QueryEngine.query doesn't work for ASK when LIMIT is specified

[balhoff]

Issue type:

• :bug: Bug

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Description:

I am using QueryEngine.query in my own application, to handle user entered queries of any type (SELECT, CONSTRUCT, ASK). I switch on the result type ('bindings', 'quads', 'boolean'). But when I submit an ASK query, 'boolean' is never encountered, and I get an error in the console: Invalid query output type: Expected 'bindings' or 'quads' but got 'boolean'.

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Environment:

My app depends on "@comunica/query-sparql": "3.1.2", but I can also reproduce this in the main Comunica query demo at https://query.comunica.dev/#datasources=https%3A%2F%2Fquery.wikidata.org%2Fbigdata%2Fldf&query=ASK%0AWHERE%20%7B%0A%20%3Fs%20%3Fp%20%3Fo%20.%0A%7D%0ALIMIT%2010

Crash log:

[github-actions[bot]]

Thanks for reporting!

[balhoff]

Aha, now that I submitted this issue, I looked back at my demo query. I had not removed the LIMIT 10 from the ASK version. Removing this makes it work correctly. I just checked the SPARQL grammar and I think it says that a LIMIT is acceptable for ASK, although I'm not sure what effect it has.

[rubensworks]

Ah, indeed, looks like our LIMIT implementation does not support ASK queries: https://github.com/comunica/comunica/blob/master/packages/actor-query-operation-slice/lib/ActorQueryOperationSlice.ts#L42-L60 Probably because there's not much value in using LIMIT on ASK, but it would be better for the engine to just ignore the LIMIT as opposed to crashing.