In computer science, arrows provide a more general interface to computation than monads. Monads essentially provide a sequential interface to computation: one can build a computation out of a value, or sequence two computations together. Arrows provide more possibilities, including expressing (true, nondeterministic) parallel computation. Indeed, all monads in Haskell are instances of arrows of a particular kind, ArrowApply. Because arrows carry more type information than just the result type, composition can be more efficient — for example, eliminating space leaks.

Arrows have found use in functional reactive programming.

External links

• Arrows: A General Interface to Computation
• “Generalising Monads to Arrows”, John Hughes, in Science of Computer Programming 37, pp67–111, May 2000.
• A New Notation for Arrows, Ross Paterson, in ICFP, Sep 2001.
• Arrows and Computation, Ross Paterson, in The Fun of Programming, Palgrave, 2003.
• Arrow notation ghc manual