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6.7.8 Records

A record type is a first class object representing a user-defined data type. A record is an instance of a record type.

Scheme Procedure: record? obj

Return #t if obj is a record of any type and #f otherwise.

Note that record? may be true of any Scheme value; there is no promise that records are disjoint with other Scheme types.

Scheme Procedure: make-record-type type-name field-names [print]

Create and return a new record-type descriptor.

type-name is a string naming the type. Currently it’s only used in the printed representation of records, and in diagnostics. field-names is a list of symbols naming the fields of a record of the type. Duplicates are not allowed among these symbols.

(make-record-type "employee" '(name age salary))

The optional print argument is a function used by display, write, etc, for printing a record of the new type. It’s called as (print record port) and should look at record and write to port.

Scheme Procedure: record-constructor rtd [field-names]

Return a procedure for constructing new members of the type represented by rtd. The returned procedure accepts exactly as many arguments as there are symbols in the given list, field-names; these are used, in order, as the initial values of those fields in a new record, which is returned by the constructor procedure. The values of any fields not named in that list are unspecified. The field-names argument defaults to the list of field names in the call to make-record-type that created the type represented by rtd; if the field-names argument is provided, it is an error if it contains any duplicates or any symbols not in the default list.

Scheme Procedure: record-predicate rtd

Return a procedure for testing membership in the type represented by rtd. The returned procedure accepts exactly one argument and returns a true value if the argument is a member of the indicated record type; it returns a false value otherwise.

Scheme Procedure: record-accessor rtd field-name

Return a procedure for reading the value of a particular field of a member of the type represented by rtd. The returned procedure accepts exactly one argument which must be a record of the appropriate type; it returns the current value of the field named by the symbol field-name in that record. The symbol field-name must be a member of the list of field-names in the call to make-record-type that created the type represented by rtd.

Scheme Procedure: record-modifier rtd field-name

Return a procedure for writing the value of a particular field of a member of the type represented by rtd. The returned procedure accepts exactly two arguments: first, a record of the appropriate type, and second, an arbitrary Scheme value; it modifies the field named by the symbol field-name in that record to contain the given value. The returned value of the modifier procedure is unspecified. The symbol field-name must be a member of the list of field-names in the call to make-record-type that created the type represented by rtd.

Scheme Procedure: record-type-descriptor record

Return a record-type descriptor representing the type of the given record. That is, for example, if the returned descriptor were passed to record-predicate, the resulting predicate would return a true value when passed the given record. Note that it is not necessarily the case that the returned descriptor is the one that was passed to record-constructor in the call that created the constructor procedure that created the given record.

Scheme Procedure: record-type-name rtd

Return the type-name associated with the type represented by rtd. The returned value is eqv? to the type-name argument given in the call to make-record-type that created the type represented by rtd.

Scheme Procedure: record-type-fields rtd

Return a list of the symbols naming the fields in members of the type represented by rtd. The returned value is equal? to the field-names argument given in the call to make-record-type that created the type represented by rtd.


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