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Re: An IRC discussion with Alexandre Bertails re SSUE-19:

From: Alexandre Bertails <bertails@w3.org>
Date: Wed, 05 Jun 2013 09:15:56 -0400
Message-ID: <51AF3A0C.6020203@w3.org>
To: Henry Story <henry.story@bblfish.net>
CC: public-ldp-wg@w3.org
On 06/05/2013 02:21 AM, Henry Story wrote:
> Where we find out that we can get from first principles of URIs, RESTful
> HTTP interactions on Resources that return representations
> (interpreted by mime types to some model - in particular a graph ),
> which said model can describe the resource referred to by the URI.
> On the difference between meaning and truth/falsehood and how one comes
> to trust a statement on the web. How one can relinquish
> trust is also treated in this this short socratic dialog with Alexandre
> yesterday evening.
>
> 12:53 bblfish: Does a URI refer to a Resource?
> 22:53 betehess: hrmmm, I've said yes twice
> 22:53 bblfish: Ok.
> 22:55 bblfish: Does an HTTP resource return a represntation of that
> resource when interacted with using GET ?
> 22:55 betehess: if it's a good web citizen, I would expect the answer to
> be Yes
> 22:55 bblfish: I think this is necessarily the case
> 22:55 betehess: but RDF does not say anything about it
> 22:56 bblfish: I have not come to RDF yet
> 22:56 betehess: and SPARQL does not care
> 22:56 betehess: right
> 22:56 bblfish: ok. don't jump the gun
> 22:56 betehess: I'll let you continue then :-)
> 22:56 betehess: but so far, Yes and Yes
> 22:56 bblfish: Good, so other than Cache corruption issues, the
> representation returned by an HTTP resource is a representation of that
> resource at that time.
> 22:57 bblfish: If the representation is in an RDF format as per mime
> type, you can interpret the representation as a Graph, right?
> 22:57 betehess: almost true
> 22:57 bblfish: where's the almost hitch?
> 22:57 betehess: what about #-URIs?
> 22:57 bblfish: #URIs are defined by the URI spec.
> 22:58 bblfish:
> https://dvcs.w3.org/hg/WebID/raw-file/tip/spec/identity-respec.html
> 22:58 bblfish: ( I don't know why we don't get the HTML anymore )
> 23:00 bblfish:   The fragment identifier component of a URI allows indirect
> 23:00 bblfish:   identification of a secondary resource by reference to
> a primary
> 23:00 bblfish:   resource and additional identifying information.  The
> identified
> 23:00 bblfish:   secondary resource may be some portion or subset of the
> primary
> 23:00 bblfish:   resource, some view on representations of the primary
> resource, or
> 23:00 bblfish:   some other resource defined or described by those
> representations.  A
> 23:00 bblfish:   fragment identifier component is indicated by the
> presence of a
> 23:00 bblfish:   number sign ("#") character and terminated by the end
> of the URI.
> 23:00 bblfish: http://tools.ietf.org/html/rfc3986#section-3.5
> 23:01 betehess: anyway, the GET gives you some information about the
> state of the resource
> 23:01 bblfish: yes, that can be interpreed as a graph if the mime type
> is correct
> 23:01 betehess: they don't necesseraly describe the resource
> 23:01 bblfish: of course you don't necessarily do that.
> 23:01 bblfish: I am looking at cases where they do.
> 23:02 bblfish: if that graph contains a triple that refers to the
> resource, then it describes that resource.
> 23:02 bblfish: as being related in some way to something else
> 23:02 bblfish: yes?
> 23:03 bblfish: RDF Semantics specifies what has to be true for the graph
> to be true.
> 23:03 betehess: I'm getting confused by words but yes, I suppose do
> 23:04 betehess: s/do/so/
> 23:04 bblfish: ok.
> 23:04 bblfish: Good so now if we define ldp:Container to be the set of
> resources which can be interacted with in a certain
> 23:04 bblfish: way and if the resource says { <> a ldp:Container . }
> 23:05 bblfish: then we can assume it is saying that certain interactions
> can be done on it: namely LDP interactions.
> 23:05 bblfish: if we can not interact that way with it then the
> statement was false.
> 23:05 betehess: that's the part I'm not sure about
> 23:05 betehess: why is it true?
> 23:05 bblfish: why is what true?
> 23:05 betehess: as far as HTTP is concerned, that's not how it works
> 23:06 bblfish: HTTP is a different layer
> 23:09 betehess: knowing for reason that [[ <> a ldp:Container ]] does
> not mean I can do it. RDF assumes that it's true, but it's only true if
> <> tells you so
> 23:10 bblfish: If you look at the RDF Semantics it will tell you when <>
> a ldp:Container is true.
> 23:10 bblfish: It is true iff <> is a resource that is part of the set
> refered to by ldp:Container.
> 23:11 betehess: for example, you can learn that [[ <foo> a ldp:Container
> ]] by dereferencing <bar>
> 23:11 betehess: it does not mean anything in practice
> 23:11 bblfish: no, it _means_ exactly that
> 23:11 betehess: <foo> may be an image
> 23:11 betehess: for RDF, yes
> 23:11 betehess: but HTTP does not care
> 23:11 bblfish: you need to distinguish meaning and truth
> 23:12 bblfish: { <foo> a ldp:Container } is true iff <foo> is an LDPC
> 23:12 bblfish: but it may not be.
> 23:12 betehess: can you instanciate  meaning and truth in my example?
> 23:12 bblfish: yes, if <foo> acts like an LDPC then the relation is true
> 23:13 bblfish: but the case is more clear cut when a resource speaks
> about itself as being an LDPC.
> 23:13 bblfish: of course it  can also be lying
> 23:13 bblfish: but you go on the presumption of innoncence
> 23:14 bblfish: Or as Donald Davidson wrote in Truth and Interpretation,
> you use a principle of charity, which is that the agents you are
> communicatiing with are truthful.
> 23:14 bblfish: but if they are not, then you stop interacting with them.
> 23:14 bblfish: and you stop linking to them.
> 23:15 bblfish: or you say things like <foo> a ldp:nonContainer
> 23:15 bblfish: so people don't follow the link
> 23:15 bblfish: or <foo> a log:Falsehood .
> 23:15 betehess: this is making everything just complex. You cannot
> convince people with such hard reasoning about how to jump from RDF
> truth (which the specs assume today) to a resource being an LDPC in practice
> 23:16 betehess: and you need to look at the content, always
> 23:16 betehess: it's the point of the Content-Type to save you here
> 23:16 bblfish: this is just what you get when you read RDF Semantics. It
> defines truth in a Tarskian way
> 23:16 bblfish: no content-type is just a way to specify the language of
> the representation.
> 23:17 betehess: yes, it would true in all realizations, which is not
> true in practice...
> 23:17 bblfish: If it is JSON content then you need to create special
> mime types
> 23:17 betehess: RDF does not say who is authoritative
> 23:17 bblfish: well not quite
> 23:17 betehess: the content-type is also about the
> semantics/interactions, not only the representation
> 23:18 bblfish: no.
> 23:18 betehess: yes
> 23:18 bblfish: no
> 23:18 bblfish: absolutely not
> 23:18 betehess: you just don't see that often at w3c because we don't
> define web apis
> 23:18 bblfish: that is the error the REST folks who use XML make
> 23:18 betehess: you mean webarch is wrong?
> 23:19 betehess: we're defining a Web API
> 23:19 bblfish: no, the REST folks only use formats where they need to
> use mime types that way
> 23:19 bblfish: ( to distinguish the meaning from other interpretations
> ) but that is their problem
> 23:19 betehess: if it was only about GETs, I wouldn't care about your
> interpretation
> 23:20 bblfish: btw { <> a ldp:Container . } also specifies how to do
> POST, etc...
> 23:20 bblfish: it creates some restrictions on HTTP: namely those
> specified in the ldp spec
> 23:21 bblfish: ( if you believe it to be true of course )
> 23:21 bblfish: or rather the other way around. It is true, if those
> interactions hold.
> 23:22 betehess: Henry, here is the thing: why should I accept { <> a
> ldp:Container . } to be true?
> 23:22 bblfish: 1. because you probably got there through a link you trust
> 23:23 bblfish: 2. if you interact and you don't get the results expected
> you come to the conclusion there is a bug or it is lying
> 23:23 betehess: and [[ if those interactions hold ]] means that it does
> not require { <> a ldp:Container . } to be true either
> 23:24 betehess: I really don't get why webarch speaks about presentation
> and interaction...
> 23:24 bblfish: true. But on the web the best resource to trust on its
> state is the resource itself
> 23:24 betehess: you're saying that interaction is implied by [[ those
> interactions hold ]]
> 23:25 betehess: anyway, I have to be 5 km away in 5 minutes...
> 23:25 bblfish: yes, http defines POST, PUT etc. LDP  vocab creates
> further restrictions on it
> 23:25 betehess: (biing)
> 23:25 bblfish: ok hope that helps

You forgot the most important comment:

Jun 04 17:25:40 <betehess>	not really, I'm not convinced

Alexandre :-)
Received on Wednesday, 5 June 2013 13:16:00 UTC

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