Next: , Previous: , Up: compiler letrec   [Index]

17.10.4 The basic transformation algorithm

The basic transformations are equivalent to: 

(letrec ((?lhs ?rhs) ...) ?body)
→ (let ((?lhs #!void) ...)
      (let ((?tmp ?rhs) ...)
        (set! ?lhs ?tmp) ...
        ?body))

(letrec* ((?lhs ?rhs) ...) ?body)
→ (let ((?lhs #!void) ...)
      (set! ?lhs ?rhs) ...
      ?body)

and in recordised code are represented as: 

(recbind ((?lhs ?rhs) ...) ?body)
→ (bind ((?lhs #!void) ...)
      (bind ((?tmp ?rhs) ...)
        (assign ?lhs ?tmp) ...
        ?body))

(rec*bind ((?lhs ?rhs) ...) ?body)
→ (bind ((?lhs #!void) ...)
      (assign ?lhs ?rhs) ...
      ?body)

as example, with these transformations the library: 

(library (optimize-letrec-basic-demo)
  (export a b c)
  (import (rnrs))
  (define (a) 1)
  (define (b) (a) 2)
  (define (c) (b) 3))

is transformed into: 

(bind ((a_0 '#!void)
       (b_0 '#!void)
       (c_0 '#!void))
 (seq
   (set! a_0 (lambda () '1))
   (set! b_0 (lambda () (seq (funcall a_0) '2)))
   (set! c_0 (lambda () (seq (funcall b_0) '3)))
   (funcall void)))