Maybe a
> 	lookupIndex :: Ord k => k -> Map k a -> Maybe Int
> 	minViewWithKey :: Map k a -> Maybe ((k,a), Map k a)
> 	maxViewWithKey :: Map k a -> Maybe ((k,a), Map k a)
> 	minView :: Map k a -> Maybe (a, Map k a)
> 	maxView :: Map k a -> Maybe (a, Map k a)
> 
> 	lookup :: Key -> IntMap a -> Maybe a
> 	maxViewWithKey :: IntMap a -> Maybe ((Key, a), IntMap a)
> 	minViewWithKey :: IntMap a -> Maybe ((Key, a), IntMap a)
> 	maxView :: IntMap a -> Maybe (a, IntMap a)
> 	minView :: IntMap a -> Maybe (a, IntMap a)
> 
> 	minView :: Set a -> Maybe (a, Set a)
> 	maxView :: Set a -> Maybe (a, Set a)
> 
> 	maxView :: IntSet -> Maybe (Int, IntSet)
> 	minView :: IntSet -> Maybe (Int, IntSet)
> 
> No information is lost, because in each case there is a single failure
> mode.
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