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(librep.info)Datums

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Datums
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   "Datums" are the mechanism by which `librep' allows Lisp programs to
define new data types, such that these types are completely distinct
from the built-in data types (i.e. they match none of the standard type
predicates).

   They also provide encapsulation, in that the data objects they
provide are completely opaque, unless a pre-defined value is known
(which was specified when the object was created, and is typically
known only by the object's creator).

 - Function: make-datum value key
     Create and return a new datum object. It has the value VALUE
     associated with it, and has type KEY.

 - Function: datum-ref arg key
     If ARG has type KEY, then return the value associated with it.
     Otherwise, an error is signalled.

 - Function: datum-set arg key value
     If ARG has type KEY, then set the value associated with it to be
     VALUE. Otherwise, an error is signalled.

 - Function: has-type-p arg key
     Return true if ARG has type KEY.

 - Function: define-datum-printer key printer
     Associate the function PRINTER with all datum objects of type KEY.
     When any such object is printed, PRINTER is applied to two
     arguments, the datum and the stream to which it should be printed
     (Note: Output Streams).