OPTIONAL support

[maelle]

• FILTER in OPTIONAL is something we'll need for spq_label().
• should there be a way to say a triple is optional when creating it via spq_mutate()/spq_filter().
• should there be a way to add a filter for an optional pattern in spq_add() but also spq_mutate()/spq_filter() and if so with what syntax? a call to spq_filter() as argument?!

[maelle]

in #142 we add filters for optional patterns.

[maelle]

related to #202

[maelle]

• for spq_label() we could try and guess where to place it based on where the variable appears but that seems brittle.
• what if the user were the ones giving the filters, labels etc for an optional pattern. How should the interface best look like.

In dm there's something called zooming https://dm.cynkra.com/reference/dm_zoom_to.html#ref-examples

[maelle]

spq_init() %>%
  spq_add(blabla) %>%
  spq_filter(blabla) %>%
  spq_optional({
    spq_add(blop) %>%
    spq_label(blop) %>%
    spq_filter(blop)
})

is maybe too close to the SPARQL syntax.

[lvaudor]

I think we could consider that for spq_label, spq_filter, spq_mutate the result is optional if the "ancestor variable" is optional.

So

1. we'd complete query$vars with a new column stating whether the variable is optional or not: the variable would be considered optional if involved only in optional triple patterns.
2. all spq_label, spq_filter, spq_mutate calls with an optional variable as the ancestor variable would be optional.

[maelle]

ok, I'll try that (maybe next week if not this week), let's see how far we can go with this.

do you have more examples?

[maelle]

I think we could consider that for spq_label, spq_filter, spq_mutate the result is optional if the "ancestor variable" is optional.

@lvaudor do you have examples for when this would happen for the filtering and mutating?