I attempted to rewrite this as a change-to-language instead of
change-to-book, and came across some problems. I don't think that the test
case is accurate; there is no specification about the possible interaction
of *print-pretty* and *print-escape*; our pretty printer, for example,
creates significantly different output *print-escape* is NIL, and so the
two pairs are not identical except for the appearance of string quotes.
I think the intent of the proposal is to force list structures to be
printed "pretty", but we've not really defined what "pretty" is ever.
Unless that's specified, the only case over which we have control is in the
interaction with user written print functions for structures.
!
Issue: FORMAT-PRETTY-PRINT
References: FORMAT (pp. 385-388), PRIN1 (p. 83), PRINC (p. 83),
WRITE (p. 382), *PRINT-PRETTY* (p. 371)
Category: CLARIFICATION
Edit history: Version 1 by Pierson 3/4/88
Version 2 by Pierson 5/24/88 (fix name typo)
Version 3 by Pierson 6/10/88 incorporate comments
Version 4 by Pierson 6/10/88 comments from Van Roggen
Version 5 by Masinter 2-Oct-88
Problem description:
The FORMAT operators, ~A and ~S are defined to print their argument as
PRINC and PRIN1 respectively. The text does not say whether or not
these FORMAT operators must obey the *PRINT-PRETTY* flag.
Proposal (FORMAT-PRETTY-PRINT:YES):
Specify that FORMAT does rebind any of the printer control
variables (*PRINT-...) except as follows:
~A
Binds *PRINT-ESCAPE* to NIL.
~S
Binds *PRINT-ESCAPE* to T.
~D
Binds *PRINT-ESCAPE* to NIL, *PRINT-RADIX* to NIL, and
*PRINT-BASE* to 10.
~B
Binds *PRINT-ESCAPE* to NIL, *PRINT-RADIX* to NIL, and
*PRINT-BASE* to 2.
~O
Binds *PRINT-ESCAPE* to NIL, *PRINT-RADIX* to NIL, and
*PRINT-BASE* to 8.
~X
Binds *PRINT-ESCAPE* to NIL, *PRINT-RADIX* to NIL, and
*PRINT-BASE* to 16.
~R
Binds *PRINT-ESCAPE* to NIL, *PRINT-RADIX* to NIL, and
*PRINT-BASE* to the value of the first argument iff a first
argument is specified.
~@C
Binds *PRINT-ESCAPE* to T.
~F,~G,~E,~$
Binds *PRINT-ESCAPE* to NIL.
Test Cases/Examples:
(LET ((TEST '#'(LAMBDA (X)
(LET ((*PRINT-PRETTY* T))
(PRINT X)
(FORMAT T "~%~S " X)
(TERPRI) (PRINC X) (PRINC " ")
(FORMAT T "~%~A " X)))))
(FUNCALL (EVAL TEST) TEST))
This should print four copies of the above lambda expression. The
first pair should appear identical and the second pair should appear
identical. The only difference between the two pairs should be the
absence of string quotes in the second pair.
Rationale:
FORMAT should interact with the printer control variables in a
predictable way. This proposal is one of the two simplest possible
definitions and provides the most flexibility to the user.
A major advantage of this proposal is that the following two
expressions are guaranteed to have exactly the same effect:
(FORMAT stream "~S" object)
(PRIN1 object stream)
Thus use or non-use of FORMAT becomes a purely stylistic matter.
Current practice:
Ibuki Lisp and the TI Explorer obey the binding of *PRINT-PRETTY*.
Lucid Common Lisp always applies ~A and ~S with *PRINT-PRETTY* bound
to NIL.
Cost to Implementors:
While changing FORMAT to not bind *PRINT-foo* variables is trivial,
there are some painful implications. How does a pretty-printing ~A
interact with MINCOL, etc? How much of the user interface depends on
FORMAT in ways that might be uglified by pretty printing?
Cost to Users:
Truely portable code shouldn't be affected because existing
implementations are inconsistent. Despite this, there are probably a
number of user programs in non-complying which will have to change
whichever way this is decided.
Cost of non-Adoption:
The interaction of FORMAT and the printer control variables (especially
*PRINT-PRETTY*) will remain undefined. This will continue to make
portable Common Lisp programming harder than it need be.
Benefits:
Life will be easier for users in two ways: (1) one more portability
problem will be removed, and (2) users will be more likely to be able
to persuade environmental features like debuggers to print in their
preferred way.
Aesthetics:
The interaction between two related parts of output will be clarified
in a simple way.
Discussion:
It is important to specify exactly what of Common Lisp's special
variables get rebound by other functions and macros in Common Lisp.
This cleanup issue addresses the interaction of FORMAT and the
*PRINT- variables. There may be other similar interactions in
Common Lisp that need clarification.
Otherwise, code that depends on FORMATs action in one implementation
will not port to others that do not have the same behavior.
CLtL might change as follows:
Add a header "Printer Control Variables" before the description of
*PRINT-ESCAPE* on page 370.
Add the following paragraph to page 386 just before the paragraph
starting with "It is an error":
"The FORMAT function by itself never binds any of the printer
control variables. Specific FORMAT directives bind exactly the
printer control variables specified in their description. While
implementations may specify the binding of new, implementation
specific printer control variables for each FORMAT directive, they
may neither bind any standard printer control variables not
specified in description of a FORMAT directive nor fail to bind
any standard printer control variables as specified in the
description."