Bill Andersen wrote:
>
> On Apr 16, 2007, at 22:04 , Waclaw Kusnierczyk wrote:
>
>>>
>>> The problem is whether 'www.nonsense.no <http://www.nonsense.no>' is
>>> a valid instance of 'host',
>>> which depends on whether 'nonsense.no' is registered with IANA to
>>> correspond to an allocated IP address. If it is registered, then I can
>>> agree that this is syntactically valid. If is not registered, one can
>>> argue that it is not even "syntactically valid", because valid domain
>>> names are determined by IANA registry, not by a production rule. That
>>> is, the requirement is not just that it is well-formed, but rather that
>>> it is in a particular list of valid strings. By way of analogy,
>>> consider a language in which symbols must be declared before they are
>>> used. If the parser encounters a well-formed lexical object that should
>>> be a declared symbol but the "symbol table lookup" fails (the symbol was
>>> never declared), is that a "syntax error"? Is that text "syntactically
>>> valid"? (I don't know that the formal languages community has
>>> consistent terminology in this area, but I haven't written a compiler in
>>> over 20 years. Maybe they do now.)
>>
>> We may be splitting hairs here.
>>
>> A formal language is a set of strings over an alphabet -- a set of
>> symbols. You may define the language ostensibly, by listing all the
>> strings, which would imply which symbols are in the alphabet. You may
>> define the language by means of a grammar, that is, specify the alphabet
>> and rules for forming strings in the language. You may do both, but the
>> definitions must agree.
>
> I agree with Wacek on this. What's being suppressed in Ed's discussion
> is precisely a difference between syntax and semantics. That
> 'www.nonsense.no' is syntactically valid is not in question - that such
> a string is a "registered" name is a semantic (and relational) property
> ... of names. (01)
So let's try to achieve some peace.
Would you agree: (02)
1. A URL is well-formed if it can be generated by the grammar of URLs,
as defined ... (03)
2. A URL is valid if it is registered by IANA. Only well-formed URLs
can be valid. (04)
3. A valid URL may be accessable. A non-valid URL cannot be accessable. (05)
Notes: (06)
'accessable' is my neologism for a URL whose associated IP points to a
resource that is accessible. (07)
1. reflects syntax, 2. semantics, 3. (in some sense) pragmatics. (08)
'well-formed' can be taken as synonymous with 'syntactically valid'
(then 'valid' should read 'semantically valid'). There can be no URL
that is well-formed but not syntactically valid. IANA registration has
nothing to do with syntactic validity. (09)
Of course, RFC and IANA documents may say different things, and they
may, in strict terms, be contradictory. (010)
If you read T. B.-L. text [1], you'll find that "A URI is an identifier
consisting of a sequence of characters matching the syntax rule named
<URI> in Section 3." and "The term "Uniform Resource Locator" (URL)
refers to the subset of URIs that, in addition to identifying a
resource, provide a means of locating the resource by describing its
primary access mechanism (e.g., its network "location")." (011)
The latter is really a syntactic constraint: it demands that a part of
the URI specifies a protocol, i.e., is of a special form. (Note that
also here the definition somewhat mixes (at least superficially)
syntactic and semantic issues. It is not the role of a URL to provide
an IP (this is what DNS does). URL does not provide a means for
locating a resource by describing its primary access mechanism; a URL
only names the mechanism (e.g., 'http:'). (012)
vQ (013)
[1] http://gbiv.com/protocols/uri/rfc/rfc3986.html#overview (014)
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