[WIP] Tree interpreter #1
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Loop bindings allow "tail calls" in loops without the need to do tail call analysis, and so they're a good quick and dirty way to make the language more useful.
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Holes are values that can be filled in later with the hole/fill
function. They're useful for manually implementing
tail-recursion-modulo-cons, which is where you fill in the second part
of a cons later so that you can make a tail call. It's possible to use
the hole function unsafely: if you make a cyclic structure it will never
be collected.
Unsafe example:
(let! rest (hole))
(let! all-ones (cons 1 rest))
(hole/fill rest all-ones)
now all-ones is a circular list!
Useful example:
(defn (list/map f xs)
(let! result (hole))
(loop ((xs xs) (spot result))
(if (nil? xs) (break (hole/fill spot nil)))
(let! new-spot (hole))
(hole/fill spot (cons (f (car xs)) new-spot))
(continue (cdr xs) new-spot))
result)
Here we define map tail recursively, and without having to do any
post-processing like reversing the list.
Pretty-print tuples as #tuple or #tuple<tag>. Add instructions for getting and setting the tuple tag. Make struct generation set the tag inside the constructor. Thanks to Roshni for pair programming on this, I had fun explaining it.
Rewrite (#tuple<cons> 1 (#tuple<cons> 2 ())) as (1 2) and (#tuple<cons> 1 2) as (1 . 2).
For instance, a symbol like a will print as 'a in the alternate printing
mode. Now the repl prints using {:#} instead of {} to get the quoting.
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Run the code by directly walking the AST