This is a new issue. It arose from an investigation of features
that are plausibly needed but missing from draft ANSI Common Lisp.
This issue seems sufficiently simple and noncontroversial that
I would like to see it on the agenda for the June X3J13 meeting.
This issue has been amended based on last minute discussion. Clarify
that "readable" is defined in terms of "similar as constants" as
defined in issue CONSTANT-COMPILABLE-TYPES. This modifies point 1a and
adds new points 1d, 1e, and 1f. The interaction between *PRINT-READABLY*
and other printer control variables has been tightened; this modifies
point 1c and deletes the old points 1d and 1e.
References: CLtL pp.360, 370, 382
Related issues: CONSTANT-COMPILABLE-TYPES
Edit history: Version 1, 9-May-89, by Moon
Version 2, 10-May-89, by Moon
(clarify ambiguities, add PRINT-UNREADABLE-OBJECT)
Version 3, 18-May-89, by Moon (respond to KMP's comments)
Version 4, 21-May-89, by Moon (almost-final cleanup)
Version 5, 22-May-89, by Pitman (``never say never'')
Version 6, 23-May-89, by Moon (final cleanup)
Version 7, 18-Jun-89, by Moon (more fixes based on
discussion in the cleanup subcommittee)
Version 8, 23-Jun-89, by Moon (fixes based on discussion)
Storing data in textual form in files, as Lisp expressions, is common
practice but has some pitfalls. Files can be unreadable if #<...> syntax
is written by the printer, or if the reader syntax or package varies
between writing and reading. Files of data intended to be carried from
one Lisp implementation to another can fail to read correctly if
implementation-dependent syntax extensions get used when not intended.
CLtL p.370 recommends that unreadable objects be printed with #<...>
syntax including implementation-dependent information. Now that users
can write their own PRINT-OBJECT methods, a way is needed for such
methods to print this syntax without any implementation-dependent coding.
1a. Add a new variable *PRINT-READABLY*. Add a corresponding keyword
argument :READABLY to WRITE. The default value of *PRINT-READABLY* is
NIL. If *PRINT-READABLY* is true, then printing any object produces a
printed representation that the reader will accept. The reader will
produce an object that is "similar as a constant" to the object that
was printed. The term "similar as a constant" is defined in the
already accepted compiler issue CONSTANT-COMPILABLE-TYPES:SPECIFY.
If *PRINT-READABLY* is true and printing a readable printed
representation is not possible, the printer signals an error of type
PRINT-NOT-READABLE rather than using an unreadable syntax such as #<...>.
The printed representation produced when *PRINT-READABLY* is true might
or might not be the same as the printed representation produced when
*PRINT-READABLY* is false.
1b. All methods for PRINT-OBJECT must obey *PRINT-READABLY*. This
includes both user-defined methods and implementation-defined methods.
1c. If *PRINT-READABLY* is true and another printer control variable
(*PRINT-LENGTH*, *PRINT-LEVEL*, *PRINT-ESCAPE*, *PRINT-GENSYM*,
*PRINT-ARRAY*, or an implementation-defined printer control variable)
would cause the requirements of point 1a to be violated, that other
printer control variable is ignored.
1d. The printing of interned symbols is not affected by *PRINT-READABLY*,
regardless of the outcome of issue COMPILE-FILE-SYMBOL-HANDLING
(referenced by issue CONSTANT-COMPILABLE-TYPES).
1e. Note that the "similar as a constant" rule for readable printing
implies that #A or #( syntax cannot be used for arrays of element-type
other than T. An implementation will have to use another syntax or
signal a PRINT-NOT-READABLE error. A PRINT-NOT-READABLE error will not
be signalled for strings or bit-vectors.
1f. Readable printing of structures and standard-objects is controlled
by their PRINT-OBJECT method, not by their MAKE-LOAD-FORM method.
"Similarity as a constant" for these objects is application dependent
and hence is defined to be whatever these methods do.
2. Add a new reader control variable, *READ-EVAL*, whose default value is
T. If *READ-EVAL* is NIL, the #. reader macro signals an error. If
*READ-EVAL* is false and *PRINT-READABLY* is true, any PRINT-OBJECT
method that would output a #. reader macro either outputs something
different or signals an error of type PRINT-NOT-READABLE.
3. Add a new macro:
WITH-STANDARD-IO-SYNTAX &body body [Macro]
Within the dynamic extent of <body>, all reader/printer control
variables, including any implementation-defined ones not specified by
Common Lisp, are bound to values that produce standard read/print
behavior. The values for Common Lisp specified variables are:
*PACKAGE* The USER package
*READTABLE* The standard readtable
The values returned by WITH-STANDARD-IO-SYNTAX are the values
of the last body form, or NIL if there are no body forms.
4. Add a new macro:
PRINT-UNREADABLE-OBJECT (object stream &key type identity) [Macro]
Output a printed representation of <object> on <stream>, beginning with
"#<" and ending with ">". Everything output to <stream> by the <body>
forms is enclosed in the angle brackets. If :type is true, the body
output is preceded by a brief description of the object's type and a
space character. If :identity is true, the body output is followed by
a space character and a representation of the object's identity,
typically a storage address.
If *PRINT-READABLY* is true, PRINT-UNREADABLE-OBJECT signals an error
of type PRINT-NOT-READABLE without printing anything.
The <object>, <stream>, :type, and :identity arguments are all evaluated
normally. :type and :identity default to false. It is valid to omit
the <body> forms. If :type and :identity are both true and there are no
<body> forms, only one space character separates the type and the identity.
The value returned by PRINT-UNREADABLE-OBJECT is NIL.
5. Add a new condition type:
Errors which occur during output while *PRINT-READABLY* is true, as a
result of attempting to output a printed representation that cannot be
read back, should inherit from this type. This is a subtype of ERROR.
The init keyword :OBJECT is supported to initialize the slot containing
the object being printed, which can be accessed using
;; Example #1: Reliable Write-Read
(WITH-OPEN-FILE (FILE pathname :DIRECTION :OUTPUT)
(PRINT DATA FILE)))
; ... Later, in another Lisp:
(WITH-OPEN-FILE (FILE pathname :DIRECTION :INPUT)
(SETQ DATA (READ FILE))))
;; Example #2: Use of PRINT-UNREADABLE-OBJECT
;; Note that in this example, the precise form of the output
;; is really implementation-dependent.
(DEFMETHOD PRINT-OBJECT ((OBJ AIRPLANE) STREAM)
(PRINT-UNREADABLE-OBJECT (OBJ STREAM :TYPE T :IDENTITY T)
(PRINC (TAIL-NUMBER OBJ) STREAM)))
#<Airplane NW0773 36000123135> ;in Implementation A
#<FAA:AIRPLANE NW0773 17> ;in Implementation B
1. *PRINT-READABLY* is important so that errors involving data with no
readable printed representation are detected when writing the file, not
later on when the file is read.
*PRINT-READABLY* is different from *PRINT-ESCAPE* because output printed
with escapes only has to be generally recognizable by humans, whereas
output printed readably has to be reliably recognizable by computers.
2. Binding *READ-EVAL* to NIL is useful when reading data that came from
an untrusted source, such as a network or a user-supplied data file, to
prevent the #. reader macro from being exploited as a "Trojan horse" to
cause arbitrary forms to be evaluated.
3. Providing the WITH-STANDARD-IO-SYNTAX macro to bind all the variables,
instead of using LET and explicit bindings of the existing variables,
ensures that nothing is overlooked and avoids problems with
implementation-defined reader/printer control variables.
If the user wishes to use a non-standard value for some variable, such as
*PACKAGE* or *READ-EVAL*, it can be bound by LET inside the body of
WITH-STANDARD-IO-SYNTAX. Similarly, if the user dislikes the somewhat
arbitrary choices of values for *PRINT-CIRCLE* and *PRINT-PRETTY*, they
can be bound to the preferred values inside the body.
4. PRINT-UNREADABLE-OBJECT allows user-written PRINT-OBEJCT methods to
adhere to implementation-specific style without requiring users to write
5. Defining a specific condition type associated with *PRINT-READABLY*
makes it possible for programs to handle the condition and recognize
the offending object.
Symbolics Genera has had these features for many years, except with
different names. For instance, WITH-STANDARD-IO-SYNTAX is named
WITH-STANDARD-IO-ENVIRONMENT and binds *PACKAGE* to a non-standard
package. The proposed new names are better than the Genera names.
Genera's WITH-STANDARD-IO-ENVIRONMENT also disables #., to prevent trojan
horses, since #. could evaluate an arbitrary form. This is particularly
important for network protocols. WITH-STANDARD-IO-SYNTAX does not bind
*READ-EVAL* to NIL, because that would prevent using #. in the printer
for common datatypes, which is current practice in some implementations
for printing PATHNAMEs or RANDOM-STATEs.
In Genera, PRINT-UNREADABLE-OBJECT is called SYS:PRINTING-RANDOM-OBJECT
and takes slightly different arguments. In PCL, PRINT-UNREADABLE-OBJECT
is called PCL:PRINTING-RANDOM-THING.
Cost to Implementors:
Very small, these features are all easy to add. If #. is output by any
system-supplied print methods, they might want to invent a different
syntax, however that is not required by this proposal.
Cost to Users:
None if they don't use the feature. Otherwise just the cost of
supporting *PRINT-READABLY* or using PRINT-UNREADABLE-OBJECT in their
Cost of non-adoption:
There will be no reliable, standard way to write data into a file.
Negligible. Entering WRITE may be slightly slower since there is
one more keyword argument to parse and one more special variable
to bind before calling PRINT-OBJECT.
Data can be written into files reliably without resorting to
Pitman and Moon support this proposal.