STEP-ENVIRONMENT, version 3

[sandra@cs.utah.edu (Sandra J Loosemore)]

As promised, here is a revised version of the STEP-ENVIRONMENT proposal.
I have removed the part of the proposal that would make STEP a special
form instead of a macro.

-Sandra



Issue:         STEP-ENVIRONMENT
 
References:    STEP (CLtL p.441)
               TIME (CLtL p.441)
 
Category:      CLARIFICATION/CHANGE
 
Edit history:  Version 1, 12-Mar-88, Moon
               Version 2, 10-Jun-88, Masinter (add discussion)
	       Version 3, 20-Jun-88, Loosemore (not a special form)

 
Problem description:
 
CLtL does not specify in what lexical environment the form given to STEP
is evaluated.  Some people think it's supposed to be evaluated in the
null environment, others think it is supposed to be evaluated in the
current environment, the one in which the STEP form was evaluated.
 
The same considerations apply to TIME.
 
 
Proposal (STEP-ENVIRONMENT:CURRENT):
 
1. Clarify that STEP and TIME evaluate the form in the current environment.

2. Clarify that calls to both STEP and TIME may be compiled, but that
in the case of STEP, it is acceptable for an implementation to
interactively step through only those parts of the evaluation that are
interpreted. 

 
Test Cases/Examples:
 
;Assuming X is not a special variable
(setq x 1)
(let ((x 2))
  (step (print x)))
 
This should print and return 2, not 1, when interpreted.
 
Rationale:
 
1. It is more useful for the lexical environment to pass transparently
through STEP and TIME than to reset to the null environment.

2. Although STEP is primarily a debugging tool, there is no reason to
prevent it from being compiled correctly.

 
Current practice:

Vax Lisp behaves as proposed.  Symbolics Common Lisp treats STEP as a special
form and does not allow it to be compiled. 

Cost to Implementors:
 
Minimal.

Cost to Users:
 
None.

Cost of non-adoption:
 
These debugging tools will continue to have vague specifications.
 
Benefits:
 
Slightly more preicse specification of Common Lisp.
 
Esthetics:
 
Slightly improved.
 
Discussion:
 
There was some discussion of separating this out into two separate
proposals, but it didn't seem useful.
 
Eric Benson contributed the definition of TIME in Lucid Common Lisp:
 
(defmacro time (form)
  `(time-internal #'(lambda () ,form)))
 
 
The function TIME-INTERNAL looks something like:
 
(defun time-internal (thunk)
  (let ((before-time (get-time-state)))
    (unwind-protect
        (funcall thunk)
      (print-time-information before-time (get-time-state)))))
 
The definition of STEP is similar.  This is just to show that it is
easy to get the right lexical environment even though TIME and STEP
are macros.

VaxLisp expands STEP into something like:

(defmacro step (form)
    `(let ((*eval-hook* #'step-command-loop))
        ,form))
 

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