xplorld / Leiopython

Yet another Scheme Interpreter in Python!

Leiopython

Yet another Scheme Interpreter in Python!

Etymology:

Leiopython is a kind of white-lipped python in New Guinea.

Known Problems

deep recursion failure - nested Enviroment

Now recursion is called by python calls. see bulitin_lambda. Enviroment is stored in Env object, which is (sort of) a reversed single-link list. When we need to resolve a NAME, Env is searched through its father Envs by literal strings. Every call to a lambda introduce a deeper level of Env. Consider code below:

	(define (zero? x)
	  (= x 0))
	(define (fac i)
	  (if (zero? i) 1 (* (fac (- i 1)) i)))
	(fac 100)

fac n computes n!. When we call, say (fac 100), the Env looks like

{"fac":<lambda object>} -> {"i":100} -> {"i":99} -> {"i":98} ...

and when we finally go down into (fac 2), the NAME fac mush searched through chained Env.get method by 98 times! In another word, n nested calls in scheme would make O(n^2) nested calls in python.

Solution

1. flattened Env

Instead of closure = Env(parent_env).set("i", 99), we have closure = parent_env.copy().set("i",99)

What about set!?

2. a global Env whose get is implemented manually instead of recursive call

Instead of closure = Env(parent_env).set("i", 99), we have closure_p = env.new_level().set("i",99)

deep recursion failure - tail recursion optimization

Even the env problem is solved, that a n nested scheme call reflects in O(n) python call, it still may yield a stack overflow.

Scheme is guranteed to have tail recursion optimization. We should implement it.

Hygienic Macros

We should implement it. 'cause it's cool.

Quasiquote and Unquote

Variable Arguments

Moriturus te saluto.