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Keyword syntax
From: |
Marius Vollmer |
Subject: |
Keyword syntax |
Date: |
Mon, 04 Oct 2004 20:27:44 +0200 |
User-agent: |
Gnus/5.1003 (Gnus v5.10.3) Emacs/21.3 (gnu/linux) |
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?
- Keyword syntax,
Marius Vollmer <=