SWI-Prolog 9.2.6 Reference Manual: Section 2.9

## 2.9 Reuse of top-level bindings

Bindings resulting from the successful execution of a top-level goal are asserted in a database if they are not too large (as defined by the Prolog flag toplevel_var_size). These values may be reused in further top-level queries as $Var. If the same variable name is used in a subsequent query the system associates the variable with the latest binding. Example:

    1 ?- maplist(plus(1), `hello`, X).
    X = [105,102,109,109,112].

    2 ?- format('~s~n', [$X]).
    ifmmp
    true.

    3 ?-

Figure 1 : Reusing top-level bindings

Note that variables may be set by executing =/2:

    6 ?- X = statistics.
    X = statistics.

    7 ?- $X.
    % Started at Fri Aug 24 16:42:53 2018
    % 0.118 seconds cpu time for 456,902 inferences
    % 7,574 atoms, 4,058 functors, 2,912 predicates, 56 modules, 109,791 VM-codes
    %
    %                     Limit   Allocated      In use
    % Local  stack:           -       20 Kb    1,888  b
    % Global stack:           -       60 Kb       36 Kb
    % Trail  stack:           -       30 Kb    4,112  b
    %        Total:    1,024 Mb      110 Kb       42 Kb
    %
    % 3 garbage collections gained 178,400 bytes in 0.000 seconds.
    % 2 clause garbage collections gained 134 clauses in 0.000 seconds.
    % Stack shifts: 2 local, 2 global, 2 trail in 0.000 seconds
    % 2 threads, 0 finished threads used 0.000 seconds
    true.