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 |