DATA-IO
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.
*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 *PRINT-ARRAY* T *PRINT-BASE* 10 *PRINT-CASE* :UPCASE *PRINT-CIRCLE* NIL *PRINT-ESCAPE* T *PRINT-GENSYM* T *PRINT-LENGTH* NIL *PRINT-LEVEL* NIL *PRINT-PRETTY* NIL *PRINT-RADIX* NIL *PRINT-READABLY* T *READ-BASE* 10 *READ-DEFAULT-FLOAT-FORMAT* SINGLE-FLOAT *READ-EVAL* T *READ-SUPPRESS* NIL *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.
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:
PRINT-NOT-READABLE [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 PRINT-NOT-READABLE-OBJECT.
;; Example #1: Reliable Write-Read
(WITH-OPEN-FILE (FILE pathname :DIRECTION :OUTPUT) (WITH-STANDARD-IO-SYNTAX (PRINT DATA FILE)))
; ... Later, in another Lisp:
(WITH-OPEN-FILE (FILE pathname :DIRECTION :INPUT) (WITH-STANDARD-IO-SYNTAX (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)))
(PRINT MY-AIRPLANE) #<Airplane NW0773 36000123135> ;in Implementation A ;or #<FAA:AIRPLANE NW0773 17> ;in Implementation B
*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.
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.
*PRINT-READABLY* or using PRINT-UNREADABLE-OBJECT in their PRINT-OBJECT methods.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.