Issue STEP-MINIMAL Writeup

Issue:        STEP-MINIMAL

Forum:	      Editorial

References:   STEP (p441)

Category:     CLARIFICATION

Edit history: 04-Mar-91, Version 1 by Pitman

Status:	      For X3J13 consideration


Problem Description:


  What is the minimal interpretation of STEP?  Specifically, must it

  pause for some user interaction, or can it just proceed without

  pausing in implementations (probably `compiled-only' implementations)

  which do not choose to implement this feature?


  In particular, are either of the following implementations valid?


  (DEFMACRO STEP (FORM) `(PROGN ,FORM))


  or


  (DEFMACRO STEP (FORM) 

    `(LET ((VALUES (MULTIPLE-VALUE-LIST ,FORM)))

       (FORMAT T "~&~S => ~{~S~^, ~}~%" ',FORM VALUES)

       (VALUES-LIST VALUES)))


Proposal (STEP-MINIMAL:PERMIT-PROGN):


  Clarify that it is permissible to implement (STEP x) as (PROGN x)

  in implementations which do not wish to provide more elaborate

  functionality.


  Rationale:


  For a delivery Lisp, this would be a minimal amount of work.


Proposal (STEP-MINIMAL:REQUIRE-PAUSE):


  Clarify that (STEP x) must, at minimum, pause in some way that permits

  user intervention.


  This also implies that in implementaions which do have a query loop, 

  some form of query must occur even for constant forms, like (STEP 5).


  Rationale:


  Users who introduce STEP into a program might do so on the assumption

  that execution will not proceed beyond that point (e.g., into some 

  dangerous code that follows) without offering them a way to intervene 

  first.


Current Practice:


  Symbolics Genera requires intervention.


  Symbolics Cloe Runtime (the part that runs on the 386) behaves like PROGN

  in the example I tried (a couple of nested arithmetic operations).


Cost to Implementors:


  REQUIRE-PAUSE:  Probably none.  If there are any minimalist 

   implementations which do not currently require a pause now, they

   would need a very small amount of additional code.


  PERMIT-PROGN:  None.


Cost to Users:


  None.  This is a debugging feature that affects -how- Lisp is used in

  subtle ways, but is not likely to be present in stable code so wouldn't 

  have a direct cost.  It might be argued that there was a slight cost

  to PERMIT-PROGN for users who wanted the pause and didn't get it in

  some implementation; there is also a risk of larger cost with 

  PERMIT-PROGN of users who expect a pause and don't get it and fall 

  through to code they didn't intend to run.


Cost of Non-Adoption:


  Unclear specification.  Implementors of certain implementations won't

  know what to do.  Users won't know what to expect.


Benefits:


  See above.


Aesthetics:


  Probably negligible.


Discussion:


  Pitman has a slight preference for REQUIRE-PAUSE, but supports any

  interpretation that makes this clear.