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.