It turns out that this combinator

f ~@~ g = (f .) . (. g)

is a powerful thing. It was abstracted from following (commonly used) pattern f . h . g where f and g are fixed.

This library not only defines ~@~ combinator, but also some derived combinators that can help us to easily define a lot of things including lenses. See lens package for details on what lenses are.

Function Data.Function.on can be implemented using ~@~ as:

on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
on f g = (id ~@~ g ~@~ g) f

If function 'on3' existed in base then it could be defined as:

on3 :: (b -> b -> b -> d) -> (a -> b) -> a -> a -> a -> d
on3 f g = (id ~@~ g ~@~ g ~@~ g) f

For more examples see documentation.

Stable releases with API documentation are available on Hackage.

• -fpedantic (disabled by default)

  Pass additional warning flags to GHC.

The BSD 3-Clause License, see LICENSE file for details.

Contributions, pull requests and bug reports are welcome! Please don't be afraid to contact author using GitHub or by e-mail (see .cabal file for that).