A.25 library(main): Provide entry point for scripts

See also

- library(prolog_stack) to force backtraces in case of an uncaught exception.
- XPCE users should have a look at library(pce_main), which starts the GUI and processes events until all windows have gone.

This library is intended for supporting PrologScript on Unix using the #! magic sequence for scripts using commandline options. The entry point main/0 calls the user-supplied predicate main/1 passing a list of commandline options. Below is a simle echo implementation in Prolog.

#!/usr/bin/env swipl

:- initialization(main, main).

main(Argv) :-
    echo(Argv).

echo([]) :- nl.
echo([Last]) :- !,
    write(Last), nl.
echo([H|T]) :-
    write(H), write(' '),
    echo(T).

main

Call main/1 using the passed command-line arguments. Before calling main/1 this predicate installs a signal handler for SIGINT (Control-C) that terminates the process with status 1.

When main/0 is called interactively it simply calls main/1 with the arguments. This allows for debugging scripts as follows:

$ swipl -l script.pl -- arg ...
?- gspy(suspect/1).		% setup debugging
?- main.			% run program

[det]argv_options(:Argv, -Positional, -Options)

Parse command line arguments. This predicate acts in one of two modes.

• If the calling module defines opt_type/3, full featured parsing with long and short options, type conversion and help is provided.
• If opt_type/3 is not defined, only unguided transformation using long options is supported. See argv_untyped_options/3 for details.

When guided, three predicates are called in the calling module. opt_type/3 must be defined, the others need not. Note that these three predicates may be defined as multifile to allow multiple modules contributing to the provided commandline options. Defining them as discontiguous allows for creating blocks that describe a group of related options.

opt_type(Opt, Name, Type)

Defines Opt to add an option Name(Value), where Value statisfies Type. Opt does not include the leading -. A single character implies a short option, multiple a long option. Long options use _ as word separator, user options may use either _ or -. Type is one of:

A | B

Disjunctive type. Disjunction can be used create long options with optional values. For example, using the type nonneg|boolean, for an option http handles --http as http(true), --no-http as http(false), --http=3000 and --http 3000 as http(3000). With an optional boolean an option is considered boolean if it is the last or the next argument starts with a hyphen (-).

boolean(Default)

boolean

Boolean options are special. They do not take a value except for when using the long --opt=value notation. This explicit value specification converts true, True, TRUE, on, On, ON, 1 and the obvious false equivalents to Prolog true or false. If the option is specified, Default is used. If --no-opt or --noopt is used, the inverse of Default is used.

integer

Argument is converted to an integer

float

Argument is converted to a float. User may specify an integer

nonneg

As integer. Requires value >= 0.

natural

As integer. Requires value >= 1.

number

Any number (integer, float, rational).

between(Low, High)

If both one of Low and High is a float, convert as float, else convert as integer. Then check the range.

atom

No conversion

oneof(List)

As atom, but requires the value to be a member of List (enum type).

string

Convert to a SWI-Prolog string

file

Convert to a file name in Prolog canonical notation using prolog_to_os_filename/2.

file(Access)

As file, and check access using access_file/2. A value - is not checked for access, assuming the application handles this as standard input or output.

term

Parse option value to a Prolog term.

term(+Options)

As term, but passes Options to term_string/3. If the option variable_names(Bindings) is given the option value is set to the pair Term-Bindings.

opt_help(Name, HelpString)

Help string used by argv_usage/1.

opt_meta(Name, Meta)

If a typed argument is required this defines the placeholder in the help message. The default is the uppercase version of the type functor name. This produces the FILE in e.g. -f FILE.

By default, -h, -? and --help are bound to help. If opt_type(Opt, help, boolean) is true for some Opt, the default help binding and help message are disabled and the normal user rules apply. In particular, the user should also provide a rule for opt_help(help, String).

[det]argv_options(:Argv, -Positional, -Options, +ParseOptions)

As argv_options/3 in guided mode, Currently this version allows parsing argument options throwing an exception rather than calling halt/1 by passing an empty list to ParseOptions. ParseOptions:

on_error(+Goal)

If Goal is halt(Code), exit with Code. Other goals are currently not supported.

options_after_arguments(+Boolean)

If false (default true), stop parsing after the first positional argument, returning options that follow this argument as positional arguments. E.g, -x file -y results in positional arguments [file, '-y']

[det]argv_usage(:Level)

Use print_message/2 to print a usage message at Level. To print the message as plain text indefault color, use debug. Other meaningful options are informational or warning. The help page consists of four sections, two of which are optional:

1. The header is created from opt_help(help(header), String). It is optional.
2. The usage is added by default. The part behind Usage: <command> is by default [options] and can be overruled using opt_help(help(usage), String).
3. The actual option descriptions. The options are presented in the order they are defined in opt_type/3. Subsequent options for the same destination (option name) are joined with the first.
4. The footer_ is created from opt_help(help(footer), String). It is optional.

The help provided by help(header), help(usage) and help(footer) are either a simple string or a list of elements as defined by print_message_lines/3. In the latter case, the construct \Callable can be used to call a DCG rule in the module from which the user calls argv_options/3. For example, we can add a bold title using

opt_help(help(header), [ansi(bold, '~w', ['My title'])]).

[det]cli_parse_debug_options(+OptionsIn, -Options)

Parse certain commandline options for debugging and development purposes. Options processed are below. Note that the option argument is an atom such that these options may be activated as e.g., --debug='http(_)'.

debug(Topic)

Call debug(Topic). See debug/1 and debug/3.

spy(Predicate)

Place a spy-point on Predicate.

gspy(Predicate)

As spy using the graphical debugger. See tspy/1.

interactive(true)

Start the Prolog toplevel after main/1 completes.

cli_enable_development_system

Re-enable the development environment. Currently re-enables xpce if this was loaded, but not initialised and causes the interactive toplevel to be re-enabled.

This predicate may be called from main/1 to enter the Prolog toplevel rather than terminating the application after main/1 completes.