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  ANSI Common Lisp   22 Printer   22.4 Dictionary of Printer

22.4.13 set-pprint-dispatch Function

Syntax:
set-pprint-dispatch type-specifier function &optional priority table    nil

Arguments and Values:
type-specifier - a type specifier.

function - a function, a function name, or nil.

priority - a real. The default is 0.

table - a pprint dispatch table. The default is the value of *print-pprint-dispatch*.

Description:
Installs an entry into the pprint dispatch table which is table.

Type-specifier is the key of the entry. The first action of set-pprint-dispatch is to remove any pre-existing entry associated with type-specifier. This guarantees that there will never be two entries associated with the same type specifier in a given pprint dispatch table. Equality of type specifiers is tested by equal.

Two values are associated with each type specifier in a pprint dispatch table: a function and a priority. The function must accept two arguments: the stream to which output is sent and the object to be printed. The function should pretty print the object to the stream. The function can assume that object satisfies the type given by type-specifier. The function must obey *print-readably*. Any values returned by the function are ignored.

Priority is a priority to resolve conflicts when an object matches more than one entry.

It is permissible for function to be nil. In this situation, there will be no type-specifier entry in table after set-pprint-dispatch returns.

Exceptional Situations:
An error is signaled if priority is not a real.

Notes:
Since pprint dispatch tables are often used to control the pretty printing of Lisp code, it is common for the type-specifier to be an expression of the form

 (cons car-type cdr-type)

This signifies that the corresponding object must be a cons cell whose car matches the type specifier car-type and whose cdr matches the type specifier cdr-type. The cdr-type can be omitted in which case it defaults to t.

Allegro CL Implementation Details:
None.

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