XQuery 3.1 introduces the arrow operator to apply a function to an item in a much more readable way. In the same spirit, I would like to apply a function to items in a sequence in such a nice and easily readable way. 

Suppose, we have a sequence ("a", "b", "c") and we want to get a sequence of upper-case letters as a result. So using the ! operator we can get each item in the sequence and using the arrow operator we can apply the upper-case function. This will result in the following code:

  ("a", "b", "c") ! (. => upper-case())

which isn't really easier readable anymore because of the required parenthesis. It would be much nicer if we could add an operator, lets say ==>, which would apply the function for each item in the sequence, so we would simply write:

  ("a", "b", "c") ==> upper-case()

This gets even more relevant if the expression is longer. Lets take the example from the spec for the arrow operator at https://www.w3.org/TR/2014/WD-xquery-31-20140424/#id-arrow-operator:

  tokenize((normalize-unicode(upper-case($string))),"\s+")

Suppose, instead of a $string we have a $sequence. Currently, we would have to write:

  $item ! (. =>upper-case()=>normalize-unicode()=>tokenize("\s+"))

With a new operator this would change to

  $item==>upper-case()=>normalize-unicode()=>tokenize("\s+")

which removes difficult to read and nested parenthesis.

So to sum up, this double arrow operator is a postfix operator that applies a function to each item in a seuqnece, using each item as the first argument to the function.] If $s is a sequence and f() is a function, then $s==>f() is equivalent to $s ! f(.) or $s ! (. => f())