Saturday, January 17, 2009

Lisp insights from learning Logo

So I was just writing a simple recursive procedure in Logo. I wanted to go down a list, cons up a new list, and return the empty list when I get to the end.

My first pass, once I found out how to return something from a function (Logo procedures aren't expressions -- you have to explicitly return) had a line like this:

if (stacks = []) (output [])

Running the procedure gave me the helpful error message:
output didn't output to if in astep
[if (stacks = [] ) (output [] )]


When I remembered that Logo's "if" wants a list of instructions, it clicked with my new nugget of knowledge that literal lists are quoted in Logo. Then I understood what was going on: this "if" is neither a macro (like in Scheme, or most languages) nor lazily evaluated (like Haskell). It's a regular function, but one that expects quoted code to evaluate on demand at runtime. Rubyists and Smalltalkers: this is something like how an if block works for you, yes?

Here's an analogy into Python, for the non-Lispers out there:

def myIf(tf, truecode, falsecode):
  code = {True : truecode, False : falsecode}
  return eval(code[tf])

>>> myIf(0 == 1, "'it was ' + 'true'", "'it was not true'.upper()")

>>> myIf(0 == 0, "'it was ' + 'true'", "'it was not true'.upper()")
'it was true'

Also, apparently Logo in general has dynamic scope -- which shouldn't surprise me, since lexical scope is relatively new in Lisp. And Berkeley Logo in particular has macros. Maybe my little compiler project is going to be harder than I expected. Honestly, I was imagining I'd be done if I could just parse it, do some simple transformations on the tree and spit JavaScript.

Sunday, January 11, 2009

my code has a side effect: learning!

So I'm working through the exercises in Real World Haskell. Here's what I've got so far. As of this writing, I'm in the middle of chapter 4. RWH totally deserves all the buzz it's been getting; it's very approachable and well-written, and my Haskell is improving rapidly. The book is still free online, but I bought the printed copy, in large part because Bryan O'Sullivan gave such a great talk at OSCON.

My earlier Haskell project had stalled out due to my not being very good at the language yet. What I want to do is build a compiler for Logo that produces JavaScript, with the turtle graphics on a canvas tag. Logo is an acceptable Lisp! I'd love to see it get more use.

Interestingly, though: