What about arrows?

Question

Reading through various tutorials about Haskell\'s various category-themed classes, we find things like Monoid, Functor, Monad and so on -

Answer 1

HXT, a library which is used for parsing XML, is a very good example for the usage of arrows (have a look how often the word Arrow occurs in the module names of this package!). You shall have a look on the great tutorial: http://adit.io/posts/2012-04-14-working_with_HTML_in_haskell.html

But it is also good to have the arrow concept for functions. For example the following code

((+1) &&& (*2)) 3 -- result is (4,6)

just works, because (->) is an instance of the arrow class (The operator &&& is defined in Control.Arrow).

Thanks to the arrow syntax you have also a great tool to write complex computations in Haskell (it works as well for functions, monads and XML filters in HXT).