Arrow
import Control.Arrow 
Overview
Arrows, or Freydcategories, are a generalization of Monads.
"They can do everything monads can do, and more. They are roughly comparable to monads with a static component." However "Arrows do have some problems". (need a more useful comparison)
For an introduction, see #External links.
Library
 Control.Arrow is the standard library for arrows.
 Arrow transformer library (see the bottom of the page) is an extension with arrow transformers, subclasses, useful data types (Data.Stream, Data.Sequence).
 arrows package
Examples
Various concepts follow here, which can be seen as concrete examples covered by the arrow concept. Not all of them provide links to Haskellrelated materials: some of them are here only to give a selfcontained material (e.g. section #Automaton gives links only to the finite state concept itself.).
Practice
Reasons, when it may be worth of solving a specific problem with arrows (instead of monads) can be read in a message from Daan Leijen. But Leijen's post is rather old (2000). Arrows are now significantly easier to understand and use than they were back then. Eg, his example might be rewritten
test = proc _ > do question < ask < "what is the question ?" answer < ask < question returnA < ("the answer to '" ++ question ++ "' is " ++ answer)
(or something vaguely like that).
Function
Arrow operations arr
and >>>
are rather straightforward. For implementing first
and related concepts, see Prelude extensions#Tuples.
Parser
The reasons why the arrow concept can solve important questions when designing a parser library are explained in Generalising Monads to Arrows written by John Hughes.
A good example of the mentioned arrow parsers can be seen in A New Notation for Arrows written by Ross Paterson: figure 2, 4, 6 (page 3, 5, 6):
is represented with arrow parsers this way:
data Expr = Plus Expr Expr  Minus Expr Expr  ...
expr :: ParseArrow () Expr
expr = proc () > do
t < term < ()
exprTail < t
exprTail :: ParseArrow Expr Expr
exprTail = proc e > do
symbol PLUS < ()
t < term < ()
exprTail < Plus e t
<+> do
symbol MINUS < ()
t < term < ()
exprTail < Minus e t
<+> returnA < e
An arrow parser library: PArrows written by Einar Karttunen.
Another arrow parser implementation: LLParser.hs written by AnttiJuhani Kaijanaho (I read the reference to it in Shae Erisson's blog / journal).
The funny thing which took a long time for me to understand arrow parsers is a sort of differential approach  in contrast to the wellknown parser approaches. (I mean, in some way wellknown parsers are of differential approach too, in the sense that they manage state transitions where the states are remainder streams  but here I mean being differential in another sense: arrow parsers seem to me differential in the way how they consume and produce values  their input and output.)
The idea of borrowing this image from mathematical analysis comes from another topic: the version control systems article Integrals and derivatives written by Martin Pool uses a similar image.
Arrows and Computation written by Ross Paterson (pages 2, 6, 7) and ProdArrows  Arrows for Fudgets
written by Magnus Carlsson (page 9) mentions that computation (e.g. state) is threaded through the operands of &&&
operation.
I mean, even the mere definition of &&&
operation
p &&& q = arr dup >>> first p >>> second q
shows that the order of the computation (the side effects) is important when using &&&
, and this can be exemplified very well with parser arrows. See an example found in PArrows written by Einar Karttunen (see module Text.ParserCombinators.PArrow.Combinator
):
  Match zero or more occurrences of the given parser.
many :: MD i o > MD i [o]
many = MStar
  Match one or more occurrences of the given parser.
many1 :: MD i o > MD i [o]
many1 x = (x &&& MStar x) >>> pure (\(b,bs) > (b:bs))
The definition of between
parser combinator can show another example for the importance of the order in which the computation (e.g. the side effects) take place using &&&
operation:
between :: MD i t > MD t close > MD t o > MD i o
between open close real = open >>> (real &&& close) >>^ fst
A more complicated example (from the same module):
  Match one or more occurrences of the given parser separated by the separator.
sepBy1 :: MD i o > MD i o' > MD i [o]
sepBy1 p s = (many (p &&& s >>^ fst) &&& p) >>^ (\(bs,b) > bs++[b])
This makes clear that the order of effects of the operands of &&&
operation can be important. But let us mention also a counterexample, e.g. nondeterministic functions arrows, or more generally, the various implementations of binary relation arrows  there is no such sequencing of effect orders. Now let us see this fact on the mere mathematical concept of binary relations (not minding how it implemented):

&&&


The picture illustrating ***
in Haskell/Understanding arrows article of Wikibooks suggests exactly such a view: order of side effects can be unimportant at some arrow instances, and the symmetry of the figure reflects this. In generally, however, the figure should use a notation for threading through side effects in a sequence.
Stream processor
The Lazy K programming language is an interesting esoteric language (from the family of pure, lazy functional languages), whose I/O concept is approached by streams.
Arrows are useful also to grasp the concept of stream processors. See details in
 ProdArrows  Arrows for Fudgets written by Magnus Carlsson, 2001.
 Generalising Monads to Arrows written by John Hughes (section 6, pages 2024)
Functional I/O, graphical user interfaces
 On the Expressiveness of Purely Functional I/O Systems written by Paul Hudak and Raman S. Sundaresh.
 Fudgets written by Thomas Hallgren and Magnus Carlsson. See also Arrows for Fudgets written by Magnus Carlsson, mentioning how these two concepts relate to each other.
 FG
 Phooey
 Grapefruit
Dataflow languages
Arrows and Computation written by Ross Paterson mentions how to mimic dataflow programming in (lazy) functional languages. See more on Lucid's own HaskellWiki page: Lucid.
Automaton
To see what the concept itself means, see the Wikipedia articles Finite state machine and also Automata theory.
How these concepts can be implemented using the concept of arrow, can be found in the introductory articles on arrows mentioned above.
Haskell XML Toolbox
HXT is an example of a real application using Arrows
External links
 Programming with Arrows  A tutorial introduction to arrows and arrow notation. Very didactic, introducing the arrow subclasses with detailed examples and rich explanations on the motivations of each decision.
 Arrows: A General Interface to Computation written by Ross Paterson.
 Generalising Monads to Arrows, and other papers
 ProdArrows  Arrows for Fudgets is also a good general material on the arrow concept (and also good for seeing, how arrows can be used to implement stream processors and Fudgets). It is written by Magnus Carlsson.
 Haskell/Arrows on Wikibooks, followed by Understanding arrows, which uses a factory/conveyor belt metaphor for arrows. We know this image for monads, but it is modified here for arrows, too.
 HaWiki's UnderstandingArrows. (An old page in the Web Archive (slow))