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Standard Generic Function PRINT-OBJECT


  • print-object object streamobject

Method Signatures

Arguments and Values


The print-object generic function 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 standard-object class and on the structure-object class. 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 structure-object class prints the object in the default #S notation; see section {\secref\PrintingStructures}.

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

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 objects is 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 {\secref\Similarity}.

Each method must implement *print-escape*.

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 \secref\PPrintDispatchTables\ and \secref\PrettyPrinterExamples.

Methods that produce output of indefinite length must obey *print-length*.

For further information, see (for example) the macros pprint-logical-block and pprint-pop. See also \secref\PPrintDispatchTables\ and \secref\PrettyPrinterExamples.

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.

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 format and expect circularities to be detected and printed using the #n# syntax. If a user-defined print-object method prints to a stream other than the one that was supplied, then circularity detection starts over for that stream. See *print-circle*.

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.



Affected By


Exceptional Situations


See Also