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Re: pretty printer
- To: firstname.lastname@example.org
- Subject: Re: pretty printer
- From: haible (Bruno Haible)
- Date: Thu, 27 Jan 94 16:43:06 +0100
Joachim Schrod <email@example.com> says:
> CLtL2, Chapter 27, describes a prettyprinting facility.
CLtL2, Chapter 27, describes the XP "pretty" printer.
> How about XP, is it available by anonymous ftp?
> Or is one in CMUCL that might be used?
XP is used in CMU CL and WCL.
> It may not be plug&play, we're not afraid of some programming work.
But decide first what you want. I will try a comparison between CLISP's
pretty printer and XP as specified in CLtL2 and implemented in xp.lisp.
XP is extensible. A programmer/user can specify how his DO, LET,
MULTIPLE-VALUE-BIND etc. forms are printed.
If a pretty-print layout becomes mandatory for every macro, then writing
macros becomes combersome.
It looks ugly when a list containing data which happens to look like
(LET ...) is printed as if it were a Lisp form. There is no way by which
the system could decide whether some object is data or program.
CLISP indents by 1 for every nesting level. XP indents by 2, which makes
up for better readability. (This could be easily fixed in io.d.)
Closing parentheses are output just below their corresponding opening
parenthesis, if *PRINT-RPARS* is T. This makes it much easier for a human
reader to find out where a certain nesting level ends.
CONTRA XP, PRO CLISP:
Heavily nested lists and structures tend to reach the right margin very
quickly in XP. This fact is even recognized by the existence of the variable
*PRINT-MISER-WIDTH*. And it is not handled satisfactorily in XP: it
produces lots of lines with much space on the left and only one token
at the right margin, often exceeding the line length.
While the *PRINT-CIRCLE* mechanism used in XP uses a large hash table
which holds every subobject of the object to be printed, CLISP's
*PRINT-CIRCLE* handling builds up a (usually much smaller) hash table
which holds only those subobjects which occur more than once.
(Detection of these subobjects is done by a low-level subroutine.)
When *PRINT-LENGTH* or *PRINT-LEVEL* is non-NIL, CLISP may print #n=
with no #n# references: #1=(A ...)
XP does this better because its decision whether to print #n= comes