CLHS: Issue FORMAT-E-EXPONENT-SIGN Writeup

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## Issue FORMAT-E-EXPONENT-SIGN Writeup

    Status:	Passed, Jan 89 X3J13
    Forum:         Cleanup
    Issue:         FORMAT-E-EXPONENT-SIGN

    References:    CLtL pp. 366, 393

    Category:      CLARIFICATION

    Edit history:  Bob Cassels, 13 Sep 88
    			Masinter,  2-Oct-88 (change issue name)

    Problem description:

        The result of (format nil "~E" 1.0) is specified in a contradictory way.
        The ambiguity is whether a plus sign should be printed in front of
        the exponent.

        The top of page 393 says, "Next, either a plus or a minus sign is
        printed, followed by e digits ... [decimal exponent]"

        Later on page 393 we see, "If all of w, d, and e are omitted, then the
        effect is ... [like prin1].

        Page 366 [presumably where prin1 is defined] doesn't explicitly say that
        the plus sign is omitted from the exponent, but all the examples (and
        usual practice) indicate that.

        So the posssibilities are:

    	A. "1.0e+0"
    	B. "1.0e0"

        The first reference implies that A is correct, the third reference
        implies that B is correct.  The second reference implies that A and B
        are the same.

    Proposal (FORMAT-E-EXPONENT-SIGN:FORCE-SIGN):

        Specify that ~E always prints a plus or minus sign in front of the
        exponent.

     This would cause the language on page 393 of CLtL to to change:

        "If all of w, d, and e are omitted, then the effect is to print the
        value using ordinary free-format exponential-notation output; PRIN1 uses
        a similar format for any non-zero number whose magnitude is less than
        10**-3 or greater than or equal to 10**7.  The only difference is that
        the ~E directive always prints a plus or minus sign in front of the
        exponent, while PRIN1 omits the plus sign if the exponent is
        non-negative."

    Test Case:

        (format nil "~E" 1.0) => "1.0e+0"

    Rationale:

        This proposal makes ~E self-consistent.  That is more important than
        making ~E consistent with PRIN1.

    Current practice:

        Symbolics Common Lisp, Ibuki Lisp, and VAX Lisp all print the plus
        sign as in the test case above.  Apollo DOMAIN Common Lisp (version
        2.10) and Xerox Common Lisp produce "1.0", which is wrong because
       it includes no exponent at all.

    Adoption Cost:

        Minimal changes to one printing routine for non-conforming
        implementations.  (No change to the three implementations mentioned
        above.)

    Cost of non-adoption:

        Minor confusion and possible incompatibility among implementations.

    Benefits:

        Less confusion, more compatibility.

    Conversion Cost:

        Minimal.  It is doubtful that any user programs depend on this
        obscure distinction.

    Esthetics:

        A matter of opinion.

    Discussion:

        Fortran ~E format requires a sign before the exponent, since the exponent
        mark character may be dropped.  Since Common Lisp ~E always prints
        the exponent marker, the exponent sign may be dropped in the case
        that it would be a plus sign.

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