SWI-Prolog -- (:<)/2

Documentation
- Reference manual
  - SWI-Prolog extensions
    - Dicts: structures with named arguments
      - Predicates for managing dicts
        
        is_dict/1
        
        is_dict/2
        
        get_dict/3
        
        get_dict/5
        
        dict_create/3
        
        dict_pairs/3
        
        put_dict/3
        
        put_dict/4
        
        del_dict/4
        
        :</2
        
        select_dict/3
        
        >:</2
        
        Destructive assignment in dicts
- Packages

Availability:built-in

[semidet]+Select :< +From

True when Select is a‘sub dict' of From: the tags must unify and all keys in Select must appear with unifying values in From. From may contain keys that are not in Select. This operation is frequently used to match a dict and at the same time extract relevant values from it. For example:

plot(Dict, On) :-
        _{x:X, y:Y, z:Z} :< Dict, !,
        plot_xyz(X, Y, Z, On).
plot(Dict, On) :-
        _{x:X, y:Y} :< Dict, !,
        plot_xy(X, Y, On).

The goal Select :< From is equivalent to select_dict(Select, From, _).