#2309: containers: specialize functions that fail in a Monad to
Maybe
Ross Paterson
ross at soi.city.ac.uk
Sat May 24 19:59:38 EDT 2008
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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