containers: specialize functions that fail in a Monad to Maybe

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.

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown