Record printing (3 of 3) John Cowan 11 Sep 2009 07:25 UTC

I'd like to propose the addition of record printing to SRFI 99.

There is no way in SRFI 99 to specify how records should be printed.
Consequently, after importing a (hypothetical) queue package and writing:

        > (define my-queue (queue 1 7 111 10))

the result of

        > my-queue

is some opaque default of the Scheme implementation such as


By allowing the implementor of the queue package to write a printer
procedure and associate it with the record type, we can get the more



The obvious approach is to check the fourth argument of
define-record-type, which currently must be a list specifying the first
slot-name and its accessor and optional mutator.  Under this extension,
if it is an identifier, it can be interpreted as the name of the record
printer procedure for this type.  Note that unlike other identifiers,
this is the name of a procedure to be implemented by define-record-type,
but the name of a procedure implemented outside define-record-type.
(This can only be done if the hygiene-breaking interpretation of a bare
identifier as a slot name is dropped from SRFI 99, as proposed in my
posting 1 of 3.)

The resulting syntax of define-record-type becomes:

(define-record-type typename (constructor args ...) predicate [printer]
        (field-tag accessor [mutator]) ...)

In the procedural library, the record printer is an additional argument
to make-rtd (before the optional magic symbols).  If the argument is #f,
a system-defined printer is used.  (I have no objection if this argument
is made optional or marked by a keyword.)

The inspection library would add a rtd-printer procedure to retrieve
the record printer.

Printers are procedures invoked by write and display with two arguments,
the record to be printed and the port to print on.

John Cowan <>   
Today an interactive brochure website, tomorrow a global content
management system that leverages collective synergy to drive "outside of
the box" thinking and formulate key objectives into a win-win game plan
with a quality-driven approach that focuses on empowering key players
to drive-up their core competencies and increase expectations with an
all-around initiative to drive up the bottom-line. --Alex Papadimoulis