#binding.pry

title pun courtesy of The Magnetic Fields

What is IRB?

IRB is what's called a REPL, or read-eval-print loop. It takes individual inputs, and returns their results. It is accessible through the terminal, and is useful for experimentation in real time. It comes packaged with a default Ruby install, and there's even a version of it available through your browser.

What is Pry?

Pry is a Ruby gem that is also a REPL, but significantly more robust than the IRB. Most notably, you can start Pry inside a running program. This allows you to a) debug a program as it runs and b) experiment with the program while retainin access to all the variables, classes, methods, etc. that you've already defined, without having to copy-paste into your Terminal nearly as much.

Filesystem-style Commands

"Pry commands are not methods; instead they are special strings that are interpreted directly by Pry before the input buffer is evaluated."

-from the Pry wiki

Pry commands allow us to manipulate things other than the expression we are trying to REPL. Here is an example I would have found extremely useful in previous labs, had I known it existed.

We can use "ls" to show a list of variables and methods in the current scope.

We can use "cd" to enter (and experiment within) a different scope.

We can use "show-source" to show the code that created the method/variable whose scope we are in.

We can use "edit" to open that code in our default text editor.

For example: I ran binding.Pry inside the creation of a new TicTacToe game. I used cd to get into the new game, then cd'd into the current (empty) board. Then, I used show-source, and it showed me the entire contents of lib/board.rb. (I've used a partial screenshot here for space's sake.)

Then I typed "edit board" which opened board.rb in Sublime Text. As an experiment, I decided that board.reset should give us an array of 9 "z"s, rather than 9 " "s.

Once I saved my changes and closed board.rb in Sublime, I ran show-source again, and it showed the revised contents.

binding.pry, two words that have the ability to freeze your program in time and in IRB, gives you a more powerful way to debug. Once you have the gem installed, you can simply put the binding.pry command anywhere in your application (usually before the line of code that is breaking your app). In IRB, you will see the pry session along with the lines of code before and after it. In this session, you can do things like check to see if your variables are accurate and you can easily test out methods.

Here are some useful Binding Pry Commands:

whereami - reprints where you are in the pry session

ls - lists all the variables, methods, modules available in the current scope

wtf - will print the most recent error stack trace (it identifies the error, from the most specific context to the most general context)

exit - binding.pry session can be used inside a loop to check each iteration for errors. Typing 'exit' while inside that current pry session will take you out of the session and trigger again.