[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Comments on LET and constructors

This page is part of the web mail archives of SRFI 76 from before July 7th, 2015. The new archives for SRFI 76 contain all messages, not just those from before July 7th, 2015.

 A small question:  Are the parent constructor arguments also in the 
 scope of the LET?  
 I think they should be.  If they are, then we can at least express one 
 of the "inexpressible" examples I listed:
 >    ;; Conditionally instantiating the parent with different arguments:
 >    ;; (actually this specific example can be expressed but would 
 >    ;;  require the test for (= y 0) to be duplicated):
 >    (define-type rational 
 >      (parent finite-rational)
 >      (constructor (lambda (x y)
 >                     (if (= y 0)
 >                         (instantiate (parent 1 0))   
 >                         (instantiate (parent x y))))))    
 as follows:
   (define-type rational (x y)
     (let ((parent-args (if (= y 0)
                            (cons 1 0)   
                            (cons x y)))))
     (parent finite-rational (car parent-args)
                             (cdr parent-args)))