[XPath] Functions that determine if a given sequence starts with another sequence or ends with another sequence

[dnovatchev]

It is surprising that we are at version 4 and still are missing:

(1) fn:starts-with-sequence($container as item()*, $maybe-start as item()*, 
                            $compare as function(item(), item()) as xs:boolean := deep-equal)
                            ) as xs:boolean

and

(2) fn:ends-with-sequence($container as item()*, $maybe-end as item()*, 
                          $compare as function(item(), item()) as xs:boolean := deep-equal)
                          ) as xs:boolean

(2) above is a shorthand for:

fn:starts-with-sequence(reverse($container), reverse($maybe-end)) 

Examples:

fn:starts-with-sequence(('a', 'b', 'c', 'd'), ('a', 'b')) returns true()

fn:starts-with-sequence(('a', 'b', 'c', 'd'), ('a', 'c')) returns false()

fn:ends-with-sequence(('a', 'b', 'c', 'd'), ('c', 'd')) returns true()

fn:ends-with-sequence(('a', 'b', 'c', 'd'), ('b', 'd')) returns false()

('a', 'b', 'c', 'd') => starts-with-sequence(('a', 'b')) returns true()

('a', 'b', 'c', 'd') => starts-with-sequence(('a', 'c')) returns false()

('a', 'b', 'c', 'd') => ends-with-sequence(('c', 'd')) returns true()

('a', 'b', 'c', 'd') => ends-with-sequence(('b', 'd')) returns false()

One possible implementation:

let $starts-with-sequence := function($seq1 as item()*, $seq2 as item()*, $self as function(*))
{
   empty($seq2)
  or
   head($seq1) eq head($seq2) and $self(subsequence($seq1, 2), subsequence($seq2, 2), $self)
}
  return
    $starts-with-sequence(('a', 'b', 'c', 'd'), ('a', 'b', 'c'), $starts-with-sequence)

[michaelhkay]

Note workarounds such as

all(for-each-pair($A, $B, my:compare#2))

[dnovatchev]

Note workarounds such as
all(for-each-pair($A, $B, my:compare#2))

And for ends-with-sequence():

   all(for-each-pair(reverse($A), reverse($B), my:compare#2))

However this is rather challenging to read and understand.

Thanks god, for strings we have fn:starts-with() and fn:ends-with(), so we will not have to write:

all(for-each-pair(string-to-codepoints($A), string-to-codepoints($B), my:compare#2)

and

all(for-each-pair(reverse(string-to-codepoints($A)), reverse(string-to-codepoints($B)), my:compare#2)

Some of us can treat this as a problem, spend some time and find a suitable XPath expression.

The whole meaning of this proposal is that doing this will no longer be a problem for anyone.

This proposal aims at achieving conceptual integrity, simplicity, readability and easy understandability / convenience

[michaelhkay]

The issue can now be closed, PR #222 was accepted on 2022-11-01
https://qt4cg.org/meeting/minutes/2022/11-01.html#pr-sequence-comparison