assoc library del_assoc predicate

[mvolkmann]

Why does the del_assoc predicate in the assoc library described at https://www.scryer.pl/assoc.html need both the key and the value to delete? It seems like supplying just the key should be enough.

[triska]

The mode indicator ? indicates that the value need not be supplied. If needed, this predicate can be used to—procedurally speaking—obtain the value.

[mvolkmann]

I tried removing that argument and get error(existence_error(procedure,del_assoc/3),del_assoc/3). So I guess it is not optional in the sense that it can be omitted, only in the sense that I don't have to supply the value that corresponds to the key being deleted. It works if I pass _ for the value. Is that what I should do if I want to delete a key/value pair based on only the key?

[triska]

Yes, exactly!