5.4.5 Implementation notes about dicts

Although dicts are designed as an abstract data type and we deliberately reserve the possibility to change the representation and even use multiple representations, this section describes the current implementation.

Dicts are currently represented as a compound term using the functor `dict`. The first argument is the tag. The remaining arguments create an array of sorted key-value pairs. This representation is compact and guarantees good locality. Lookup is order log( N ), while adding values, deleting values and merging with other dicts has order N. The main disadvantage is that changing values in large dicts is costly, both in terms of memory and time.

Future versions may share keys in a separate structure or use a binary trees to allow for cheaper updates. One of the issues is that the representation must either be kept canonical or unification must be extended to compensate for alternate representations.