hashtable:ht_update/4

swipl
- library

ht_update(+HT, +Key, ?Old, +New) is semidet

True when HT holds Key-Old before and Key-New after this call. Note that it is possible to update to a variable and the instantiate this. For example, a word-count update could be implemented as:

update_word_count(HT, Word) :-
    (   ht_update(HT, Word, Old, New)
    ->  New is Old+1
    ;   ht_put(HT, Word, 1)
    ).