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On Wed, 30 Apr 2003, Michael Burschik wrote:
>> (Besides tradition, I've nothing against returning useful values from
>> destructive ops, though.)
>
>I totally agree with you, but the procedures in question do not require an
>iterator. I am thinking of (list-remove! (list 1 2 3) 44), for instance.
actually there was a long tradition of useful values returned
from mutators before scheme went and made them all "Undefined."
The schema as I recall went like this....
When you were setting a subvalue within a structure, the former
subvalue got returned. Thus something like
(define pr (cons 'a 'b))
(set-car! pr 'c) => b
and
(define vc (vector 'a 'b 'c 'd))
(vector-set! vc 2 'e) => c
When you defined something, the return value was its name. Thus:
(define foo 'bar) ==> foo
This convention is still followed in most schemes.
When you were deleting something from a structure, you got back
a value that was still available for deleting. Thus
(define (liszt (list 'a 'b 'c 'd 'e 'f 'g)))
(list-remove! liszt 'b) => c
;; returned the "next" cell after the deleted one
(list-remove! liszt 'z) => g
;; returned the "last" cell after an unsuccessful search
(list-remove! liszt 'g) => f
;; returned the "first" cell when the last one was deleted
Many modern schemes return a special value labeled "undefined",
which it is an error to refer to or store. But I think MITscheme
still does the old-school value returns.
Bear