SWI-Prolog 7.4.2 Reference Manual: Section 2.8

## 2.8 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. 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.
    28.00 seconds cpu time for 183,128 inferences
    4,016 atoms, 1,904 functors, 2,042 predicates, 52 modules
    55,915 byte codes; 11,239 external references

                          Limit    Allocated       In use
    Heap         :                                624,820 Bytes
    Local  stack :    2,048,000        8,192          404 Bytes
    Global stack :    4,096,000       16,384          968 Bytes
    Trail  stack :    4,096,000        8,192          432 Bytes
    true.