(An ((Even Better) Lisp) Interpreter (in Python))(norvig.com)

140 pointsby cgopalan15 anni fa17 comments

(An ((Even Better) Lisp) Interpreter (in Python))

http://norvig.com/lispy2.html

5 comments

If you like this sort of thing and want to learn Haskell, consider looking at "Write Yourself a Scheme in 48 Hours"[1]. Of course, writing a Scheme interpreter in Scheme is also classic; SICP[2] is also worth perusing.

[1]: http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_H...

[2]: http://mitpress.mit.edu/sicp/

benbscholz15 anni fa

I have been having a lot of fun hacking together a simple Scheme interpreter in C. This is my guide: http://michaux.ca/articles/scheme-from-scratch-bootstrap-v0_...

The implementation of an interpreter has always interested me, but I never thought it was within my reach as a weekend project. Norvig's original lis.py post made me look around for other implementations. This follow-up is a real treat.

tptacek15 anni fa

My favorite HN posting of the year so far.

There's no good reason for the label "lambda"!

thisrod15 anni fa

The footnote was interesting. Instead of (lambda (x) (* x x)), Smalltalk writes [:x | x*x]. Apparently Church's original lambda notation was very close to that.

This helps more than you'd think. For example, (if condition (lambda () foo) (lambda () bar)) is too verbose, but (if condition [foo] [bar]) is fine. Simplifying lambda gives you lazy evaluation, and then you don't need syntax for conditionals.

waterhouse15 anni fa

An alternative solution: if you're willing to allow non-ASCII characters into your source code, then DrRacket provides a keyboard shortcut to insert a λ character. I'll illustrate with anonymous recursion.

  > (define sum
      (λ (n)
        ((λ (f) (f f n 0))
         (λ (f n total)
           (if (zero? n)
               total
               (f f (- n 1) (+ n total)))))))
  > (sum 10)
  55

Jach15 anni fa

You can go one step further and start giving things like color meaning. (example: http://www.colorforth.com/ide.html written with colorForth http://www.colorforth.com/cf.htm ) Of course, alternatives should exist for someone to write on a US-keyboard in any editor. Clojure supports two ways to do anonymous closures, though neither of them use a lambda symbol.

sneak15 anni fa

> if you're willing to allow non-ASCII characters into your source code

Blasphemy.

Raphael15 anni fa

You could use any letter, really. Perhaps l or f.

pvg15 anni fa

Poked me in the eye as well! It's easy to find a lot of interesting and illuminating writing about the importance and influence of notation and symbols, especially in famous cases (say, Leibniz's vs Newton's) but why a specific foo is named foo can be an oddly tricky and inconclusive nerd googlegame (nerd googlegame may be highly redundant).

Gamma function, gamma constant, gamma correction, gamma radiation, gamma distribution, and there went an hour I could have been wasting on today's best catpictures.

pjscott15 anni fa

A sequel to this article, where Norvig explains how to make a really minimal (but ridiculously clear and elegant) Lisp interpreter in 90 lines of Python:

http://norvig.com/lispy.html

william4215 anni fa

From the URLs, it looks like a prequel.

webspiderus15 anni fa

just looks like "this" needs to be properly disambiguated :)

pjscott15 anni fa

Wow, that's kind of embarrassing. To eliminate ambiguity, here they are in order:

http://norvig.com/lispy.html

http://norvig.com/lispy2.html

Raphael15 anni fa

2 comes after? I don't know...

pepijndevos15 anni fa

It doesn't implement real call/cc and continuations, which is what I want to learn. Still great stuff.

spacemanaki15 anni fa

Lisp in Small Pieces covers call/cc and much more, and I highly recommend it. It's deep and rich, so be warned that it may completely devour your time.

Another look at call/cc is in Marc Feeley's 90 Minute Scheme to C Compiler:

http://news.ycombinator.com/item?id=2633841 (submission that incudes a link to slides and video)

swah15 anni fa

I feel like:

Interpreter in C: an implementation. Interpreter in Python: an exercise.