wip: abstract stack/continuation machine

2026-08-17 22:31:45 -06:00
parent c3c4866fa8
commit 745277ed1a
16 changed files with 723 additions and 22 deletions
@@ -0,0 +1,53 @@
+#+title: assorted notes on compilation
+
+* letrec
+
+consider:
+
+#+begin_src scheme
+  (letrec ((even? (lambda (n)
+                    (if (zero? n)
+                        #t
+                        (odd? (- n 1)))))
+           (odd? (lambda (n)
+                   (if (zero? n)
+                       #f
+                       (even? (- n 1))))))
+    (even? 12))
+#+end_src
+
+#+RESULTS:
+: #t
+
+since ~letrec~ is a primitive construct in the CPS language, the translation of mutually recursive functions is straightforward:
+
+#+begin_src scheme
+  (define (-& x y k) (k (- x y)))
+  (define (zero?& x k) (k (zero? x)))
+  (define (halt x) x)
+
+  (letrec ((even? (lambda (n ktail)
+                    (zero?& n
+                            (lambda (x1)
+                              (if x1
+                                  #t
+                                  (-& n 1
+                                      (lambda (x2)
+                                        (odd? x2 ktail))))))))
+           (odd? (lambda (n ktail)
+                   (zero?& n
+                           (lambda (x1)
+                             (if x1
+                                 #f
+                                 (-& n 1
+                                     (lambda (x2)
+                                       (even? x2 ktail)))))))))
+    (even? 12 halt))
+#+end_src
+
+#+RESULTS:
+: #t
+
+however, Scheme permits ~letrec~-expressions with non-lambda right-hand sides, while the CPS language permits only kappa and lambda forms. thus, the handling of these forms is less trivial.
+
+for now we'll just reject any ~letrec~ forms with non-lambda right-hand sides, lol. they aren't very important.