Keyword syntax

* Keyword syntax
@ 2004-10-04 18:27 Marius Vollmer
  2004-10-04 18:44 ` Roland Orre
  2004-10-04 18:51 ` Paul Jarc
  0 siblings, 2 replies; 4+ messages in thread
From: Marius Vollmer @ 2004-10-04 18:27 UTC (permalink / raw)

Hi,

I'm considering to change keywords are read or written by Guile, to
make the two operations more consistent with each other.

Right now, keywords are read as a single token and a part of that
token is made the keyword name, which is a symbol.

For example, a token can be "#:foo" (without the double quotes) and
the name of the keyword is the symbol named "foo".  Also, "#:12" gives
a keyword with a symbol as the name that itself has the name "12".
Now, "12" is not a valid way to write this symbol in Scheme, and there
for Guile writes the keyword #:12 in the following funny way:

    guile> #:12
    #:#{12}#

The "#{12}#" is the syntax to write a symbol with an arbitrary name.

However, reading this printed form gives a different keyword.

    guile> #:#{12}#
    #:#{\#{12}\#}#

I can see two ways to fix this: 1) changing the way keywords are read and
2) changing the way they are printed.

As to 1), the simplest change would be to just do the equivalent of
(symbol->keyword (read port)).  Thus, the name of a keyword is read as
a general Scheme datum, and is then validated to be a symbol and
converted to a keyword.

For 2), we would have to print keyword without using the symbol
printer and would have to explicitely deal with 'weird' keyword names
ourselves.

I have implemented 1) in CVS head as an experiment.  Below is a sample
session, showing the differences.

I like 1) more than 2) (by a small margin) although it is a more
radical change since keywords right now are defined to have symbols as
names anyway and because it is equivalent to doing

    (read-hash-extend #\: 
      (lambda (chr port)
        (symbol->keyword (read port))))

which I like because it is very straightforward.

    Previously

        guile> #:12
        #:#{12}#
        guile> #:#{12}#
        #:#{\#{12}\#}#
        guile> #:(a b c)
        #:#{}#
        ERROR: In expression (a b c):
               Unbound variable: a
        guile> #: foo
        #:#{}#
        ERROR: Unbound variable: foo

    With 1)

        guile> #:12
        ERROR: Wrong type (expecting symbol): 12
        guile> #:#{12}#
        #:#{12}#
        guile> #:(a b c)
        ERROR: Wrong type (expecting symbol): (a b c)
        guile> #: foo
        #:foo

Opinions?

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