[r6rs-discuss] [Formal] defer lambda rather than define rhs when expanding body

[Per Bothner]

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This message is a formal comment which was submitted to formal-comment at r6rs.org, following the requirements described at: http://www.r6rs.org/process.html
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Type: Simplification and enhancement
Priority: Minor
Component: Concepts (?)
Version: 5.92

When expanding a body the current specification defers expanding
the rhs of a define.  I suggest it would be more consistent and
natural to defer expanding lambda-expressions instead.

* It improves consistency.  Consider:

    #|1|# (define VAR (MAC))
    #|2|# (set! VAR (MAC))
    #|3|# (list (MAC))
    #|4|# (MAC)
    #|5|# (define VAR (lambda () (MAC)))
    #|6|# (set! VAR (lambda () (MAC)))
    #|7|# (define-syntax MAC ...)

Which of #1-#6 are valid when followed by #7?
By my reading only #1 and #5 are allowed.  But that doesn't seem natural
to me.  If #5 is allowed, then #6 should be allowed.
Similarly for #1 and #2.

In my proposal, #1-#4 are invalid while #5-#6 are valid.

* There are also anomalies with expanding top-level forms.  In
the above example, the expansion of MAC is deferred in #1, #2, #3,
but not #4.  This could lead to subtle differences.  In my proposal
all of these would be expanded eagerly, which removes surprises.

* An informal reason is that this makes the expansion
process more similar to evaluation: Evaluating define means
evaluating its rhs, while evaluating a lambda expression
does not evaluate its body.

* The above reason leads to better support for read-eval-print
loops and other interactive modes where expansion is interleaved
with evaluation: If I type a define in repl, an implementation
can defer expanding a lambda (until it is called), but it can't
defer expanding the rhs of a define in general.

Original informal discussion:
http://lists.r6rs.org/pipermail/r6rs-discuss/2007-February/001612.html
-- 
	--Per Bothner
per at bothner.com   http://per.bothner.com/