Issue FORMAT-OP-C Writeup

Issue:        FORMAT-OP-C

References:   WRITE-CHAR (p384), ~C (p389)

Category:     CHANGE/CLARIFICATION

Edit History: 23-Feb-87, Version 1 by Pitman

              29-Apr-87, Versions 2,3 by Pitman

               5-Jun-87, Version 4 by Masinter (copy-editing)

              11-Jun-87, Version 5 release to X3J13

					(remove confusing paragraph)


Problem Description:


CLtL is not adequately specific about the function of the format

operation ~C. The description on p389 says that "~C prints the character

in an implementation-dependent abbreviated format. This format should be

culturally compatible with the host environment." This description is

not very useful in practice.


Presumably the authors intended the `cultural compatibility' part to

gloss issues like how the SAIL character set printed, but unfortunately

another completely reasonable (albeit unplanned) interpretation arose:

some implementations have (FORMAT NIL "~C" #\Space) => "Space" rather

than " ".


Since the behavior of ~A is also vague on characters (a separate

proposal will address this), the only way to safely output a literal

character is to WRITE-CHAR; currently, FORMAT does not suffice.


Proposal (FORMAT-OP-C:WRITE-CHAR):


Change the behavior of ~C to say that, when given a character with zero

bits, it will perform the same action as WRITE-CHAR. (This proposal

leaves the behavior of ~C with non-zero bits incompletely specified.)

For example, the description of the ~C format directive on p389 of CLTL

might read:


  ~C prints the character using WRITE-CHAR if it has zero bits.

  Characters with bits are not necessarily printed as WRITE-CHAR

  would do, but are displayed in an implementation-dependent

  abbreviated format that is culturally compatible with the host

  environment.


Test Case:


(EQUAL (FORMAT NIL "~C" #\Space) " ")


Rationale:


This was probably the intent of the Common Lisp designers. 


It makes things clear enough that programmers can know what to expect in

the normal case (standard characters with zero bits).


Users can use (FORMAT NIL "~:C" #\Space) to get "Space" if they want it.

It seems as if the implementations which return "Space" treat ~C and ~:C

equivalently or very similarly.


Current Practice:


Implementations are divided. Some implementations have

     (FORMAT NIL "~C" #\Space) => "Space".

Others have the same form return " ".


Adoption Cost:


Those implementations which did not already implement ~C as WRITE-CHAR

would require an incompatible change.


Benefits:


User code that uses ~C would have a chance of being portable. As things

stand, users who use ~C can't reliably port their code.


~C and ~:C would perform usefully distinct operations.


Conversion Cost:


Standard ``Query Replace'' technology for finding occurrences of "~C"

and changing them to "~:C" semi-automatically should suffice.


Aesthetics:


Making ~C do something well-defined will probably be perceived as a

simplification.


Discussion:


The cleanup committee generally supports this clarification.


The original version of this proposal (which tried to make WRITE-CHAR

and ~C identical in all cases) prompted the following comment:


"I believe the error in CLtL is that it was not stated explicitly that

the `implementation-dependent abbreviated format' applies only to

characters with non-zero char-bits. Thus instead of removing the

mumbling about cultural compatibility, I suggest simply adding a

sentence saying that ~C is the same as WRITE-CHAR for characters with

zero char-bits.  I don't think we want to require ~C and write-char to

do the same thing for characters with bits."