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Re: optional user-specified end-delimiters

This page is part of the web mail archives of SRFI 109 from before July 7th, 2015. The new archives for SRFI 109 contain all messages, not just those from before July 7th, 2015.

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/