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Re: optional user-specified end-delimiters

On 04/16/2013 02:46 PM, John Cowan wrote:
Per Bothner scripsit:

It seems reasonable to allow an integer label, so we allow starting
with a digit.  Thus for simplicity we also allow hyphen, underscore,
or period.

I think starting with a digit is just an extra complication, and
we shouldn't have it.  It's a special case that needs to be tested,
documented, and learned separately.

I'm fine with that.

extended-string-literal ::= "&" string-starttag?
    "{" initial-ignored? string-literal-part* "}!" string-endtag?
string-starttag ::= "!" label
string-endtag ::= "!" label

The string-endtag is required if the string-starttag is specified,
and of course the labels must match.

I think the string-endtag should be label followed by "!" instead.
That way all the content, including both labels, is between "!" and "!".

I assume you mean:

See below why I think that is awkward.

Note this chance means that an extended-string-literal must be
followed by a delimiter or end of input.

For a named quasi-literal, we can use the constructor-name as an end-tag:


I am very much against this, for reasons given earlier:  "}example" should not
be distinct from "} example", since "}" is a delimiter.

Not sure I understand why.  (I don't remember seeing the earlier
reasons.) I don't see "}" listed as a <delimiter> in either R6RS
or R7RS draft 8.  Regardless, whether it is a <delimiter> is
irrelevant - the question is what can follow the "}".  Your
suggested syntax does have "}TAG!" different from "} TAG!".

You can add an explicit label:

I think an explicit label is the way to go.


I'd change this to "}23!", modulo not allowing digit-strings.

It would be nice to be able use "example" as an end-label, without
having to explicitly declare it.  Many markup languages have this,
though not optional as I'm suggesting:

@end example

or XML:


If you really dislike:


Then we could require "!":


With an explicit label either:




This gets a little messy, I think, which is why I think what I
proposed works better, IMO:

&example!label{content}example!label  ; probably less useful

If may occasionally be useful to make labels available for
semantic information.  One  example is as implicit "id" attributes.

I think that is a very bad mistake.  The labels should be purely syntactic
and discardable, like the numbers used in #2# and #2= datum labels; it's
all the same if you write #2=(a . #2#) or #333=(a . #333#), and the Lisp
system does not have to distinguish.

Given the lack of a strong use-case, I'm fine leaving this out.
We can always add it later.

There are some plausible alternatives.  For example we can put the
end-tag just before the right brace.  For named constructors we
could use:


That won't fly, because "&cname" is a perfectly cromulent substring.

No, it's not, since "&" is an escape character in <content>.

I really dislike your other alternatives as well.

I agree - let's focus on the label *following* the "}".
	--Per Bothner
per@xxxxxxxxxxx   http://per.bothner.com/