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Fri, 12 Oct 2012

Getting Scheme to do REPL in Emacs

I took a college course last year, half of which was learning Lisp and functional programming. I don't feel I learned that much about either Lisp or functional programming in the course. I had taken a previous course with the same instructor in graphing theory where I felt I did learn a lot. Especially in subsequent courses where I had to learn tree data structures and the like.

Anyhow, I decided to take another crack at Lisp and functional programming. Some of the great and/or successful programmers have a fondness for Lisp and recommend it, even if you don't see it around much any more. As Paul Graham says about his usage of Lisp, "Everyone else was writing their software in C++ or Perl. But we also knew that that didn't mean anything. If you chose technology that way, you'd be running Windows."

Structure and Interpretation of Computer Programs is often touted as a must-read book. When I first browsed through it a few years ago it seemed confusing. I'm not sure why that is, when I look at it now it mostly seems simple and clear. I'm still reading the first of the five chapters. They're very heavy on the "interpretation" part of their title, going into evaluation and eval etc. It's not yet clear to me why they're emphasizing this so much, but perhaps I'll understand as I read through the book.

My college course used Common Lisp. I understand CL is more of the real-world one, with more libraries, but also more cruft and less simplicity.

Scheme is simpler, more elegant, and easier to understand. Scheme defines functions with the symbol define. CL defines functions with the symbol defun. That alone tells you a lot about the dialects.

One thing I like about Scheme is it seems to have a small number of primitive expressions, with a few more derived/library expressions built on those primitive expressions. I like this simplicity. While these Scheme expressions deal with abstraction and things like that, it reminds me of how almost all number-theoretic functions on the natural numbers all derive from three primitive functions - constant, successor and projection, and by doing the composition and primitive recursion operations on those functions. And the computations that can't be done with these three functions and two operations are rather offbeat, like George Cantor's ones, which do little other than disprove you can't do every natural number computation with those rules.

I also like that Scheme clearly marks non-functional procedures and forms with exclamation points.

Especially since Lisp is not heavily used nowadays, it seems obvious to me that people should first learn Scheme as that seems the best language to learn in. If they want to do some real world stuff and feel CL has better libraries or whatnot, they can then shift to CL.

So anyhow I've been going through SICP. The initial expressions could mostly be done with the Scheme interpreter guile. It does not have readline support by default like CL does, so I put into my .guile profile:

(use-modules (ice-9 readline))
(activate-readline)

As the programs became more complex, I wanted a more sophisticated REPL. Emacs seems to be what Lisp programmers use. I am not well-acquainted with emacs, even though I first started using it twenty years ago! I usually use vi, or nano, or Gnome gedit, or Eclipse's editor, or the like. Anyhow, doing elisp with Emacs is easy enough, but using Scheme is a little bit more work. I spent some time looking at it today and got it put together. Oddly, there's not really one place on the web which tells you how to do this.

In my emacs init file I now have:

(setq scheme-program-name "guile")
(global-set-key (kbd "C-x C-e") 'scheme-send-last-sexp)

I also have:

(split-window-vertically)

Just so I don't have to do "Control-x -> 2" when I start Emacs. If I start using Emacs more for editing, perhaps I'll comment that line out.

So I click the bottom window, type "Escape-x" and then "run-scheme". Then I click the top window and start typing in expressions. I usually do "Control-x Control-e" after each one to evaluate it. It evaluates in the bottom window which runs guile. I had the scheme-program-name set to scm and was running that for a bit, but switched to guile. Don't know much about either aside from that both seem to be copyrighted by the FSF, but the FSF seems to push guile more, and also guile has a nice (help) facility.

Anyhow it is running well enough for now. I'd like to improve my Scheme REPL environment a little more, but it is working OK for now.

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