Go to the previous, next section.
MIT Scheme provides hooks for specifying that certain kinds of objects have special written representations. There are no restrictions on the written representations, but only a few kinds of objects may have custom representation specified for them, specifically: records (see section Records), vectors that have special tags in their zero-th elements (see section Vectors), and pairs that have special tags in their car fields (see section Lists). There is a different procedure for specifying the written representation of each of these types.
procedure+: set-record-type-unparser-method! record-type unparser-method
Changes the unparser method of the type represented by record-type to be unparser-method, and returns an unspecified value. Subsequently, when the unparser encounters a record of this type, it will invoke unparser-method to generate the written representation.
procedure+: unparser/set-tagged-vector-method! tag unparser-method
Changes the unparser method of the vector type represented by tag to be unparser-method, and returns an unspecified value. Subsequently, when the unparser encounters a vector with tag as its zero-th element, it will invoke unparser-method to generate the written representation.
procedure+: unparser/set-tagged-pair-method! tag unparser-method
Changes the unparser method of the pair type represented by tag to be unparser-method, and returns an unspecified value. Subsequently, when the unparser encounters a pair with tag in its car field, it will invoke unparser-method to generate the written representation.
An unparser method is a procedure that is invoked with two arguments: an unparser state and an object. An unparser method generates a written representation for the object, writing it to the output port specified by the unparser state. The value yielded by an unparser method is ignored. Note that an unparser state is not an output port, rather it is an object that contains an output port as one of its components. Application programs generally do not construct or examine unparser state objects, but just pass them along.
There are two ways to create an unparser method (which is then
registered by one of the above procedures). The first, and easiest, is
to use standard-unparser-method. The second is to define your
own method using the procedure with-current-unparser-state. We
encourage the use of the first method, as it results in a more uniform
appearance for objects. Many predefined datatypes, for example
procedures and environments, already have this appearance.
procedure+: standard-unparser-method name procedure
Returns a standard unparser method. Name may be any object, and
is used as the name of the type with which the unparser method is
associated; name is usually a symbol. Procedure must be
#f or a procedure of two arguments.
If procedure is #f, the returned method generates an
external representation of this form:
#[name hash]
Here name is the external representation of the argument
name, as generated by write,(21) and hash is the external
representation of an exact non-negative integer unique to the object
being printed (specifically, it is the result of calling hash on
the object). Subsequently, the expression
#@hash
is notation for the object.
If procedure is supplied, the returned method generates a slightly different external representation:
#[name hash output]
Here name and hash are as above, and output is the output generated by procedure. The representation is constructed in three stages:
"#[",
name, " ", and hash.
The following procedure is useful for writing more general kinds of unparser methods.
procedure+: with-current-unparser-state unparser-state procedure
This procedure calls procedure with one argument, the output port
from unparser-state. Additionally, it arranges for the remaining
components of unparser-state to be given to the printer when they
are needed. The procedure generates some output by writing to the
output port using the usual output operations, and the value yielded by
procedure is returned from with-current-unparser-state.
The port passed to procedure should only be used within the dynamic extent of procedure.
Go to the previous, next section.