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Message from discussion How do you print readably?
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Richard M Kreuter  
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 More options Mar 7 2007, 1:03 pm
Newsgroups: comp.lang.lisp
From: Richard M Kreuter <kreu...@progn.net>
Date: Wed, 07 Mar 2007 13:03:26 -0500
Local: Wed, Mar 7 2007 1:03 pm
Subject: Re: How do you print readably?
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Tim Bradshaw <t...@tfeb.org> writes:
> On 2007-03-06 14:05:59 +0000, Richard M Kreuter <kreu...@progn.net> said:

>> If the PRINT emits output with a custom read macro, then the READ will
>> error, since WITH-STANDARD-IO-SYNTAX uses the standard readtable.

> Yes. If you want a special readtable in place but otherwise want to be
> assured of standard syntax you need to put stuff inside
> WITH-STANDARD-IO-SYNTAX to make that be the case.

Well sure, you can use WITH-STANDARD-IO-SYNTAX as a base on which to
build custom I/O syntaxes.  

I was looking that the examples in the WITH-STANDARD-IO-SYNTAX
dictionary entry as constraints on what PRINT-OBJECT methods can do,
since the examples won't work in general if PRINT-OBJECT methods can
emit arbitrary output,

Now that I look at it, the dictionary entry for *PRINT-READABLY* is
clear enough on its own:

| If ‘*print-readably*’ is true, some special rules for printing
| objects go into effect.  Specifically, printing any object O₁
| produces a printed representation that, when seen by the Lisp reader
| while the standard readtable is in effect, will produce an object O₂
| that is similar to O₁.

So a PRINT-OBJECT method /must not/ emit anything unreadable in the
standard syntax when *PRINT-READABLY* is true.  Provided that all
methods PRINT-OBJECT conform to this protocol WITH-STANDARD-IO-SYNTAX
turns out to be reliable, after all.

--
RmK


 
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