SWI-Prolog has a set of flags that let you examine and configure the Prolog execution runtime. You can set Prolog flags from Emacs directly with the following command:
Set the value of a Prolog flag.
This command let’s you interactively configure the embedded Prolog execution environment by changing the values of Prolog flags. It prompts you for a Prolog flag, with completion candidates annotated with their current values. Then, it prompts again for a Prolog term and sets the flag’s value to that term.
For more information about Prolog flags in SWI-Prolog, see Environment Control in the SWI-Prolog manual.
As an example, the Prolog flag double_quotes controls the
interpretation of double quotes in Prolog code. By default,
double_quotes is set to string, so for instance
"foo" is read as a SWI-Prolog string. You can easily validate
this in the Sweep top-level:
?- A = "foo". A = "foo".
You can change the interpretation of double quotes to denote lists of
character codes, by setting the value the double_quotes flag to
codes with M-x sweeprolog-set-prolog-flag RET
double_quotes RET codes RET. Evaluating A = "foo"
again exhibits the different interpretation:
?- A = "foo". A = [102, 111, 111].
Note that some flags have a thread-local value, and
sweeprolog-set-prolog-flag always operates only on the main
thread. To set flags in an existing top-level thread, use the
predicate set_prolog_flag/2 directly in that top-level.