Pretty-printing records

[Per Bothner]

On 1/1/19 12:41 PM, Duncan Mak wrote:
> The new output
> 
>      #|kawa:4|# path2
>      #<<pathname> device: unspecific directory: (absolute usr) name:
> morris type: false version: unspecific>
> 
> Like you wrote, it's not exactly pretty-printing, but at least the
> fields of the record are visible in the REPL now.
> 
> I know that in Scheme48, they have a DEFINE-RECORD-DISCLOSER form, it
> looks like this:
> http://s48.org/1.9.2/manual/manual-Z-H-6.html#node_sec_5.9
> 
> In MIT Scheme, their DEFINE-STRUCTURE form is more complicated, and
> includes a PRINT-PROCEDURE option.
> 
> SRFI 9 records are very close to S48 records, maybe
> DEFINE-RECORD-DISCLOSER is the way to go?

That only helps somewhat. It assumes you want something pretty-printed as if
it were a list, except using curly braces.   One issue is if you want
keywords - you'd want each keyword-value pair to be a logical group
(preferably on the same line), without extra parentheses.

There is also the issue of how to manage and look-up the discloser function.

The MIT-Scheme define-structure with print-procedure is more flexible, as
the procedure takes a port, which means you can do flexible pretty-printing.
However, it seems more complicated to use - and to implement.

The simplest for both user and implementor is to have the Record class
implement Printable.  That would give a nice default output.  Having
the ability to customize the output is nice, but gets into more complicated
design issues.

Kawa does have a framework for type-specific printing customization:
See the standardFormat variable in DisplayFormat.java.  However, there
is no simple hook to use it from Scheme without knowing a lot about how it works.
It would be nice to provide such a Scheme wrapper.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/