pry--pry/README.markdown

Pry
=============

(C) John Mair (banisterfiend) 2011

_attach an irb-like session to any object at runtime_

Pry is a simple Ruby REPL (Read-Eval-Print-Loop) that specializes in the interactive
manipulation of objects during the running of a program.

In some sense it is the opposite of IRB in that you bring a REPL
session to your code (with Pry) instead of bringing your code to a
REPL session (as with IRB).

It is not based on the IRB codebase, and implements some unique REPL
commands such as `show-method`, `show-doc`, `ls` and `cd` (type `help`
to get a full list).

Pry is also fairly flexible and allows significant user
[customization](http://rdoc.info/github/banister/pry/master/file/wiki/Customizing-pry.md). It
is trivial to set it to read from any object that has a `readline` method and write to any object that has a
`puts` method - many other aspects of Pry are also configurable making
it a good choice for implementing custom shells.

Pry now comes with an executable so it can be invoked at the command line.
Just enter `pry` to start. A `.pryrc` file in the user's home directory will
be loaded if it exists. Type `pry --help` at the command line for more information.

* Install the [gem](https://rubygems.org/gems/pry): `gem install pry`
* Read the [documentation](http://rdoc.info/github/banister/pry/master/file/README.markdown)
* See the [source code](http://github.com/banister/pry)

Pry also has `rubygems-test` support; to participate, first install
Pry, then:

1. Install rubygems-test: `gem install rubygems-test`
2. Run the test: `gem test pry`
3. Finally choose 'Yes' to upload the results. 

Example: Interacting with an object at runtime
---------------------------------------

With the `Object#pry` method we can pry (open an irb-like session) on
an object. In the example below we open a Pry session for the `Test` class and execute a method and add
an instance variable. The current thread is taken over by the Pry REPL loop for the duration of the session.

    require 'pry'

    class Test
      def self.hello() "hello world" end
    end

    Test.pry

    # Pry session begins on stdin
    Beginning Pry session for Test
    pry(Test)> self
    => Test
    pry(Test)> hello
    => "hello world"
    pry(Test)> @y = 20
    => 20
    pry(Test)> exit
    Ending Pry session for Test

    # program resumes here

If we now inspect the `Test` object we can see our changes have had
effect:

    Test.instance_variable_get(:@y) #=> 20

### Alternative Syntax

You can also use the `Pry.start(obj)` or `pry(obj)` syntax to start a pry session on
`obj`. e.g

    Pry.start(5)
    Beginning Pry session for 5
    pry(5)>

OR

    pry(6)
    beginning Pry session for 6
    pry(6)>

Example: Pry sessions can nest
-----------------------------------------------

Here we will begin Pry at top-level, then pry on a class and then on
an instance variable inside that class:

    # Pry.start() without parameters begins a Pry session on top-level (main)
    Pry.start
    Beginning Pry session for main
    pry(main)> class Hello
    pry(main)*   @x = 20
    pry(main)* end
    => 20
    pry(main)> cd Hello
    Beginning Pry session for Hello
    pry(Hello):1> instance_variables
    => [:@x]
    pry(Hello):1> cd @x
    Beginning Pry session for 20
    pry(20:2)> self + 10
    => 30
    pry(20:2)> cd ..
    Ending Pry session for 20
    pry(Hello):1> cd ..
    Ending Pry session for Hello
    pry(main)> cd ..
    Ending Pry session for main

The number after the `:` in the pry prompt indicates the nesting
level. To display more information about nesting, use the `nesting`
command. E.g

    pry("friend":3)> nesting
    Nesting status:
    0. main (Pry top level)
    1. Hello
    2. 100
    3. "friend"
    => nil

We can then jump back to any of the previous nesting levels by using
the `jump-to` command:

    pry("friend":3)> jump-to 1
    Ending Pry session for "friend"
    Ending Pry session for 100
    => 100
    pry(Hello):1>

If we just want to go back one level of nesting we can of course
use the `quit` or `exit` or `back` commands.

To break out of all levels of Pry nesting and return immediately to the
calling process use `exit-all`:

    pry("friend":3)> exit-all
    Ending Pry session for "friend"
    Ending Pry session for 100
    Ending Pry session for Hello
    Ending Pry session for main
    => main

    # program resumes here

Features and limitations
------------------------

Pry is an irb-like clone with an emphasis on interactively examining
and manipulating objects during the running of a program.

Its primary utility is probably in debugging, though it may have other
uses (such as implementing a quake-like console for games, for example). Here is a
list of Pry's features along with some of its limitations given at the
end.

###Features:

* Pry can be invoked at any time and on any object in the running program.
* Pry sessions can nest arbitrarily deeply -- to go back one level of nesting type 'exit' or 'quit' or 'back'
* Pry comes with syntax highlighting on by default just use the `toggle-color` command to turn it on and off.
* Use `_` to recover last result.
* Use `_pry_` to reference the Pry instance managing the current session.
* Pry supports tab completion.
* Pry has multi-line support built in.
* Use `^d` (control-d) to quickly break out of a session.
* Pry has special commands not found in many other Ruby REPLs: `show-method`, `show-doc`
`jump-to`, `ls`, `cd`, `cat`
* Pry gives good control over nested sessions (important when exploring complicated runtime state)
* Pry is not based on the IRB codebase.
* Pry allows significant customizability.
* Pry uses the [method_source](https://github.com/banister/method_source) gem; so
this functionality is available to a Pry session.
* Pry uses [RubyParser](https://github.com/seattlerb/ruby_parser) to
validate expressions in 1.8, and [Ripper](http://rdoc.info/docs/ruby-core/1.9.2/Ripper) for 1.9.
* Pry implements all the methods in the REPL chain separately: `Pry#r`
for reading; `Pry#re` for eval; `Pry#rep` for printing; and `Pry#repl`
for the loop (`Pry.start` simply wraps `Pry.new.repl`). You can
invoke any of these methods directly depending on exactly what aspect of the functionality you need.

###Limitations:

* Some Pry commands (e.g `show-command`) do not work in Ruby 1.8.
* `method_source` functionality does not work in JRuby.
* 1.9 support requires `Ripper` - some implementations may not support this.

Commands
-----------

### The Pry API:

* `Pry.start()` Starts a Read-Eval-Print-Loop on the object it
receives as a parameter. In the case of no parameter it operates on
top-level (main). It can receive any object or a `Binding`
object as parameter. `Pry.start()` is implemented as `Pry.new.repl()`
* `obj.pry` and `pry(obj)` may also be used as alternative syntax to
`Pry.start(obj)`.

  However there are some differences. `obj.pry` opens
a Pry session on the receiver whereas `Pry.start` (with no parameter)
will start a Pry session on top-level. The other form of the `pry`
method: `pry(obj)` will also start a Pry session on its parameter.

  The `pry` method invoked by itself, with no explict receiver and no
parameter will start a Pry session on the implied receiver. It is
perhaps more useful to invoke it in this form `pry(binding)` or
`binding.pry` so as to get access to locals in the current context.

  Another difference is that `Pry.start()` accepts a second parameter
that is a hash of configuration options (discussed further, below).

* If, for some reason you do not want to 'loop' then use `Pry.new.rep()`; it
only performs the Read-Eval-Print section of the REPL - it ends the
session after just one line of input. It takes the same parameters as
`Pry#repl()`
* Likewise `Pry#re()` only performs the Read-Eval section of the REPL,
it returns the result of the evaluation or an Exception object in
case of error. It also takes the same parameters as `Pry#repl()`
* Similarly `Pry#r()` only performs the Read section of the REPL, only
returning the Ruby expression (as a string). It takes the same parameters as all the others.
* `Pry.run_command COMMAND` enables you to invoke Pry commands outside
of a session, e.g `Pry.run_command "ls -m", :context => MyObject`. See
docs for more info.

### Session commands

Pry supports a few commands inside the session itself. These commands are
not methods and must start at the beginning of a line, with no
whitespace in between.

If you want to access a method of the same name, prefix the invocation by whitespace.

* Typing `!` on a line by itself will clear the input buffer - useful for
  getting you out of a situation where the parsing process
  goes wrong and you get stuck in an endless read loop.
* `status` shows status information about the current session.
* `whereami` shows the code context of the session.
* `version` Show Pry version information
* `help` shows the list of session commands with brief explanations.
* `toggle-color` turns on and off syntax highlighting.
* `simple-prompt` toggles the simple prompt mode.
* `exit` or `quit` or `back` or `^d` (control-d) will end the current Pry session and go
  back to the calling process or back one level of nesting (if there
  are nested sessions).
* `ls [OPTIONS] [VAR]` returns a list of local variables, instance variables, and
  methods, etc. Highly flexible. See `ls --help` for more info.
* `cat VAR` Calls `inspect` on `VAR`
* `cd VAR` Starts a `Pry` session on the variable VAR. E.g `cd @x`
(use `cd ..` to go back).
* `show-method [OPTIONS] METH` Displays the sourcecode for the method
  `METH`. e.g `show-method hello`. See `show-method --help` for more info.
* `show-doc [OPTIONS] METH` Displays comments for `METH`. See `show-doc
  --help` for more info.
* `show-command COMMAND` Displays the sourcecode for the given Pry
  command. e.g: `show-command cd`
* `jump-to NEST_LEVEL`  Unwinds the Pry stack (nesting level) until the appropriate nesting level is reached.
* `exit-all` breaks out of all Pry nesting levels and returns to the
  calling process.
* You can type `Pry.start(obj)` or `obj.pry` to nest another Pry session within the
  current one with `obj` as the receiver of the new session. Very useful
  when exploring large or complicated runtime state.

Syntax Highlighting
--------------------

Syntax highlighting is on by default in Pry. You can toggle it on and
off in a session by using the `toggle-color` command. Alternatively,
you can turn it off permanently by putting the line `Pry.color =
false` in your `~/.pryrc` file.

Bindings and objects
--------------------

Pry ultimately operates on `Binding` objects. If you invoke Pry with a
Binding object it uses that Binding. If you invoke Pry with anything
other than a `Binding`, Pry will generate a Binding for that
object and use that.

If you want to open a Pry session on the current context and capture
the locals you should use: `binding.pry`. If you do not care about
capturing the locals you can simply use `pry` (which will generate a
fresh `Binding` for the receiver).

Top-level is a special case; you can start a Pry session on top-level
*and* capture locals by simply using: `pry`. This is because Pry
automatically uses `TOPLEVEL_BINDING` for the top-level object (main).

Example Programs
----------------

Pry comes bundled with a few example programs to illustrate some
features, see the `examples/` directory.

* `example_basic.rb`             - Demonstrate basic Pry functionality
* `example_input.rb`             - Demonstrates how to set the `input` object.
* `example_output.rb`            - Demonstrates how to set the `output` object.
* `example_hooks.rb`             - Demonstrates how to set the `hooks` hash.
* `example_print.rb`             - Demonstrates how to set the `print` object.
* `example_prompt.rb`            - Demonstrates how to set the `prompt`.
* `example_input2.rb`            - An advanced `input` example.
* `example_commands.rb`          - Implementing a mathematical command set.
* `example_commands_override.rb` - An advanced `commands` example.
* `example_image_edit.rb`        - A simple image editor using a Pry REPL (requires `Gosu` and `TexPlay` gems).

Customizing Pry
---------------

Pry allows a large degree of customization. 

[Read how to customize Pry here.](http://rdoc.info/github/banister/pry/master/file/wiki/Customizing-pry.md)

Contact
-------

Problems or questions contact me at [github](http://github.com/banister)