print-object (original) (raw)

ANSI Common Lisp 22 Printer 22.4 Dictionary of Printer

Syntax:

print-object object stream object

Method Signatures:

print-object (object standard-object) stream

print-object (object structure-object) stream

Arguments and Values:

object - an object.

stream - a stream.

Description:

The generic function print-object writes the printed representation of _object_to stream. The function print-object is called by the Lisp printer; it should not be called by the user.

Each implementation is required to provide a method on the class standard-object and on the class structure-object. In addition, each implementation must provide methods on enough other classes_so as to ensure that there is always an applicable method. Implementations are free to add methods for other classes. Users may write methods for print-object for their own_classes if they do not wish to inherit an implementation-dependent method.

The method on the class structure-object prints the object in the default #S notation; see Section 22.1.3.12 Printing Structures.

Methods on print-object are responsible for implementing their part of the semantics of the printer control variables, as follows:

• *print-readably*
  All methods for print-object must obey *print-readably*. This includes both user-defined methods and implementation-defined methods. Readable printing of structures and standard objectsis controlled by their print-object method, not by their make-load-form method.Similarity for these objects is application dependent and hence is defined to be whatever these methods do; see Section 3.2.4.2 Similarity of Literal Objects.
• *print-escape*
  Each method must implement *print-escape*.
• *print-pretty*
  The method may wish to perform specialized line breaking or other output conditional on the value of *print-pretty*. For further information, see (for example) the macro pprint-fill. See also Section 22.2.1.4 Pretty Print Dispatch Tables and Section 22.2.2 Examples of using the Pretty Printer.
• *print-length*
  Methods that produce output of indefinite length must obey*print-length*. For further information, see (for example) the macros pprint-logical-blockand pprint-pop. See also Section 22.2.1.4 Pretty Print Dispatch Tables and Section 22.2.2 Examples of using the Pretty Printer.
• *print-level*
  The printer takes care of *print-level* automatically, provided that each method handles exactly one level of structure and calls write (or an equivalent function) recursively if there are more structural levels. The printer's decision of whether an_object_ has components (and therefore should not be printed when the printing depth is not less than *print-level*) is_implementation-dependent_. In some implementations its print-object method is not called; in others the method is called, and the determination that the object has components is based on what it tries to write to the stream.
• *print-circle*
  When the value of *print-circle* is true, a user-defined print-object _method_can print objects to the supplied _stream_using write,prin1,princ, or formatand expect circularities to be detected and printed using the #n# syntax. If a user-definedprint-object methodprints to a stream other than the one that was supplied, then circularity detection starts over for thatstream. See *print-circle*.
• *print-base*,*print-radix*,*print-case*, *print-gensym*, and *print-array*
  These printer control variables apply to specific types of _objects_and are handled by the methods for those objects.

If these rules are not obeyed, the results are undefined.

In general, the printer and the print-object methods should not rebind the print control variables as they operate recursively through the structure, but this is implementation-dependent.

In some implementations the stream argument passed to a print-object method is not the original stream, but is an intermediate stream that implements part of the printer.methods should therefore not depend on the identity of this stream.

See Also:

pprint-fill,pprint-logical-block,pprint-pop,write,*print-readably*,*print-escape*,*print-pretty*,*print-length*, Section 22.1.3 Default Print-Object Methods, Section 22.1.3.12 Printing Structures, Section 22.2.1.4 Pretty Print Dispatch Tables, Section 22.2.2 Examples of using the Pretty Printer

Allegro CL Implementation Details:

None.