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Command line

The clausal command runs a Clausal Prolog (.clausal) or ISO Prolog (.pl) program. It is installed with the package (pip install clausal); python -m clausal is the same command.

clausal [-g GOAL]... [--once] [--test] FILE [-- ARGS...]
clausal                      # no FILE: the interactive REPL

Running a program

Given only a file, clausal loads it and runs main/0:

% hello.clausal
:- module(hello, []).

main :- write('Hello, world'), nl.

:- end_module(hello).

$ clausal hello.clausal
Hello, world

The exit status is 0 when main succeeds and 1 when it fails or raises. main/0 need not be exported. A program with no main/0 is only loaded, so clausal lib.clausal is a quick load check: it reports a syntax or load error and exits 1, or exits 0.

Clausal Prolog refuses :- initialization(...), so main/0 is the entry point by name.

Running goals: -g

-g GOAL runs a goal in the program's module, instead of main/0, and prints each answer the way a Prolog toplevel does:

$ clausal family.clausal -g "grandparent(tom, X)"
X = ann.
X = pat.
$ clausal family.clausal -g "grandparent(tom, ann)"
true.
$ clausal family.clausal -g "grandparent(ann, X)"
false.
  • -g can be given more than once; the goals run in order.
  • --once (or -1) prints only the first answer of each goal.
  • Variables whose names start with _ are not printed.
  • A goal can use the operators the program has: its own op/3 directives and those of the libraries it imports, such as #= after :- use_module(library(clpz)).
  • A goal that fails prints false. and ends the run with status 1. A goal that raises prints the error term on standard error and exits 1. A goal that does not parse is a usage error, status 2.

Running tests: --test

clausal --test FILE runs the program's test/1 and test/2 clauses, the same as python -m clausal.testing FILE. See Testing.

Program arguments: --

Everything after -- is passed to the program, as a list of atoms in the read-only argv flag:

main :- current_prolog_flag(argv, Args), write(Args), nl.

$ clausal echo.clausal -- one two 3
[one,two,3]

Each argument is an atom, so the 3 above is the atom '3', not a number. Without --, argv is [].

Exit status

Status Meaning
0 main/0 or every -g goal succeeded, the tests passed, or the program loaded
1 a goal failed or raised, a test failed, or the program did not load
2 a usage error: a missing or unsupported FILE, a goal that does not parse, conflicting options

--test returns the test runner's own statuses; see Testing.