N4 (notation)

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N4 is another notation for RDF like languages. It is named N4 because it is "after" N3.

Contents

1 The Syntax 2 A short example: 3 Comments about the example 4 Metaclasses 5 WebResourceDescriptionLanguage

The Syntax

The N4 syntax is intentionally very simple:

N4 = '(' *(uri (uri | String | N4)) ')'

A short example:

(
    n4ns:n4 http://n4.org/n4/ 
    n4ns:p http://x.org/personal/
    n4ns: http://x.org/docs/1/

    p:Person (a n4:Class 
       n4:dof (li x:address li x:name)
    )
    p:Address (a n4:Class
       n4:dof (li p:city li p:street)
    )
   
    (li p:city li p:street)(a n4:Property n4:range n4:String)
    
    :me (a p:Person p:id "me"
       p:address (a p:Address id :me/address 
           p:city "Paris"
       )
    )
    :me/address (p:street "tur")
)

Comments about the example

• it is a parenthesized expression
• namespaces are used as shorthands for uris
• the empty namespace is the current n4 document

• each n4 expression is a sequence of attribute-value pairs
• An attribute name can be a n4 expression
• a n4 expression denotes an "object", i.e. an uri with assertions on it.
• keywords 'a', 'li' and 'id' are "specials" uris
  • 'a' types an expression
  • 'li' introduces an element of a list
  • 'id' names an expression in a namespace
• a blind list in subject position is a list a of subjects

Metaclasses

RDF based notations use a metaclass based design which can be used to represent things in a very concise way. Lets recall that a Metaclass is a class whose instances are also classes.

Metaclasses allow to define class variables, i.e. slot values hold by classes. Class variables are naturally shared by all instances of the class.

:mc (a n4:Class n4:subClassOf n4:Class)
(li :fcolor li :bcolor) (a n4:Property n4:domain :mc)
:c (a :mc :fcolor "red")
:o1 (a :c)
:o2 (a :c)

•  :o1 and :o2 are instances of c and share the value of the slot :fcolor.
•  :c is a class whose class is a metaclass
•  :c is also a regular object.

Shared slots allow to use an interesting idiom : only define objects which are classes, and derive them following the two relations is-instance-of AND is-subclass-of. Lets note this double relation by the keyword "as".

:c1 (as :c :bcolor "white")
:c2 (as :c1 :fcolor "red")
:c3 (as :c1 :fcolor "blue")
:c4 (as :c3 :bcolor "red")

WebResourceDescriptionLanguage

Here is presented the WRSDL format, which allows to type web resources on a given site, and to map various url schemes to these resources.

An example:

(n4ns: http://n4.org/wrsdl/s1
n4ns:r http://n4.org/wrsdl/1.0/schema

:site (a r:Site r:name "example site"

   li (a r:Resource r:name "home" id :home
      li (a r:Get r:description "the home page "
         li (a r:Accept r:mime "text/html")
         li (a r:Accept r:mime "application/n4" r:class :Home)
      )
   )
   li (a r:Resource r:name "welcome" id :welcome r:description "splash"
      li (a r:Get 
         li (a r:Accept r:mime "image/gif" id :welcome/gif)
         li (a r:Accept r:mime "image/png" id :welcome/png)
      )
   )
)

:xmap (a r:Mappings r:host "n4.org:4445" r:context "/example/s1")
:map/home (as :xmap 
   li (a r:Mapping r:of :home
      li (a r:Path r:value "/Home")
   )
)
:map/welcome (as x:map 
   li (a r:Mapping r:of :welcome
      li (a r:Path r:value "/Welcome")
      li (a r:Parameter r:name "size" r:type :sizes)
   ) 
)

:sizes (a r:Enumeration li "small" li "big" li "huge")

:Home (a r:LinkSequence)