NAME
CLI::Framework::Tutorial - "HOWTO" develop CLIF applications using best
practices

CLIF DOCUMENTATION
This is a guide to developing CLIF applications. It is a supplement to
the documentation in CLI::Framework::Application and
CLI::Framework::Command, which have more thorough coverage of some finer
points.

It is suggested that new users start by reading this document, then use
the other documentation for reference as necessary.

INTRODUCTION
Developers have been reluctantly writing ad-hoc, disposable scripts for
too long or struggling to decide how not to do so. There is a better
alternative.

CLI::Framework::Application enumerates many advantages to using CLIF
instead of writing yet-another-getopt-based-script. CLIF comes with a
lot of documentation, but don't take that to mean that using CLIF is
complicated. CLIF apps with simple needs are very easy to build. Apps
with complex needs are a bit more work, but much easier to build (and
far easier to test and maintain) than doing that work from scratch.

This document will first demonstrate a very simple CLIF application.
Next, a complete application will be shown to demonstrate more advanced
CLIF features.

Think of a typical command-line script. It needs to parse command-line
options and arguments, check that any required external resources
(files, databases, etc.) have been supplied, fail nicely if something is
missing or inconsistent, then do something that depends on the options,
arguments, and external resources.

What happens when new scripts are created to do something similar? All
too often, they end up with different option names for conceptually the
same purpose. It is common for functionality needed by several scripts
to be duplicated in each similar script. This rapidly gets out of hand,
becoming a maintenance frustration. Your team members are not "on the
same page" and new people learning your tools have to have lengthy,
verbal one-on-one code tours.

Instead, a set of related scripts could be combined into a CLIF
application. Consistent naming conventions and sharing of common code is
naturally encouraged. The commands are easy to test. New commands can be
added with ease.

FROM P.O.S. TO CLIF IN A FEW EASY STEPS
A "P.O.S." is a "Plain Old Script." This section shows you how to reform
that old P.O.S., creating a shiny new CLIF application!

Please see working code for this example included with the
"CLI::Framework" distribution (examples/demo-simple.pl).

This example demonstrates the following features:

* inline application definition

* basics (app, app args, commands, command options and args)

* the relationship between plain scripts and CLIF applications
(including how to convert between them)

To understand CLIF commands, imagine converting a legacy script to a
CLIF application. First, create a Perl class that inherits from
CLI::Framework::Command. Place the main body of the script in a "run()"
method. Add the functions that the script defines, if any.

# Your Command Subclass...
package Converted::Script::Command::LegacyScript;
use base qw( CLI::Framework::Command );

sub run { ... }

Next, create a Perl class (creating a separate package file for the
class is totally optional) that inherits from
CLI::Framework::Application and define a method, "command_map()", that
links command names with command classes that implement the commands:

# Your Application Class...
package Converted::Script;
use base qw( CLI::Framework::Application );

sub command_map {
{
'legacy-script' => 'Converted::Script::Command::LegacyScript',
}
}

The code that provides a friendly usage message (if the legacy script
provided one) can be replaced by defining the "usage_text" method:

sub usage_text {
qq{
$0 [--verbose|v] [--help|h]: work all manner of mischief devised by long-departed miscreants
}
}

Back in your Command Subclass, the option/argument processing code will
be replaced with a method defining what options will be recognized (the
data structure to be returned is exactly as explained in
Getopt::Long::Descriptive):

sub option_spec {
[ 'help|h' => 'show help' ],
[ 'verbose|v' => 'be verbose' ],
}

...and that's all it takes to convert a simple script to a CLIF app.
This contrived example demonstrates the mechanics, but let me point out
a few advantages (see DESIGN GOALS AND FEATURES in the
CLI::Framework::Applicaton documentation for the long list):

Clear division of responsibilities
Using packages, subroutines, and separate files (if desired), CLIF
apps follow established convention and provide a new pattern for
creating tools.

Easy to test
Now that functional units of code are subroutines in packages, you
can unit test each component independently.

Easy to maintain
Instead of puzzling over a several-thousand-line script, maintaining
a CLIF application is like maintaining any other well-engineered
application code.

Easy to extend
Related tools frequently occur in groups. Instead of awkwardly
forcing loosely-related behaviors into the same script, CLIF makes
it easy to add additional commands.

WHEN NOT TO USE CLIF
CLIF could be used for the simplest of needs, but it may be overkill in
very simple situations.

You may want to avoid CLIF for very basic scripts that have a single
behavior and are completely independent from other such tools. However,
if there's a chance that the scripts might grow to become more complex
or if you would simply like a pattern to follow, it may still be worth
considering.

CONCEPTS AND DEFINITIONS
See CONCEPTS AND DEFINITIONS in the CLI::Framework::Application
documentation.

UNDERSTANDING THE APPLICATION RUN SEQUENCE
See APPLICATION RUN SEQUENCE in the CLI::Framework::Application
documentation.

Understanding this is important to building more complex apps. You need,
at the least, to understand how CLIF differentiates between options/args
that are meant for the application itself and those options/args that
are meant for individual commands.

The following examples demonstrate the alternative command request
forms. Note that in all cases, any number of (sub)command options and
arguments can be passed (these examples show only one of each for
brevity).

FORM #1 (without subcommands) -- command requests that involve NO
subcommands take the following form:

<app> [--app-opt] <cmd> [--cmd-opt] [cmd-arg] ...

(notice how the position of options and arguments determines whether
they are meant for the application as a whole or for the specific
command).

FORM #2 (with subcommands) -- Command requests that involve A SINGLE
subcommand take this form:

<app> [--app-opt] <cmd> [--cmd-opt] <subcmd> [--subcmd-opt] [subcmd-arg] ...

Command requests that involve MULTIPLE subcommands follow the same form:

<app> [--app-opt] <cmd> [--cmd-opt] <subcmd1> [--subcmd1-opt] <subcmd2> [--subcmd2-opt] [subcmd2-arg] ...

(notice that the final arguments apply to the final subcommand. In this
form, the only command that can receive arguments is the final
subcommand).

A MORE INVOLVED EXAMPLE
Please see working code for this example included with the
"CLI::Framework" distribution (examples/queue).

The next example demonstrates the following features:

* inline application definition

* basics (app, app args, commands, command options and args)

* subcommands

* validation of application and command arguments

* interactive mode

* non-interactive mode

Suppose we need to write a command-line app that provides an interface
to a queue. Strings can be added to or removed from the queue, queue
contents can be displayed, and queue "properties" can be set to restrict
the contents added to the queue. The interface should work
interactively.

The following usage demonstrates the desired behavior:

# ---- interactive mode ----
1) dequeue
2) cmd-list
3) enqueue
4) print
5) alias
6) property

> help enqueue

enqueue [--tag=<tag1> [--tag=<tag2> [...] ] ] <item1> [<item2> ... <itemN>]: add item(s) to queue

> enqueue --tag=x "something"

> property set --evens

> e 1 21 514 937 18

The working example in examples/queue accomplishes this goal in a single
inline application containing the Application Class and multiple Command
Classes.

This application is created in fundamentally the same way as the simple
one presented earlier. It uses more commands, more Application
Class/Command Class hooks, and subcommands. The code is much longer but
almost all of it is for business logic -- little additional
CLIF-specific code is needed.

The example code shows how various commands can be managed by an
Application subclass. The code is commented thoroughly to explain the
various hooks that are available for Application Class and Command
Classes.

Of course, CLIF applications can always be used in non-interactive mode:

# ---- non-interactive mode ----
$ examples/queue --qout=/tmp/qfile enqueue 'first'
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue --tag=x --tag=y 'second'
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property list
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property set --evens
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile property list
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 17
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 4
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 2
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile dequeue
$ examples/queue --qin=/tmp/qfile --qout=/tmp/qfile enqueue 3
$ examples/queue --qin=/tmp/qfile print

PLANNING A COMPLEX CLIF APPLICATION
Little additional thought (beyond that needed for business logic) is
required to create a basic CLIF app -- the strategy explained in "FROM
P.O.S. TO CLIF IN A FEW EASY STEPS" demonstrates how CLIF differs from a
"Plain Old Script".

A more complicated command line application will benefit from a wider
variety of the features CLIF provides. The extra features are easy to
use, but the additional complexity warrants some planning.

Here are some considerations:

Basic interface
What commands and subcommands should be available? What options and
arguments will they support? What kind of validation should be done
on the provided command requests? Which built-in commands will be
used? Will an interactive mode be provided? If so, will a custom
menu be created? Do any commands need to directly access or modify
the application itself or the other commands (these will be
metacommands)?

High-level code layout
Which components of the application will be defined in their own
package files? Which will be defined inline?

Separation of concerns using MVC strategy
How will the model be separated from the rest of the application?
What about the view?

Data sharing between application and commands
What data will data be shared between the application and the
commands? Will this be arranged by using the cache, using a Command
superclass (a generic command class that all of your commands
inherit from), or by some other means?

Read on for possible answers to some of these questions.

HOW CAN I ...?
This section explains how CLIF could be used to support various common
goals. Even if your particular situation does not appear here, reading
this section will give you an understanding of how CLIF could be set up
to support novel cases.

How can I quickly create a very simple application?
For a demonstration of how to create a very simple CLIF app, see "FROM
P.O.S. TO CLIF IN A FEW EASY STEPS". CLIF applications require, at the
minimum:

* An Application Class that inherits from CLI::Framework::Application.
For anything useful to happen, it should override the
"command_map()" hook and include a new command.

* A Command Class that inherits from CLI::Framework::Command. It
should override the "run()" hook (or have a subcommand that
overrides "run()").

* An Application Script that calls the "run()" method in your
application.

These can all be defined in one file or each class can be placed in a
separate file. Do whatever works best for your particular needs.

How can I include logging in my application?
In your Application Class, define "init()" to initialize your logging
object and save the resulting object in the cache, where the object will
be available to your application and command objects.

How can I include database connectivity in my application?
In your Application Class, define "init()" to connect to your database
and save the resulting object or database handle in the cache, where the
object/handle will be available to your application and command objects.

Of course, for proper Separation of Concerns, you should not simply
store a connected database handle in the cache and use it directly in
your Command classes. You should instead store an object of another
class that encapsulates your data model layer code. An example of this
is the model class for the demo journal application included with CLIF
tests: t/lib/My/Journal/Model.pm.

How can I support an application configuration file?
In your Application Class, define "init()" to load your configuration
file and save the resulting configuration object in the cache using the
cache(), where the object will be available to your application and
command objects.

How can I use templates for more flexible output?
In your Application Class, override the "render()" method.

For instance, you could write an application where all commands return a
data structure to be used in processing a template. Your "render()"
method could determine which template file to process (e.g. based on
which command is being run) and then process it using the received data
structure.

How can I use alternative CLI prompting techniques and terminal I/O convenience functions?
You may, for example, want to present a menu of options from a variety
of choices based on content from a database. Or perhaps you want to
prompt the user for a list of numbers and you want to support a
comma-separated list with ranges, etc.

Create a CLI::Framework::Command subclass (say, "Your::Command") that
implements your convenience functions or uses a CPAN module such as
Term::Prompt. Then all of your commands can inherit from "Your::Command"
and will all have access to the functions.

You may also want to override read_cmd().

How can I create an app without a 'help' command?
The 'help' command is fundamental to most applications. If you really
want to build an application without a 'help' command, simply create a
custom Help command with an empty "run" method.

TROUBLESHOOTING
The following problems and solutions may be helpful when working with
CLIF.

* Don't forget to inherit from CLI::Framework::Application in your
Application class and CLI::Framework::Command in your command class

* Don't forget to define (override) command_map() in your Application
class

* Don't forget to define (override) run() in your Command class

* If in doubt, run "perl -wc <your command class file>"

If a user-defined command class does not compile, your CLIF
application will fail silently. Running "perl -wc Class.pm" will
report compilation problems for Class.pm.

This is free software; you can redistribute it and/or modify it under
the same terms as Perl itself. See perlartistic.

AUTHOR
Karl Erisman (karl.erisman@icainformatics.com)