Re: Difference letrec & environment binding

[dsmich]

---- Hans Aberg <address@hidden> wrote: 
> What is the practical difference between the two ways of doing the same thing 
> (letrec or an environment). Is letrec more efficient in the implementation of 
> Guile?
> 
> Hans
> 
> 
> (letrec (
>   (even? (lambda (n)
>     (if (zero? n) #t (odd? (- n 1)))))
>   (odd? (lambda (n)
>     (if (zero? n) #f (even? (- n 1))))))
>   (even? 1000))
> 
> ((lambda ()
>   (define even? (lambda (n)
>     (if (zero? n) #t (odd? (- n 1)))))
>   (define odd? (lambda (n)
>     (if (zero? n) #f (even? (- n 1)))))
>   (even? 1000)))
> 

No difference at all.  Those generate identical code.  I believe the second 
form is transformed into the first during compilation.

scheme@(guile-user)> ,x a
Disassembly of #<procedure a ()>:

   0    (assert-nargs-ee/locals 8)      
   2    (br :L204)                      ;; -> 25              at (unknown 
file):15:9
   6    (local-ref 0)                   ;; `n'
   8    (make-int8:0)                   ;; 0                  at (unknown 
file):16:8
   9    (ee?)                           
  10    (br-if-not :L205)               ;; -> 16              at (unknown 
file):16:4
  14    (make-true)                     ;; #t
  15    (return)                        
  16    (local-ref 0)                   ;; `n'
  18    (sub1)                                                at (unknown 
file):16:27
  19    (local-set 0)                   ;; `n'
  21    (br :L206)                      ;; -> 29              at (unknown 
file):16:21
  25    (br :L207)                      ;; -> 48              at (unknown 
file):17:8
  29    (local-ref 0)                   ;; `n'
  31    (make-int8:0)                   ;; 0                  at (unknown 
file):18:8
  32    (ee?)                           
  33    (br-if-not :L208)               ;; -> 39              at (unknown 
file):18:4
  37    (make-false)                    
  38    (return)                        
  39    (local-ref 0)                   ;; `n'
  41    (sub1)                                                at (unknown 
file):18:28
  42    (local-set 0)                   ;; `n'
  44    (br :L209)                      ;; -> 6               at (unknown 
file):18:21
  48    (make-int16 3 232)              ;; 1000
  51    (local-set 0)                   
  53    (br :L209)                      ;; -> 6               at (unknown 
file):19:2

scheme@(guile-user)> ,x b
Disassembly of #<procedure b ()>:

   0    (assert-nargs-ee/locals 8)      
   2    (br :L216)                      ;; -> 25              at (unknown 
file):23:16
   6    (local-ref 0)                   ;; `n'
   8    (make-int8:0)                   ;; 0                  at (unknown 
file):24:8
   9    (ee?)                           
  10    (br-if-not :L217)               ;; -> 16              at (unknown 
file):24:4
  14    (make-true)                     ;; #t
  15    (return)                        
  16    (local-ref 0)                   ;; `n'
  18    (sub1)                                                at (unknown 
file):24:27
  19    (local-set 0)                   ;; `n'
  21    (br :L218)                      ;; -> 29              at (unknown 
file):24:21
  25    (br :L219)                      ;; -> 48              at (unknown 
file):25:15
  29    (local-ref 0)                   ;; `n'
  31    (make-int8:0)                   ;; 0                  at (unknown 
file):26:8
  32    (ee?)                           
  33    (br-if-not :L220)               ;; -> 39              at (unknown 
file):26:4
  37    (make-false)                    
  38    (return)                        
  39    (local-ref 0)                   ;; `n'
  41    (sub1)                                                at (unknown 
file):26:28
  42    (local-set 0)                   ;; `n'
  44    (br :L221)                      ;; -> 6               at (unknown 
file):26:21
  48    (make-int16 3 232)              ;; 1000
  51    (local-set 0)                   
  53    (br :L221)                      ;; -> 6               at (unknown 
file):27:2

-Dale