@@ -76,3 +76,45 @@ for BEGIN and IF; later, other special forms may follow. According to R*RS, the last expression inside a lambda body is also in tail position, but in our implementation lambda bodies can only contain one expression, so it doesn't apply here. (For multiple expressions use BEGIN, which *does* use TCO.) + +# +# CONTINUATIONS + +Work like this: + +(+ 1 (call/cc (lambda (k) (+ 2 (k 3))))) +($$ 1 (call/cc ...)) + +(<+> $$ (call/cc ...)) 1 +(<+> 1 $$) (call/cc (lambda (k) (+ 2 (k 3)))) +(<+> 1 $$) ($$ (lambda (k) (+ 2 (k 3)))) call/cc +(<+> 1 $$) (<call/cc> $$) (lambda (k) (+ 2 (k 3))) +(<+> 1 $$) (<call/cc> $$) <lambda> +(<+> 1 $$) (<call/cc> <lambda>) + +At this point, we execute (<call/cc> <lambda>)... + +Now what? +- We create a continuation (i.e. a snapshot of the current stack) +- We create a function that takes an argument x, and that, when called, restores + the call stack to the snapshot stored in the continuation, and then returns x +- We replace the (<call/cc> <lambda>) with the lambda body, associated + with an environment which has the lambda's variable bound to said function + +(<+> 1 $$) (+ 2 (k 3)) with k = {continuation} +(<+> 1 $$) ($$ 2 (k 3)) + +(<+> 1 $$) (<+> $$ (k 3)) 2 +(<+> 1 $$) (<+> 2 $$) (k 3) +(<+> 1 $$) (<+> 2 $$) ($$ 3) k +(<+> 1 $$) (<+> 2 $$) ({cont} $$) 3 +(<+> 1 $$) (<+> 2 $$) ({cont} 3) + +At this point, we're ready to call (k 3). As said, it restores the stack to +the way it was, then collapses the value 3. So: + +(<+> 1 $$) 3 +(<+> 1 3) +=> 4 + +The other, partially evaluated expressions, DO NOT MATTER as soon as we call +(k 3). + README @@ -39,8 +39,7 @@ TODO: - Rewrite some parts in a more functional style, esp. where we change expressions in place (placeholder etc). Instead, we may be able to just use a new expression. That way, we won't have to use copy.deepcopy... -- Add a few more special forms like AND and OR -- Continuations? +- Add a few more special forms like QUOTE, AND and OR """ dollop.py 642c20837de9469778d68a26962a3fe2a460ce51 Hans Nowak zephyrfalcon@gmail.com http://github.com/zephyrfalcon/dollop/commit/5a9d5ec3aa4a34adf12a70c82beb107993e8e82c 5a9d5ec3aa4a34adf12a70c82beb107993e8e82c 2009-06-16T19:36:09-07:00 2009-06-16T19:36:09-07:00 add some comments/docs for clarification 008ae0a2c766b9387dc1622f2eda917439680906 Hans Nowak zephyrfalcon@gmail.com