This module helps in building a command-line interface to an application. In particular, it provides functions that take an option specification and a list of atoms, probably given to the program on the command line, and return a parsed representation (a list of the customary Key(Val) by default; or optionally, a list of Func(Key, Val) terms in the style of current_prolog_flag/2). It can also synthesize a simple help text from the options specification.
The terminology in the following is partly borrowed from python, see
http://docs.python.org/library/optparse.html#terminology
. Very briefly,
arguments is what you provide on the command line and for many
prologs show up as a list of atoms Args in current_prolog_flag(argv, Args).
For a typical prolog incantation, they can be divided into
Positional arguments are in particular used for mandatory arguments without which your program won't work and for which there are no sensible defaults (e.g,, input file names). Options, by contrast, offer flexibility by letting you change a default setting. Options are optional not only by etymology: this library has no notion of mandatory or required options (see the python docs for other rationales than laziness).
The command-line arguments enter your program as a list of atoms, but the programs perhaps expects booleans, integers, floats or even prolog terms. You tell the parser so by providing an options specification. This is just a list of individual option specifications. One of those, in turn, is a list of ground prolog terms in the customary Name(Value) format. The following terms are recognized (any others raise error).
current_prolog_flag(Key, Value). This term is mandatory (an
error is thrown if missing).
-s , -K, etc). Uppercase
letters must be quoted. Usually ListOfFlags will be a
singleton list, but sometimes aliased flags may be convenient.
--verbose, --no-debug, etc). They are
basically a more readable alternative to short flags, except
--flag value or
--flag=value (but not as --flagvalue); short
flags as
-f val or -fval (but not -f=val)
--bool-flag or --bool-flag=true
or --bool-flag true; and they can be negated as --no-bool-flag
or --bool-flag=false or
--bool-flag false.
Except that shortflags must be single characters, the distinction
between long and short is in calling convention, not in namespaces.
Thus, if you have shortflags([v]), you can use it as -v2
or -v 2 or --v=2 or --v 2 (but
not
-v=2 or --v2).
Shortflags and longflags both default to []. It can be
useful to have flagless options -- see example below.
x:integer=3, interest:float=0.11). It may be
useful to have named variables (x, interest)
in case you wish to mention them again in the help text. If not given
the Meta: part is suppressed -- see example below.
boolean, atom, integer, float, term.
The corresponding argument will be parsed appropriately. This term is
optional; if not given, defaults to term.
Long lines are subject to basic word wrapping -- split on white
space, reindent, rejoin. However, you can get more control by supplying
the line breaking yourself: rather than a single line of text, you can
provide a list of lines (as atoms). If you do, they will be joined with
the appropriate indent but otherwise left untouched (see the option mode
in the example below).
Absence of mandatory option specs or the presence of more than one for a particular option throws an error, as do unknown or incompatible types.
As a concrete example from a fictive application, suppose we want the
following options to be read from the command line (long flag(s),
short
flag(s), meta:type=default, help)
--mode -m atom=SCAN data gathering mode,
one of
SCAN: do this
READ: do that
MAKE: make numbers
WAIT: do nothing
--rebuild-cache -r boolean=true rebuild cache in
each iteration
--heisenberg-threshold -t,-h float=0.1 heisenberg threshold
--depths, --iters -i,-d K:integer=3 stop after K
iterations
--distances term=[1,2,3,5] initial prolog term
--output-file -o FILE:atom=_ write output to FILE
--label -l atom=REPORT report label
--verbosity -v V:integer=2 verbosity level,
1 <= V <= 3
We may also have some configuration parameters which we currently
think not needs to be controlled from the command line, say
path('/some/file/path').
This interface is described by the following options specification (order between the specifications of a particular option is irrelevant).
ExampleOptsSpec =
[ [opt(mode ), type(atom), default('SCAN'),
shortflags([m]), longflags(['mode'] ),
help([ 'data gathering mode, one of'
, ' SCAN: do this'
, ' READ: do that'
, ' MAKE: fabricate some numbers'
, ' WAIT: don''t do anything'])]
, [opt(cache), type(boolean), default(true),
shortflags([r]), longflags(['rebuild-cache']),
help('rebuild cache in each iteration')]
, [opt(threshold), type(float), default(0.1),
shortflags([t,h]), longflags(['heisenberg-threshold']),
help('heisenberg threshold')]
, [opt(depth), meta('K'), type(integer), default(3),
shortflags([i,d]),longflags([depths,iters]),
help('stop after K iterations')]
, [opt(distances), default([1,2,3,5]),
longflags([distances]),
help('initial prolog term')]
, [opt(outfile), meta('FILE'), type(atom),
shortflags([o]), longflags(['output-file']),
help('write output to FILE')]
, [opt(label), type(atom), default('REPORT'),
shortflags([l]), longflags([label]),
help('report label')]
, [opt(verbose), meta('V'), type(integer), default(2),
shortflags([v]), longflags([verbosity]),
help('verbosity level, 1 <= V <= 3')]
, [opt(path), default('/some/file/path/')]
].
The help text above was accessed by opt_help(ExamplesOptsSpec, HelpText).
The options appear in the same order as in the OptsSpec.
Given ExampleOptsSpec, a command line (somewhat
syntactically inconsistent, in order to demonstrate different calling
conventions) may look as follows
ExampleArgs = [ '-d5'
, '--heisenberg-threshold', '0.14'
, '--distances=[1,1,2,3,5,8]'
, '--iters', '7'
, '-ooutput.txt'
, '--rebuild-cache', 'true'
, 'input.txt'
, '--verbosity=2'
].
opt_parse(ExampleOptsSpec, ExampleArgs, Opts, PositionalArgs)
would then succeed with
Opts = [ mode('SCAN')
, label('REPORT')
, path('/some/file/path')
, threshold(0.14)
, distances([1,1,2,3,5,8])
, depth(7)
, outfile('output.txt')
, cache(true)
, verbose(2)
],
PositionalArgs = ['input.txt'].
Note that path('/some/file/path') showing up in Opts has
a default value (of the implicit type’term'), but no corresponding
flags in OptsSpec. Thus it can't be set from the command line. The rest
of your program doesn't need to know that, of course. This provides an
alternative to the common practice of asserting such hard-coded
parameters under a single predicate (for instance setting(path, '/some/file/path')),
with the advantage that you may seamlessly upgrade them to command-line
options, should you one day find this a good idea. Just add an
appropriate flag or two and a line of help text. Similarly, suppressing
an option in a cluttered interface amounts to commenting out the flags.
opt_parse/5 allows more control through an additional argument list as shown in the example below.
?- opt_parse(ExampleOptsSpec, ExampleArgs, Opts, PositionalArgs,
[ output_functor(appl_config)
]).
Opts = [ appl_config(verbose, 2),
, appl_config(label, 'REPORT')
...
]
This representation may be preferable with the empty-flag configuration parameter style above (perhaps with asserting appl_config/2).
term, which subsumes integer, float, atom,
it may be possible to get away cheaper (e.g., by only giving booleans).
However, it is recommended practice to always specify types: parsing
becomes more reliable and error messages will be easier to interpret.
-sbar is taken to mean -s bar,
not -s -b -a -r, that is, there is no clustering of flags.
-s=foo is disallowed. The rationale is that although
some command-line parsers will silently interpret this as -s =foo,
this is very seldom what you want. To have an option argument start with’='
(very un-recommended), say so explicitly.
depth twice: once as
-d5 and once as --iters 7. The default when
encountering duplicated flags is to keeplast (this
behaviour can be controlled, by ParseOption duplicated_flags).
Opts is a list of parsed options in the form Key(Value).
Dashed args not in OptsSpec are not permitted and will raise
error (see tip on how to pass unknown flags in the module description).
PositionalArgs are the remaining non-dashed args after each
flag has taken its argument (filling in true or false
for booleans). There are no restrictions on non-dashed arguments and
they may go anywhere (although it is good practice to put them last).
Any leading arguments for the runtime (up to and including’--')
are discarded.
opt_parse(OptsSpec, ApplArgs, Opts, PositionalArgs, []).Opts is a list of parsed options in the form Key(Value),
or (with the option functor(Func) given) in the form
Func(Key, Value). Dashed args not in OptsSpec are not
permitted and will raise error (see tip on how to pass unknown flags in
the module description).
PositionalArgs are the remaining non-dashed args after each
flag has taken its argument (filling in true or false
for booleans). There are no restrictions on non-dashed arguments and
they may go anywhere (although it is good practice to put them last).
ParseOptions are
keepfirst, keeplast, keepall with
the obvious meaning. Default is keeplast.
[] or absent).
Flagless options cannot be manipulated from the command line and will
not show up in the generated help. This is useful when you have (also)
general configuration parameters in your OptsSpec, especially
if you think they one day might need to be controlled externally. See
example in the module overview.
allow_empty_flag_spec(false) gives the more customary
behaviour of raising error on empty flags.