Record printing (3 of 3)

[John Cowan <cowan@xxxxxxxx>]

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

        #<queue:0xDEADBEEF>

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
sensible

        #<queue:1,7,111,10>

instead.

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 <cowan@xxxxxxxx>             http://www.ccil.org/~cowan
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