programming musings
14 Jan 2006

the joy of repl

Back in the old days i was a macho C++ programmer, one of those sneering at Java or any other language but C, willing to manage my memory and pointers and mystified by the complexity of the template syntax (it was difficult and cumbersome, ergo it had to be good). Everyone has a past.

Things began to change when i decided to add Guile extensibility to GNU MDK. I was using the project as an excuse to learn everything one has to learn to write free software, from parsers with flex and bison, to documentation generators like texinfo and doxygen or localisation via gettext. Next thing was scriptability, and those days Scheme was still the way to extend your GNU programs (last time i checked, GNOME was full of XML, a windows-like registry and, oh my, C#… no Scheme (or good taste) to be seen).

So, when i first encountered Scheme i was high on static type checking, object oriented programming in its narrow C++ flavour, and all that jazz. I didn't understand immediately what was so cool about having an interpreter, and defining functions without the compiler checking their signature at every single call made me feel uneasy. I was told that i still had strong type checking in Lisp, but that it is deferred to run time, instead of at the apparently safer compile phase. I didn't get it. Thanks god, SICP was so fun that i kept on learning, and i kept wondering for a while what was so great about interpreters and dynamic typing.

Problem was, i was writing C programs in Scheme. In a compiled language (a la C) and, to some degree, in any statically typed one, your code is dead. You write pages and pages of inert code. You compile it. Still dead. Only when you launch that binary does it come to life, only that it lives elsewhere, beyond your reach. Admittedly, i'm exaggerating: you can reach it in a convoluted way via a debugger. But still. A debugger is an awkward beast, and it will only work with the whole lot: all your program compiled, linked and whatnot.

Enter a dynamic language. Enter its REPL. When you have a, say, Lisp interpreter at your disposal you don't write your code first and load it later (that's what i was doing at first). You enter your code piecewise, function by function, variable by variable at that innocent looking prompt. You develop incrementally, and, at every single moment, your objects and functions are alive: you can access them, inspect them, modify them. Your code becomes an organic creature, plastic. Its almost not programming, but experimenting.

Maybe you're raising a skeptical eyebrow. Maybe you have one of those modern visual-something debugger that lets you modify your compiled code on the fly and continue running your code using the new definitions and you think that's what i'm talking about… Well, no, sorry, that's only part of what i'm talking about. To begin with, you continue executing your program. I can do whatever i want. But that's not all. We are talking about a dynamically typed language. That means that me and my little REPL have much more leeway to modify the living code, and thus much more margin to grow up and evolve the code.

At the end of the day, dynamically typed languages give me freedom. Programming is a creative process and greatly benefits from that freedom. At first, abandoning the safety net provided by static typing was a little bit scary, but as i grew up as a programmer i felt more and more confident, and gradually the initially uneasy feeling morphed into joy. The joy of REPL.

Richard P. Gabriel has made a far better job in beautifully conveying what i'm trying to express in his excellent introduction to David Lamkins' book Successful Lisp, entitled The Art of Lisp and Writing. Unfortunately, i haven't found it online – you can read the first few pages in's "look inside this book" section for this book. And also in his essay Do Programmers Need Seat Belts?. Paul Graham has famously argued in favour of bottom-up development in many of his essays, and specially in Programming Bottom-Up:

It's worth emphasizing that bottom-up design doesn't mean just writing the same program in a different order. When you work bottom-up, you usually end up with a different program. Instead of a single, monolithic program, you will get a larger language with more abstract operators, and a smaller program written in it. Instead of a lintel, you'll get an arch.

Finally, please note that i'm well aware that the static vs. dynamic typing debate is still open, and that decent type systems like those in Haskell and ML have, arguably, much to offer in the way to solid software engineering. Type theory has also a powerful and beautiful mathematical foundation. The above is just my gut feeling and current position on these issues, and i don't pretend to have backed it with solid technical argumentation. Nor was it my goal. I'm more interested here in programming as a creative activity than as engineering.

Tags: lisp scheme auld
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