Special Locals

Eliot Sykes edited this page Jul 10, 2017 · 22 revisions

The Special Locals

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Pry injects a number of local variables into a session. Among them is the familiar 'last result' local - _ from IRB.

The role of a special local is to streamline the user interface by making useful information readily available to the programmer.

Last result

local name: _

The _ local always stores the result of the last expression, allowing you to recall it for use in new calculations.


pry(main)> 5 + 5
=> 10
pry(main)> _ + 10
=> 20

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Last exception

local name: _ex_

Pry gives you access to the most recently caught exception in a local variable called _ex_. Unlike Ruby's builtin $!, this persists until another exception is raised, so can be used for detailed digging.

You can also use the the wtf? command to view the exception's backtrace, and cat --ex to view the code that raised the exception. For more information on exceptions check out the Exception handling section.


pry(main)> 4 + "j"
TypeError: String can't be coerced into Fixnum
from (pry):2:in `+'
pry(main)> _ex_
=> #<TypeError: String can't be coerced into Fixnum>
pry(main)> _ex_.backtrace.first(4)
=> ["(pry):2:in `+'",
 "(pry):2:in `<main>'",
 "/Users/john/ruby/projects/pry/lib/pry/pry_instance.rb:229:in `eval'",
 "/Users/john/ruby/projects/pry/lib/pry/pry_instance.rb:229:in `re'"]

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The input and output cache

local names: _in_ and _out_

Input expressions and output results are automatically stored in array-like data structures accessible from the _in_ and _out_ locals respectively. These lists only contain pure Ruby code and so exclude Pry commands and their results. Also note that the _in_ cache stores full expressions not just lines of code. The size of the _in_ and _out_ cache is determined by Pry.config.memory_size, when this limit is reached newer entries begin to overwrite the older ones.

Note that in the examples below the input/output cache index is displayed on the far left; some prompts have this by default, such as the Pry::NAV_PROMPT

If we want to replay an expression from the _in_ cache it is best to use the play command. We use it like this: play --in 5.

Example: The _out_ cache.

pry(main)> 1 + 1
=> 2
pry(main)> 2 + 3
=> 5
pry(main)> _out_[-1] + _out_[-2]
=> 7

Example: The _in_ cache.

[5] pry(main)> def poem
[5] pry(main)*   puts "come back to me brutal, empty room"
[5] pry(main)*   puts "thin byzantine face"
[5] pry(main)* end;  
[6] pry(main)> play --in 5 --lines 2..3
come back to me brutal, empty room
thin byzantine face
=> nil

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Last file, and last directory

local names: _file_ and _dir_

Some commands such as show-method, show-doc, show-source, stat and cat update the _file_ and _dir_ local variables after they run. These locals contain the full path to the file involved in the last command as well as the directory containing that file.

You can then use these special locals in conjunction with Pry commands to do such things as change directory into the directory containing the file, open the file in an editor, display the file using cat, and so on. __LINE__ may also come in handy here, if you need to access the current line number.


pry(main)> show-method pry

From: /Users/john/ruby/projects/pry/lib/pry/core_extensions.rb @ line 19:
Number of lines: 3

def pry(target=self)
pry(main)> _file_
=> "/Users/john/ruby/projects/pry/lib/pry/core_extensions.rb"
pry(main)> cat #{_file_} -e 2
class Object
  # Start a Pry REPL.
pry(main)> shell-mode
pry main:/ $ .cd #{_dir_}
pry main:/Users/john/ruby/projects/pry/lib/pry $

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Current Pry instance

local name: _pry_

The _pry_ local stores a reference to the Pry instance managing the current session. By interacting with this instance we can introspect on the machinery behind the REPL itself and further configure/customize our session at runtime. See the Customization and configuration section for more information.


pry(main)> _pry_.prompt = proc { "> " };
> puts "look at my new prompt!"
look at my new prompt!
=> nil

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