Editing Arrow tutorial (section)

== Some other arrow operations ==
Now let's define some operations that are generic to all arrow types:

* <code>split</code> is an arrow value that splits a single value into a pair of duplicate values:

<haskell>
split :: (Arrow a) => a b (b, b)
split = arr (\x -> (x,x))
</haskell>

* <code>unsplit</code> is an arrow value that takes a pair of values and combines them to return a single value:

<haskell>
unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d
unsplit = arr . uncurry       
     -- = \op -> arr (\(x,y) -> x `op` y)
</haskell>

* <code>(***)</code> combines two arrow values by running them on a pair (the first arrow value on the first component of the pair; the second arrow value on the second component of the pair):

<haskell>
f *** g = first f >>> second g
</haskell>

* <code>(&&&)</code> combines two arrow values by running them with the same input:

<haskell>
f &&& g = split >>> first f >>> second g
     -- = split >>> f *** g
</haskell>

* <code>liftA2</code> makes a new arrow value that combines the output from two other arrow values using a binary operation.  It works by splitting a value and operating on both halves and then combining the result:

<haskell>
liftA2 :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d
liftA2 op f g = split >>> first f >>> second g >>> unsplit op
           -- = f &&& g >>> unsplit op
</haskell>