NAME¶
Getopt::Tabular - table-driven argument parsing for Perl 5
SYNOPSIS¶
use Getopt::Tabular;
(or)
use Getopt::Tabular qw/GetOptions
SetHelp SetHelpOption
SetError GetError/;
...
&Getopt::Tabular::SetHelp (long_help, usage_string);
@opt_table = (
[section_description, "section"],
[option, type, num_values, option_data, help_string],
...
);
&GetOptions (\@opt_table, \@ARGV [, \@newARGV]) || exit 1;
DESCRIPTION¶
Getopt::Tabular is a Perl 5 module for table-driven argument parsing,
vaguely inspired by John Ousterhout's Tk_ParseArgv. All you really need to do
to use the package is set up a table describing all your command-line options,
and call &GetOptions with three arguments: a reference to your option
table, a reference to @ARGV (or something like it), and an optional third
array reference (say, to @newARGV). &GetOptions will process all arguments
in @ARGV, and copy any leftover arguments (i.e. those that are not options or
arguments to some option) to the @newARGV array. (If the @newARGV argument is
not supplied, "GetOptions" will replace @ARGV with the stripped-down
argument list.) If there are any invalid options, "GetOptions" will
print an error message and return 0.
Before I tell you all about why Getopt::Tabular is a wonderful thing, let me
explain some of the terminology that will keep popping up here.
- argument
- any single word appearing on the command-line, i.e. one element of the
@ARGV array.
- option
- an argument that starts with a certain sequence of characters; the default
is "-". (If you like GNU-style options, you can change this to
"--".) In most Getopt::Tabular-based applications, options can
come anywhere on the command line, and their order is unimportant (unless
one option overrides a previous option). Also, Getopt::Tabular will allow
any non-ambiguous abbreviation of options.
- option argument
- (or value) an argument that immediately follows certain types of
options. For instance, if "-foo" is a scalar-valued integer
option, and "-foo 3" appears on the command line, then 3 will be
the argument to "-foo".
- option type
- controls how "GetOptions" deals with an option and the arguments
that follow it. (Actually, for most option types, the type interacts with
the "num_values" field, which determines whether the option is
scalar- or vector-valued. This will be fully explained in due
course.)
FEATURES¶
Now for the advertising, i.e. why Getopt::Tabular is a good thing.
- •
- Command-line arguments are carefully type-checked, both by pattern and
number---e.g. if an option requires two integers, GetOptions makes sure
that exactly two integers follow it!
- •
- The valid command-line arguments are specified in a data structure
separate from the call to GetOptions; this makes it easier to have very
long lists of options, and to parse options from multiple sources (e.g.
the command line, an environment variable, and a configuration file).
- •
- Getopt::Tabular can intelligently generate help text based on your option
descriptions.
- •
- The type system is extensible, and if you can define your desired argument
type using a single Perl regular expression then it's particularly easy to
extend.
- •
- To make your program look smarter, options can be abbreviated and come in
any order.
- •
-
You can parse options in a "spoof" mode that has no side-effects -- this
is useful for making a validation pass over the command line without
actually doing anything.
In general, I have found that Getopt::Tabular tends to encourage programs with
long lists of sophisticated options, leading to great flexibility, intelligent
operation, and the potential for insanely long command lines.
BASIC OPERATION¶
The basic operation of Getopt::Tabular is driven by an
option table,
which is just a list of
option descriptions (otherwise known as option
table entries, or just entries). Each option description tells
"GetOptions" everything it needs to know when it encounters a
particular option on the command line. For instance,
["-foo", "integer", 2, \@Foo, "set the foo values"]
means that whenever "-foo" is seen on the command line,
"GetOptions" is to make sure that the next two arguments are
integers, and copy them into the caller's @Foo array. (Well, really into the
@Foo array where the option table is defined. This is almost always the same
as "GetOptions"' caller, though.)
Typically, you'll group a bunch of option descriptions together like this:
@options =
(["-range", "integer", 2, \@Range,
"set the range of allowed values"],
["-file", "string", 1, \$File,
"set the output file"],
["-clobber", "boolean", 0, \$Clobber,
"clobber existing files"],
...
);
and then call "GetOptions" like this:
&GetOptions (\@options, \@ARGV) || exit 1;
which replaces @ARGV with a new array containing all the arguments left-over
after options and their arguments have been removed. You can also call
"GetOptions" with three arguments, like this:
&GetOptions (\@options, \@ARGV, \@newARGV) || exit 1;
in which case @ARGV is untouched, and @newARGV gets the leftover arguments.
In case of error, "GetOptions" prints enough information for the user
to figure out what's going wrong. If you supply one, it'll even print out a
brief usage message in case of error. Thus, it's enough to just "exit
1" when "GetOptions" indicates an error by returning 0.
Detailed descriptions of the contents of an option table entry are given next,
followed by the complete run-down of available types, full details on error
handling, and how help text is generated.
OPTION TABLE ENTRIES¶
The fields in the option table control how arguments are parsed, so it's
important to understand each one in turn. First, the format of entries in the
table is fairly rigid, even though this isn't really necessary with Perl. It's
done that way to make the Getopt::Tabular code a little easier; the drawback
is that some entries will have unused values (e.g. the "num_values"
field is never used for boolean options, but you still have to put something
there as a place-holder). The fields are as follows:
- option
- This is the option name, e.g. "-verbose" or
"-some_value". For most option types, this is simply an option
prefix followed by text; for boolean options, however, it can be a little
more complicated. (The exact rules are discussed under "OPTION
TYPES".) And yes, even though you tell Getopt::Tabular the valid
option prefixes, you still have to put one onto the option names in the
table.
- type
- The option type decides what action will be taken when this option is seen
on the command line, and (if applicable) what sort of values will be
accepted for this option. There are three broad classes of types: those
that imply copying data from the command line into some variable in the
caller's space; those that imply copying constant data into the caller's
space without taking any more arguments from the command line; and those
that imply some other action to be taken. The available option types are
covered in greater detail below (see "OPTION TYPES"), but
briefly: "string", "integer", and "float"
all imply copying values from the command line to a variable;
"constant", "boolean", "copy",
"arrayconst", and "hashconst" all imply copying some
pre-defined data into a variable; "call" and "eval"
allow the execution of some arbitrary subroutine or chunk of code; and
"help" options will cause "GetOptions" to print out
all available help text and return 0.
- num_values
- for "string", "integer", and "float"
options, this determines whether the option is a scalar (
num_values = 1) or vector ( num_values > 1) option. (Note
that whether the option is scalar- or vector-valued has an important
influence on what you must supply in the option_data field!) For
"constant", "copy", "arrayconst", and
"hashconst" option types, num_values is a bit of a
misnomer: it actually contains the value (or a reference to it, if array
or hash) to be copied when the option is encountered. For "call"
options, num_values can be used to supply extra arguments to the
called subroutine. In any case, though, you can think of num_values
as an input value. For "boolean" and "eval" options,
num_values is ignored and should be "undef" or 0.
- option_data
- For "string", "integer", "float",
"boolean", "constant", "copy",
"arrayconst", and "hashconst" types, this must be a
reference to the variable into which you want "GetOptions" to
copy the appropriate thing. The "appropriate thing" is either
the argument(s) following the option, the constant supplied as
num_values, or 1 or 0 (for boolean options).
For "boolean", "constant", "copy", and
scalar-valued "string", "integer", and
"float" options, this must be a scalar reference. For
vector-valued "string", "integer", and
"float" options ( num_values > 1), and for
"arrayconst" options, this must be an array reference. For
"hashconst" options, this must be a hash reference.
Finally, option_data is also used as an input value for
"call" and "eval" options: for "call", it
should be a subroutine reference, and for "eval" options, it
should be a string containing valid Perl code to evaluate when the option
is seen. The subroutine called by a "call" option should take at
least two arguments: a string, which is the actual option that triggered
the call (because the same subroutine could be tied to many options), and
an array reference, which contains all command line arguments after that
option. (Further arguments can be supplied in the num_values
field.) The subroutine may freely modify this array, and those
modifications will affect the behaviour of "GetOptions"
afterwards.
The chunk of code passed to an "eval" option is evaluated in the
package from which "GetOptions" is called, and does not have
access to any internal Getopt::Tabular data.
- help_string
- (optional) a brief description of the option. Don't worry about formatting
this in any way; when "GetOptions" has to print out your help,
it will do so quite nicely without any intervention. If the help string is
not defined, then that option will not be included in the option help
text. (However, you could supply an empty string -- which is defined -- to
make "GetOptions" just print out the option name, but nothing
else.)
- arg_desc
- (optional) an even briefer description of the values that you expect to
follow your option. This is mainly used to supply place-holders in the
help string, and is specified separately so that "GetOptions"
can act fairly intelligently when formatting a help message. See
"HELP TEXT" for more information.
OPTION TYPES¶
The option type field is the single-most important field in the table, as the
type for an option "-foo" determines (along with
num_values)
what action "GetOptions" takes when it sees "-foo" on the
command line: how many following arguments become "-foo"'s
arguments, what regular expression those arguments must conform to, or whether
some other action should be taken.
As mentioned above, there are three main classes of argument types:
- argument-driven options
- These are options that imply taking one or more option arguments from the
command line after the option itself is taken. The arguments are then
copied into some variable supplied (by reference) in the option table
entry.
- constant-valued options
- These are options that have a constant value associated with them; when
the option is seen on the command line, that constant is copied to some
variable in the caller's space. (Both the constant and the value are
supplied in the option table entry.) Constants can be scalars, arrays, or
hashes.
- other options
- These imply some other action to be taken, usually supplied as a string to
"eval" or a subroutine to call.
Argument-driven option types¶
- string, integer, float
- These are the option types that imply "option arguments", i.e.
arguments after the option that will be consumed when that option is
encountered on the command line and copied into the caller's space via
some reference. For instance, if you want an option "-foo" to
take a single string as an argument, with that string being copied to the
scalar variable $Foo, then you would have this entry in your option table:
["-foo", "string", 1, \$Foo]
(For conciseness, I've omitted the help_string and argdesc
entries in all of the example entries in this section. In reality, you
should religiously supply help text in order to make your programs easier
to use and easier to maintain.)
If num_values is some n greater than one, then the
option_data field must be an array reference, and n
arguments are copied from the command line into that array. (The array is
clobbered each time "-foo" is encountered, not appended to.) In
this case, "-foo" is referred to as a vector-valued
option, as it must be followed by a fixed number of arguments.
(Eventually, I plan to add list-valued options, which take a
variable number of arguments.) For example an option table like
["-foo", "string", 3, \@Foo]
would result in the @Foo array being set to the three strings immediately
following any "-foo" option on the command line.
The only difference between string, integer, and float
options is how picky "GetOptions" is about the value(s) it will
accept. For string options, anything is OK; for integer
options, the values must look like integers (i.e., they must match
"/[+-]?\d+/"); for float options, the values must look
like C floating point numbers (trust me, you don't want to see the regexp
for this). Note that since string options will accept anything, they might
accidentally slurp up arguments that are meant to be further options, if
the user forgets to put the correct string. For instance, if
"-foo" and "-bar" are both scalar-valued string
options, and the arguments "-foo -bar" are seen on the
command-line, then "-bar" will become the argument to
"-foo", and never be processed as an option itself. (This could
be construed as either a bug or a feature. If you feel really strongly
that it's a bug, then complain and I'll consider doing something about
it.)
If not enough arguments are found that match the required regular
expression, "GetOptions" prints to standard error a clear and
useful error message, followed by the usage summary (if you supplied one),
and returns 0. The error messages look something like "-foo option
must be followed by an integer", or "-foo option must be
followed by 3 strings", so it really is enough for your program to
"exit 1" without printing any further message.
- User-defined patterns
- Since the three option types described above are defined by nothing more
than a regular expression, it's easy to define your own option types. For
instance, let's say you want an option to accept only strings of
upper-case letters. You could then call
&Getopt::Tabular::AddPatternType as follows:
&Getopt::Tabular::AddPatternType
("upperstring", "[A-Z]+", "uppercase string")
Note that the third parameter is optional, and is only supplied to make
error messages clearer. For instance, if you now have a scalar-valued
option "-zap" of type "upperstring":
["-zap", "upperstring", 1, \$Zap]
and the user gets it wrong and puts an argument that doesn't consist of all
uppercase letters after "-zap", then "GetOptions" will
complain that "-zap option must be followed by an uppercase
string". If you hadn't supplied the third argument to &AddType,
then the error message would have been the slightly less helpful
"-zap option must be followed by an upperstring". Also, you
might have to worry about how "GetOptions" pluralizes your
description: in this case, it will simply add an "s", which
works fine much of the time, but not always. Alternately, you could supply
a two-element list containing the singular and plural forms:
&Getopt::Tabular::AddPatternType
("upperstring", "[A-Z]+",
["string of uppercase letters", "strings of uppercase letters"])
So, if "-zap" instead expects three "upperstring"s, and
the user goofs, then the error message would be (in the first example)
"-zap option must be followed by 3 uppercase strings" or
"-zap option must be followed by three strings of uppercase
letters" (second example).
Of course, if you don't intend to have vector-valued options of your new
type, pluralization hardly matters. Also, while it might seem that this is
a nice stab in the direction of multi-lingual support, the error messages
are still hard-coded to English in other places. Maybe in the next
version...
Constant-valued option types¶
- boolean
- For boolean options, option_data must be a scalar reference;
num_values is ignored (you can just set it to "undef" or
0). Booleans are slightly weird in that every boolean option implies
two possible arguments that will be accepted on the command line,
called the positive and negative alternatives. The positive alternative
(which is what you specify as the option name) results in a true value,
while the negative alternative results in false. Most of the time, you can
let "GetOptions" pick the negative alternative for you: it just
inserts "no" after the option prefix, so "-clobber"
becomes "-noclobber". (More precisely, "GetOptions"
tests all option prefixes until one of them matches at the beginning of
the option name. It then inserts "no" between this prefix and
the rest of the string. So, if you want to support both GNU-style options
(like "--clobber") and one-hyphen options ("-c"), be
sure to give "--" first when setting the option patterns
with &SetOptionPatterns. Otherwise, the negative alternative to
"--clobber" will be "-no-clobber", which might not be
what you wanted.) Sometimes, though, you want to explicitly specify the
negative alternative. This is done by putting both alternatives in the
option name, separated by a vertical bar, e.g.
"-verbose|-quiet".
For example, the above two examples might be specified as
["-clobber", "boolean", undef, \$Clobber],
["-verbose|-quiet", "boolean", undef, \$Verbose],...);
If "-clobber" is seen on the command line, $Clobber will be set to
1; if "-noclobber" is seen, then $Clobber will be set to 0.
Likewise, "-verbose" results in $Verbose being set to 1, and
"-quiet" will set $Verbose to 0.
- const
- For const options, put a scalar value (not reference) in
num_values, and a scalar reference in option_data. For
example:
["-foo", "const", "hello there", \$Foo]
On encountering "-foo", "GetOptions" will copy
"hello there" to $Foo.
- arrayconst
- For arrayconst options, put an array reference (input) (not
an array value) in num_values, and another array reference (output)
in option_data. For example:
["-foo", "arrayconst", [3, 6, 2], \@Foo]
On encountering "-foo", "GetOptions" will copy the array
"(3,6,2)" into @Foo.
- hashconst
- For hashconst options, put a hash reference (input) (not a
hash value) in num_values, and another hash reference (output) in
option_data. For example:
["-foo", "hashconst", { "Perl" => "Larry Wall",
"C" => "Dennis Ritchie",
"Pascal" => "Niklaus Wirth" },
\%Inventors]
On encountering "-foo", "GetOptions" will copy into
%Inventors a hash relating various programming languages to the culprits
primarily responsible for their invention.
- copy
- copy options act just like const options, except when
num_values is undefined. In that case, the option name itself will
be copied to the scalar referenced by option_data, rather than the
"undef" value that would be copied under these circumstances
with a const option. This is useful when one program accepts
options that it simply passes to a sub-program; for instance, if
prog1 calls prog2, and prog2 might be run with the
-foo option, then prog1's argument table might have this option:
["-foo", "copy", undef, \$Foo,
"run prog2 with the -foo option"]
and later on, you would run prog2 like this:
system ("prog2 $Foo ...");
That way, if "-foo" is never seen on prog1's command line,
$Foo will be untouched, and will expand to the empty string when building
the command line for prog2.
If num_values is anything other than "undef", then
copy options behave just like constant options.
Other option types¶
- call
- For call options, option_data must be a reference to a
subroutine. The subroutine will be called with at least two arguments: a
string containing the option that triggered the call (because the same
subroutine might be activated by many options), a reference to an array
containing all remaining command-line arguments after the option, and
other arguments specified using the num_values field. (To be used
for this purpose, num_values must be an array reference; otherwise,
it is ignored.) For example, you might define a subroutine
sub process_foo
{
my ($opt, $args, $dest) = @_;
$$dest = shift @$args; # not quite right! (see below)
}
with a corresponding option table entry:
["-foo", "call", [\$Foo], \&process_foo]
and then "-foo" would act just like a scalar-valued string option
that copies into $Foo. (Well, almost ... read on.)
A subtle point that might be missed from the above code: the value returned
by &process_foo does matter: if it is false, then
"GetOptions" will return 0 to its caller, indicating failure. To
make sure that the user gets a useful error message, you should supply one
by calling "SetError"; doing so will prevent
"GetOptions" from printing out a rather mysterious (to the end
user, at least) message along the lines of "subroutine call
failed". The above example has two subtle problems: first, if the
argument following "-foo" is an empty string, then
"process_foo" will return the empty string---a false
value---thus causing "GetOptions" to fail confusingly. Second,
if there no arguments after "-foo", then "process_foo"
will return "undef"---again, a false value, causing
"GetOptions" to fail.
To solve these problems, we have to define the requirements for the
"-foo" option a little more rigorously. Let's say that any
string (including the empty string) is valid, but that there must be
something there. Then "process_foo" is written as follows:
sub process_foo
{
my ($opt, $args, $dest) = @_;
$$dest = shift @$args;
(defined $$dest) && return 1;
&Getopt::Tabular::SetError
("bad_foo", "$opt option must be followed by a string");
return 0;
}
The "SetError" routine actually takes two arguments: an error
class and an error message. This is explained fully in the "ERROR
HANDLING" section, below. And, if you find yourself writing a lot of
routines like this, "SetError" is optionally exported from
"Getopt::Tabular", so you can of course import it into your main
package like this:
use Getopt::Tabular qw/GetOptions SetError/;
- eval
- An eval option specifies a chunk of Perl code to be executed
("eval"'d) when the option is encountered on the command line.
The code is supplied (as a string) in the option_data field; again,
num_values is ignored. For example:
["-foo", "eval", undef,
'print "-foo seen on command line\n"']
will cause "GetOptions" to print out (via an "eval") the
string "-foo seen on the command line\n" when -foo is seen. No
other action is taken apart from what you include in the eval string. The
code is evaluated in the package from which "GetOptions" was
called, so you can access variables and subroutines in your program
easily. If any error occurs in the "eval",
"GetOptions" complains loudly and returns 0.
Note that the supplied code is always evaluated in a "no strict"
environment---that's because Getopt::Tabular is itself "use
strict"-compliant, and I didn't want to force strictness on every
quick hack that uses the module. (Especially since eval options
seem to be used mostly in quick hacks.) (Anyone who knows how to fetch the
strictness state for another package or scope is welcome to send me
hints!) However, the -w state is untouched.
- section
- section options are just used to help formatting the help text. See
"HELP TEXT" below for more details.
ERROR HANDLING¶
Generally, handling errors in the argument list is pretty transparent:
"GetOptions" (or one of its minions) generates an error message and
assigns an error class, "GetOptions" prints the message to the
standard error, and returns 0. You can access the error class and error
message using the "GetError" routine:
($err_class, $err_msg) = &Getopt::Tabular::GetError ();
(Like "SetError", "GetError" can also be exported from
Getopt::Tabular.) The error message is pretty simple---it is an
explanation for the end user of what went wrong, which is why
"GetOptions" just prints it out and forgets about it. The error
class is further information that might be useful for your program; the
current values are:
- bad_option
- set when something that looks like an option is found on the command line,
but it's either unknown or an ambiguous abbreviation.
- bad_value
- set when an option is followed by an invalid argument (i.e., one that
doesn't match the regexp for that type), or the wrong number of
arguments.
- bad_call
- set when a subroutine called via a call option or the code
evaluated for an eval option returns a false value. The subroutine
or eval'd code can override this by calling "SetError"
itself.
- bad_eval
- set when the code evaluted for an eval option has an error in
it.
- help
- set when the user requests help
Note that most of these are errors on the end user's part, such as bad or
missing arguments. There are also errors that can be caused by you, the
programmer, such as bad or missing values in the option table; these generally
result in "GetOptions" croaking so that your program dies
immediately with enough information that you can figure out where the mistake
is.
bad_eval is a borderline case; there are conceivably cases where
the end user's input can result in bogus code to evaluate, so I grouped this
one in the "user errors" class. Finally, asking for help isn't
really an error, but the assumption is that you probably shouldn't continue
normal processing after printing out the help---so "GetOptions"
returns 0 in this case. You can always fetch the error class with
"GetError" if you want to treat real errors differently from help
requests.
HELP TEXT¶
One of Getopt::Tabular's niftier features is the ability to generate and format
a pile of useful help text from the snippets of help you include in your
option table. The best way to illustrate this is with a couple of brief
examples. First, it's helpful to know how the user can trigger a help display.
This is quite simple: by default, "GetOptions" always has a
"-help" option, presence of which on the command line triggers a
help display. (Actually, the help option is really your preferred option
prefix plus "help". So, if you like to make GNU-style options to
take precedence as follows:
&Getopt::Tabular::SetOptionPatterns qw|(--)([\w-]+) (-)(\w+)|;
then the help option will be "--help". There is only one help option
available, and you can set it by calling &SetHelpOption (another optional
export).
Note that in addition to the option help embedded in the option table,
"GetOptions" can optionally print out two other messages: a
descriptive text (usually a short paragraph giving a rough overview of what
your program does, possibly referring the user to the fine manual page), and a
usage text. These are both supplied by calling &SetHelp, e.g.
$Help = <<HELP;
This is the foo program. It reads one file (specified by -infile),
operates on it some unspecified way (possibly modified by
-threshold), and does absolutely nothing with the results.
(The utility of the -clobber option has yet to be established.)
HELP
$Usage = <<USAGE;
usage: foo [options]
foo -help to list options
USAGE
&Getopt::Tabular::SetHelp ($Help, $Usage)
Note that either of the long help or usage strings may be empty, in which case
"GetOptions" simply won't print them. In the case where both are
supplied, the long help message is printed first, followed by the option help
summary, followed by the usage. "GetOptions" inserts enough blank
lines to make the output look just fine on its own, so you shouldn't pad
either the long help or usage message with blanks. (It looks best if each ends
with a newline, though, so setting the help strings with here-documents---as
in this example---is the recommended approach.)
As an example of the help display generated by a typical option table, let's
take a look at the following:
$Verbose = 1;
$Clobber = 0;
undef $InFile;
@Threshold = (0, 1);
@argtbl = (["-verbose|-quiet", "boolean", 0, \$Verbose,
"be noisy"],
["-clobber", "boolean", 0, \$Clobber,
"overwrite existing files"],
["-infile", "string", 1, \$InFile,
"specify the input file from which to read a large " .
"and sundry variety of data, to which many " .
"interesting operations will be applied", "<f>"],
["-threshold", "float", 2, \@Threshold,
"only consider values between <v1> and <v2>",
"<v1> <v2>"]);
Assuming you haven't supplied long help or usage strings, then when
"GetOptions" encounters the help option, it will immediately stop
parsing arguments and print out the following option summary:
Summary of options:
-verbose be noisy [default]
-quiet opposite of -verbose
-clobber overwrite existing files
-noclobber opposite of -clobber [default]
-infile <f> specify the input file from which to read a large and
sundry variety of data, to which many interesting
operations will be applied
-threshold <v1> <v2>
only consider values between <v1> and <v2> [default: 0 1]
There are a number of interesting things to note here. First, there are three
option table fields that affect the generation of help text:
option,
help_string, and
argdesc. Note how the
argdesc strings
are simply option placeholders, usually used to 1) indicate how many values
are expected to follow an option, 2) (possibly) imply what form they take
(although that's not really shown here), and 3) explain the exact meaning of
the values in the help text.
argdesc is just a string like the help
string; you can put whatever you like in it. What I've shown above is just my
personal preference (which may well evolve).
A new feature with version 0.3 of Getopt::Tabular is the inclusion of default
values with the help for certain options. A number of conditions must be
fulfilled for this to happen for a given option: first, the option type must
be one of the "argument-driven" types, such as "integer",
"float", "string", or a user-defined type. Second, the
option data field must refer either to a defined scalar value (for
scalar-valued options) or to a list of one or more defined values (for
vector-valued options). Thus, in the above example, the "-infile"
option doesn't have its default printed because the $InFile scalar is
undefined. Likewise, if the @Threshold array were the empty list
"()", or a list of undefined values "(undef,undef)", then
the default value for "-threshold" also would not have been printed.
The formatting is done as follows: enough room is made on the right hand side
for the longest option name, initially omitting the argument placeholders.
Then, if an option has placeholders, and there is room for them in between the
option and the help string, everything (option, placeholders, help string) is
printed together. An example of this is the "-infile" option: here,
"-infile <f>" is just small enough to fit in the 12-character
column (10 characters because that is the length of the longest option, and 2
blanks), so the help text is placed right after it on the same line. However,
the "-threshold" option becomes too long when its argument
placeholders are appended to it, so the help text is pushed onto the next
line.
In any event, the help string supplied by the caller starts at the same column,
and is filled to make a nice paragraph of help. "GetOptions" will
fill to the width of the terminal (or 80 columns if it fails to find the
terminal width).
Finally, you can have pseudo entries of type
section, which are important
to make long option lists readable (and one consequence of using
Getopt::Tabular is programs with ridiculously long option lists -- not
altogether a bad thing, I suppose). For example, this table fragment:
@argtbl = (...,
["-foo", "integer", 1, \$Foo,
"set the foo value", "f"],
["-enterfoomode", "call", 0, \&enter_foo_mode,
"enter foo mode"],
["Non-foo related options", "section"],
["-bar", "string", 2, \@Bar,
"set the bar strings (which have nothing whatsoever " .
"to do with foo", "<bar1> <bar2>"],
...);
results in the following chunk of help text:
-foo f set the foo value
-enterfoomode enter foo mode
-- Non-foo related options ---------------------------------
-bar b1 b2 set the bar strings (which have nothing
whatsoever to do with foo
(This example also illustrates a slightly different style of argument
placeholder. Take your pick, or invent your own!)
SPOOF MODE¶
Since callbacks from the command line ("call" and "eval"
options) can do anything, they might be quite expensive. In certain cases,
then, you might want to make an initial pass over the command line to ensure
that everything is OK before parsing it "for real" and incurring all
those expensive callbacks. Thus, "Getopt::Tabular" provides a
"spoof" mode for parsing a command line without side-effects. In the
simplest case, you can access spoof mode like this:
use Getopt::Tabular qw(SpoofGetOptions GetOptions);
.
.
.
&SpoofGetOptions (\@options, \@ARGV, \@newARGV) || exit 1;
and then later on, you would call "GetOptions" with the
original @ARGV (so it can do what "SpoofGetOptions" merely
pretended to do):
&GetOptions (\@options, \@ARGV, \@newARGV) || exit 1;
For most option types, any errors that "GetOptions" would catch should
also be caught by "SpoofGetOptions" -- so you might initially think
that you can get away without that "|| exit 1" after calling
"GetOptions". However, it's a good idea for a couple of reasons.
First, you might inadvertently changed @ARGV -- this is usually a bug and a
silly thing to do, so you'd probably want your program to crash loudly rather
than fail mysteriously later on. Second, and more likely, some of those
expensive operations that you're initially avoiding by using
"SpoofGetOptions" might themselves fail -- which would cause
"GetOptions" to return false where "SpoofGetOption"
completes without a problem. (Finally, there's the faint possiblity of bugs in
"Getopt::Tabular" that would cause different behaviour in spoof mode
and real mode -- this really shouldn't happen, though.)
In reality, using spoof mode requires a bit more work. In particular, the whole
reason for spoof argument parsing is to avoid expensive callbacks, but since
callbacks can eat any number of command line arguments, you have to emulate
them in some way. It's not possible for "SpoofGetOptions" to do this
for you, so you have to help out by supplying "spoof" callbacks. As
an example, let's say you have a callback option that eats one argument (a
filename) and immediately reads that file:
@filedata = ();
sub read_file
{
my ($opt, $args) = @_;
warn ("$opt option requires an argument\n"), return 0 unless @$args;
my $file = shift @$args;
open (FILE, $file) ||
(warn ("$file: $!\n"), return 0);
push (@filedata, <FILE>);
close (FILE);
return 1;
}
@options =
(['-read_file', 'call', undef, \&read_file]);
Since "-read_file" could occur any number of times on the command
line, we might end up reading an awful lot of files, and thus it might be a
long time before we catch errors late in the command line. Thus, we'd like to
do a "spoof" pass over the command line to catch all errors. A
simplistic approach would be to supply a spoof callback that just eats one
argument and returns success:
sub spoof_read_file
{
my ($opt, $args) = @_;
(warn ("$opt option requires an argument\n"), return 0)
unless @$args;
shift @$args;
return 1;
}
Then, you have to tell "Getopt::Tabular" about this alternate callback
with no side-effects (apart from eating that one argument):
&Getopt::Tabular::SetSpoofCodes (-read_file => \&spoof_read_file);
("SetSpoofCodes" just takes a list of key/value pairs, where the keys
are "call" or "eval" options, and the values are the
"no side-effects" callbacks. Naturally, the replacement callback for
an "eval" option should be a string, and for a "call"
option it should be a code reference. This is not actually checked, however,
until you call "SpoofGetOptions", because "SetSpoofCodes"
doesn't know whether options are "call" or "eval" or
what.)
A more useful "spoof_read_file", however, would actually check if the
requested file exists -- i.e., we should try to catch as many errors as
possible, as early as possible:
sub spoof_read_file
{
my ($opt, $args) = @_;
warn ("$opt option requires an argument\n"), return 0
unless @$args;
my $file = shift @$args;
warn ("$file does not exist or is not readable\n"), return 0
unless -r $file;
return 1;
}
Finally, you can frequently merge the "real" and "spoof"
callback into one subroutine:
sub read_file
{
my ($opt, $args, $spoof) = @_;
warn ("$opt option requires an argument\n"), return 0 unless @$args;
my $file = shift @$args;
warn ("$file does not exist or is not readable\n"), return 0
unless -r $file;
return 1 if $spoof;
open (FILE, $file) ||
(warn ("$file: $!\n"), return 0);
push (@filedata, <FILE>);
close (FILE);
return 1;
}
And then, when specifying the replacement callback to "SetSpoofCodes",
just create an anonymous sub that calls "read_file" with $spoof
true:
&Getopt::Tabular::SetSpoofCodes
(-read_file => sub { &read_file (@_[0,1], 1) });
Even though this means a bigger and more complicated callback, you only need
one such callback -- the alternative is to carry around both
"read_file" and "spoof_read_file", which might do
redundant processing of the argument list.
AUTHOR¶
Greg Ward <greg@bic.mni.mcgill.ca>
Started in July, 1995 as ParseArgs.pm, with John Ousterhout's Tk_ParseArgv.c as
a loose inspiration. Many many features added over the ensuing months;
documentation written in a mad frenzy 16-18 April, 1996. Renamed to
Getopt::Tabular, revamped, reorganized, and documentation expanded 8-11
November, 1996.
Copyright (c) 1995-97 Greg Ward. All rights reserved. This is free software; you
can redistribute it and/or modify it under the same terms as Perl itself.
BUGS¶
The documentation is bigger than the code, and I still haven't covered option
patterns or extending the type system (apart from pattern types). Yow!
No support for list-valued options, although you can roll your own with
call options. (See the demo program included with the distribution for
an example.)
Error messages are hard-coded to English.