878 Proposed extension to subsequence

[michaelhkay]

Following discussion under issue #844, I decided to explore the possibility of extending subsequence() with optional parameters, with the aim of making the quartet of items-before/after/starting-with/ending-with unnecessary.

This is the spec that results. I feel it's a good trade-off; by adding three optional parameters to fn:subsequence, we can eliminate 4 functions that we are having trouble finding names for. The examples feel to me to be intuitive and readable; and there is more capability in the new function than we had before, for example by combining a predicate for the start position with an integer for the length.

I haven't explored arity-2 callbacks - these certainly need some notes and examples.

[ChristianGruen]

Suggested change

	are supplied, he function returns a sequence comprising those items of
	are supplied, the function returns a sequence comprising those items of

[ChristianGruen]

We should probably raise an error…

• if both $start and $from are supplied,
• if more than one of $length, $while, or $until is supplied, or
• optionally, if none of the 5 arguments is supplied (unless we want to allow fn:subsequence($seq) returning $seq).

[michaelhkay]

Yes, we should add specific error conditions for invalid combinations of arguments.

Yes, it is intended that if start and length are both defaulted, you get the whole sequence back. It's not a useful combination of options, but since it falls out naturally, there's no point disallowing it.

[ChristianGruen]

Looks good. One or two examples and an explanatory note – or some equivalent code – could be added for the positional argument ($pos).

[michaelhkay]

For an interesting use case where the new function might be convenient, see

https://stackoverflow.com/questions/77724793

[ChristianGruen]

My summarized comments:

1. We should raise an error…

• if both $start and $from are supplied, or
• if more than one of $length, $while, or $until is supplied.

2. One or two examples and an explanatory note – or some equivalent code – should be added for the positional argument ($pos).

[michaelhkay]

Notes from discussion on 9th Jan. There was a feeling the proposed function was too complex. I agreed to explore the idea of a simpler subsequence-where() function.

Perhaps subsequence-where($input, $from as predicate := true#0, $to as predicate := false#0), with the semantics being something like

let $start := index-where($input, $from)[1] otherwise number('+INF')
let $tail := subsequence($input, $start)
let $end := index-where($tail, $to)[1] otherwise number('+INF')
return subsequence($tail, 1, $end)

[ChristianGruen]

Perhaps subsequence-where($input, $from as predicate := true#0, $to as predicate := false#0), with the semantics being something like

Related: #844 (comment)

[ChristianGruen]

Dropped, in favor of #940.