Traversable package:indexed-traversable

A Traversable with an additional index. An instance must satisfy a (modified) form of the Traversable laws:

itraverse (const Identity) ≡ Identity
fmap (itraverse f) . itraverse g ≡ getCompose . itraverse (\i -> Compose . fmap (f i) . g i)

FunctorWithIndex, FoldableWithIndex, TraversableWithIndex This package provides three useful generalizations:

class Functor f => FunctorWithIndex i f | f -> i where
imap :: (i -> a -> b) -> f a -> f b

class Foldable f => FoldableWithIndex i f | f -> i where
ifoldMap :: Monoid m => (i -> a -> m) -> f a -> m

class (FunctorWithIndex i t, FoldableWithIndex i t, Traversable t) => TraversableWithIndex i t | t -> i where
itraverse :: Applicative f => (i -> a -> f b) -> t a -> f (t b)

This package contains instances for types in GHC boot libraries. For some additional instances see indexed-traversable-instances. The keys package provides similar functionality, but uses (associated) TypeFamilies instead of FunctionalDependencies.