FunctorWithIndex, FoldableWithIndex, TraversableWithIndex

LTS Haskell 20.23:
Stackage Nightly 2023-06-04:
Latest on Hackage:

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BSD-2-Clause licensed by Edward Kmett
Maintained by Oleg Grenrus
This version can be pinned in stack with:indexed-traversable-,2545

Module documentation for

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.

Changes [2022-12-28]

0.1.2 [2021-10-30]

  • Changed (<$>) + (<*>) to liftA2 to potentially avoid extra fmap.
  • Add (#..), coercive composition after a 2-parameter function.
  • Switched references to lens ‘Of’- functions to base functions.

Thanks to wygulmage for contributions.

0.1.1 [2020-12-27]

0.1 [2020-12-15]

  • Split out and combine this package functionality from lens and optics