CLHS: Issue DRIBBLE-TECHNIQUE Writeup

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## 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.

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