Editing Arrow tutorial (section)

== The <code>Arrow</code> class ==
A value of type <code>(Arrow a) => a b c</code> (commonly referred to as just an <i>arrow</i>) represents a process that takes as input a value of type <code>b</code> and outputs a value of type <code>c</code>.

The class includes the following methods:

* <code>arr</code> builds an arrow value out of a function:

:<haskell>
arr :: (Arrow a) => (b -> c) -> a b c
</haskell>

* <code>(>>>)</code> composes two arrow values to form a new one by "chaining" them together, one after another:

:<haskell>
(>>>) :: (Arrow a) => a b c -> a c d -> a b d
</haskell>

* <code>first</code> and <code>second</code> make a new arrow value out of an existing one.  They perform a transformation (given by their argument) on either the first or the second item of a pair:

:<haskell>
first  :: (Arrow a) => a b c -> a (b, d) (c, d)
second :: (Arrow a) => a b c -> a (d, b) (d, c)
</haskell>

:<code>first</code> and <code>second</code> may seem pretty strange at first, but they'll make sense in a few minutes.