Re: Define in let

[taylanbayirli@gmail.com (Taylan Ulrich B.)]

Dmitry Bogatov <KAction@gnu.org> writes:

> It seems following is invalid:
>
>    (let ((a 2))
>         (define (foo x) (+ a x)))
>
> I prefer to reduce scope of variable as much as possible, so
> I find this restriction unconvinent. Is is part of standard or technical
> limitation? Is it any workaround?
>
> Please, keep in CC, I am not subscribed.
>
> --
> Best regards, Dmitry Bogatov <KAction@gnu.org>,
> Free Software supporter and netiquette guardian.
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No Scheme standard so far has supported such a thing, and neither Guile.

It would be neat, I had the idea too (it would allow re-use of the
semantics of `begin' and thus be coherent/orthogonal in a way), but I
think it's a non-trivial change, probably both to the implementation of
Guile and the semantics for `let' that have been well-understood ever
since very old Lisps: that `let' is a thin wrapper around a call to an
in-place `lambda'.

(let ((a x)
      (b y))
  ...)

is just

((lambda (a b)
   ...)
 x y)

in pretty much any Lisp.

Note that you can do

(define foo
  (let ((a 2))
    (lambda (x) (+ a x))))

and if you want to define multiple things that use that `a' binding, you
could use `define-values', but Guile doesn't have that yet (there's a
hacky in-Scheme definition in R7RS-small if you want something quick):

(define-values (foo bar)
  (let ((a 2))
    (values
     (lambda (x) (+ a x))
     (lambda (y) (* a y)))))

Taylan