Our life around REPL – Read–Eval–Print Loop

Whenever I see my niece so comfortable with iPhone, Mac, Advance Toys; I usually wonder almost similar feeling our parents must have pondered about us. But what is the biggest disadvantage of being part of GEN X. What? We have best things in the world but the irony is that we don’t enjoy or embrace the inventions/creations done by our forefathers generation. For us TV, Radio, Telephone, Mobile, Desktop was little obvious, now for next generation iPhone, iPad, Smartphone, DSLR, Laptops, Tablets et al are very obvious. Its clearly visible our parents struggle with our generation widgets and I have now started to struggle with next-generation gadgets.

You must be thinking what is it had to do with REPL. During college days, I started coding in C, bit of Java (to clear job interviews incase required), then majorly on PHP, RUBY, PYTHON, JAVASCRIPT, BASH. The moment I started coding on these languages, which are derived from their forefathers C & Lisp, I sensed a sheer bliss while working on them and started promoting them in peer group as soon as I realized the language is always available and it was obvious not to look back at C, C++, Java and many others. We can easily compare it to Gramophone, cassettes, VCR , VCP, Landline but these inventions were the foundation of where we stand today in our life or in programming world.

I was not clear about the term REPL when I started working on these languages and thought of it as a jargon of programming world. Recently, wrote a smallest code on ruby and a REPL was created. This is when I realized the power of REPL and thought to read about their forefathers. Let’s first have a sneak-peek into that smallest piece of ruby repl code.REPL

#!/usr/bin/env ruby
loop {p eval gets }


Now read this piece of code in reverse order, READ (gets), EVAL, PRINT (p), LOOP. Save this file as repl.rb and run as`ruby repl.rb`

─$ ruby repl.rb 1 ↵
"akshay Gupta".upcase

Congratulations, now you understand REPL and underlying fact of  language is always available i.e. no real distinction between read-time, compile-time and run-time. Now, when I start working on any new language, it’s easier to pick up if it’s REPL based and I truly thank Mr. John McCarthy father of Artificial Intelligence, creator of LISP (where lies the roots of REPL) for his immense contribution in the field of computer science, AI, programming languages and so many programming paradigm. If i’m not wrong, he is also the inventor of if-then-else conditional construct and we take these for granted now.

Running code at read-time lets users reprogram; running code at compile-time is the basis of macros; compiling at runtime is the basis of Lisp’s use as an extension language in programs like Emacs; and reading at runtime enables programs to communicate using expressions.

There have been two really clean, consistent models of programming so far: the C model and the Lisp model. As computers have grown more powerful, the new languages being developed have been moving steadily toward the Lisp model. A popular recipe for new programming languages in the past 20 years has been to take the C model of computing and add some parts taken from the Lisp model, like runtime typing and garbage collection.


It’s equally important to look forward and innovate in coming time and respect, embrace the efforts of our remarkable forefathers. To read and practice different languages that support REPL please follow the below links. I primarily depend heavily upon irb and PRY.

In our everyday life we do nothing more than READ (consume knowledge), EVAL ( between right and wrong), PRINT (discuss around), LOOP (day everyday) i.e. REPL 🙂