meaning of local define

[CPH%OZ.AI.MIT.EDU@XX.LCS.MIT.EDU]

    Date: Thursday, 27 March 1986  15:40-EST
    From: Jonathan A Rees <JAR at MC.LCS.MIT.EDU>

    This can make a difference when a user tries to figure out whether
    expressions like the following will return a well-defined result:

      (define (foo x)
        (define bar (1+ x))
        (define baz (* bar bar))
        (/ baz 6))

    According to the documentation of DEFINE, this is equivalent to

      (define (foo x)
        (letrec ((bar (1+ x))
		 (baz (* bar bar)))
          (/ baz 6)))

    I'm not a big fan of local define's, but I think that if they are
    implemented, they should have right-to-left semantics.  I believe
    local-DEFINE partisans will agree with this.

I (sort of) like local-DEFINE.  If I understand correctly, you would
like to have them be evaluated sequentially because each appears,
syntactically, to be a side-effect.  I feel funny about this, because
the example above is precisely the kind of thing that constantly
screws up beginning users in 6.001.

I'm used to thinking of local-DEFINE as a syntax for LETREC (which
takes less horizontal space!), and not having any dependence on order
of evaluation.  And in fact I consider that very, very bad style.
The example above should be written using LET.

In fact, the only useful values in local-DEFINE or LETREC are
procedure definitions and delayed evaluations, because nothing else
can need the mutual recursion -- every other kind of expression can
always be written using LET.

Anyway, to get to the point: as I recall, local-DEFINE was included in
the standard only because of S&ICP.  I would favor leaving the current
description of its semantics, perhaps with a prominent note to that
effect, and recommending LETREC for new code.

I should probably learn to use LETREC instead, but I fear I'm too
attached to DEFINE by now.