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Issue DATA-IO Writeup

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.

Issue: DATA-IO

References: CLtL pp.360, 370, 382


Category: ADDITION

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)

Problem description:

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-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]

&body body

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.


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




; ... Later, in another Lisp:





;; Note that in this example, the precise form of the output

;; is really implementation-dependent.





#<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

implementation-dependent code.

5. Defining a specific condition type associated with *PRINT-READABLY*

makes it possible for programs to handle the condition and recognize

the offending object.

Current practice:

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.


and takes slightly different arguments. In PCL, PRINT-UNREADABLE-OBJECT


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.

Performance impact:

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

implementation-specific programming.


Mildly improved.


Pitman and Moon support this proposal.

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