Issue DRIBBLE-TECHNIQUE Writeup

Issue:        DRIBBLE-TECHNIQUE

References:   DRIBBLE (p443)

Category:     CLARIFICATION

Edit history: 06-Dec-87, Version 1 by Pitman

              14-Feb-88, Version 2 by Masinter


Problem Description:


The intent and effect of DRIBBLE is not very clearly specified. Users have

complained that DRIBBLE behaves quite differently in different implementations.


Proposal (DRIBBLE-TECHNIQUE:MAKE-EXPLICITLY-VAGUE):


Clarify that DRIBBLE is intended primarily for interactive debugging and that

its effect cannot be relied upon from programs.


Test Case:


 #1: (PROGN (DRIBBLE "temp")

	    (PRINT 'FOO)

            (DRIBBLE))


 #2: (DRIBBLE "temp")

     (PROGN (PRINT 'FOO)

            (DRIBBLE)

	    (PRINC 'BAR))	 


Rationale:


Users of DRIBBLE have been surprised about how differently DRIBBLE behaves in

different implementations. This makes the status quo much more explicit and will

lead to less surprise.


Current Practice:


Some implementations implement DRIBBLE as a function which simply assigns

streams such as *STANDARD-OUTPUT* to broadcast streams (or the approximate

equivalent thereof).  Even within this camp, there is not widespread agreement

about which streams are affected. However, typically test case #1 will capture

the output of (PRINT 'FOO) in the file "temp" and will execute (PRINT 'BAR) but

not capture its output.


Some implementations (eg, Symbolics) push to a recursive command loop when

DRIBBLE is called with an argument, and return from that command loop when

DRIBBLE is called with no argument. In these implementations, test case #1 will

enter a recursive command loop upon the first call to DRIBBLE and will not

execute the (PRINT 'FOO) until (DRIBBLE) is done interactively. When the second

(DRIBBLE) in test case #1 is executed, DRIBBLE will complain that output is not

currently being recorded. In test case #2, the output of (PRINT 'FOO) will be

captured, but the form (PRINT 'BAR) will never be executed because (DRIBBLE)

does not return normally (it throws).


Cost to implementors:


None. This is just a clarification.


Cost to users:


Anyone who currently uses DRIBBLE in code that is believed to be portable is

kidding himself. If such code works in some implementations, it can continue to

work.


Benefits:


Users will be aware that they cannot rely on DRIBBLE in portable code.


Aesthetics:


No apparent effect.


Discussion:


DRIBBLE, like other environment features, is a standard interface to a

non-standard feature. As such, there is some question as to its place in the

ANSI standard. However, if DRIBBLE remains in the standard, its behavior is made

explicitly vague by this proposal.