Interscript
An Interchange Standard for Editable Documents
History, Motivation, Goals, and Constraints Mitchell
The Base Language: A Travelogue Mitchell
Short Break Everyone
The Layout Model Ayers
Some Standard Constructs Ayers
Interscript Seminar November, 1983
History, Motivation, Goals, and Constraints
History
Motivation and Goals
What properties should Interscript have?
What Interscript is not
What is in the first version of Interscript?
Interscript Seminar November, 1983
History
May '81:
Project starts in CSL at Lampson's suggestion.
(Horning, Mitchell, McGregor, Paxton, Reid)
Aug '81:
SISB authorizes development of standard;
SDD joins the party.
(above plus Ayers, Curry, de la Beaujardiere, Karlton)
Sept '81 Feb '83:
The work grinds on. (Ayers, Horning, Guttag, Lampson, Mitchell)
Mar '83 May '83:
The "March" Interscript Offensive
(Ayers, Horning, Lampson, Mitchell)
June '83 Sept '83:
First Draft Standard is produced (Ayers, Mitchell)
Oct '83:
Interscript test implementation for the PICS
publishing project begins (ABrown, Ayers)
Interscript Seminar November, 1983
Motivation and Goals
Motivation
Xerox needs an interchange standard for editable documents
for product compatibility
for product evolution
It is an interesting technical problem
External standards organizations are active in this area
formatting language focus
Goals
A standard that supports
WYSIWYG editors
formatting languages
A standard that can span the Xerox product range
A standard that can evolve
Interscript Seminar November, 1983
What properties should Interscript have?
(or for that matter, any interchange standard)
It must be "as simple as possible, but no simpler"
It must enable interchange among editors of varying capabilities
It must be simple to internalize a script
Both form and content must be representable
Structure must be representable and preservable
It must be simple to externalize a document
The encoding should be efficient in space and time
It must be extensible
Interscript Seminar November, 1983
What Interscript is not
It is not a file format
It is not a standard for editing
It is not an editing system itself
combining documents is not an Interscript function
It is not a substitute for other corporate standards
e.g., character codes, font naming
It is not intended for human communication
Interscript Seminar November, 1983
What is in the first version of Interscript?
A processing model
internalization editing externalization
A (solid) basis for the standard
description language for scripts
tiny syntax; small, precise semantics
Standardized document constructs
TEXT (FONT, TAB)
TEXTFIELD
TABLE (TABLEROW, TABLEENTRY, COLUMNSPLIT)
PARAGRAPH
INTERPRESS
Standardized layout constructs
description not prescription: TEX-like
a general layout model
Interscript Seminar November, 1983
The Base Language: A Travelogue
Syntax and Semantics by Example
When are Two Scripts Equivalent?
Tags and Node Invariants
Ignorance-Based Editing
Interscript Seminar November, 1983
Syntax and Semantics by Example (1)
A Script is a tree of nodes
{ "This string is the only content of this node" }
A Script is a tree of nodes
INTERSCRIPT/INTERCHANGE/1.0
{
DOCUMENT$
{ TEXT$ "This is the first subnode of the document" }
{ TEXT$ "This string is the only content of this node" }
} ENDSCRIPT
Basic values are atoms, numbers, strings, and quoted expressions
Atoms: TEXT DOCUMENT SimpleId any.qualified.name
Numbers: 0 1 2 3.14159 1.7E3
Strings: "just characters in quotes"
Quoted Expressions: '3+5' '{ PARAGRAPH$ leftMargin� }'
Interscript Seminar November, 1983
Syntax and Semantics by Example (2)
An attribute is a name-value pair
leftMargin ← 36
Expressions are very simple
leftMargin^
leftMargin^ LT (rightMargin^20)
leftMargin^+rightMargin^20
The only kind of structured value is a node
A content-only node is like a vector
a ← { "Now " "is " "the " "time " "for " . . . }
a!0
An attribute-only node is like a record
origin ← { CARTESIAN$ x𡤀 y𡤀 z𡤀}
origin^.y
A node may hold both contents and attributes
{ TEXT$ font ← TimesRoman^ --an attribute--
"This string is the only content of this node" }
Interscript Seminar November, 1983
Syntax and Semantics by Example (3)
A node's semantics are based on its tags
{ TABLE$ heading←{TEXT$ "Frooble production by territory"}
{ TABLEROW$
{ TABLEENTRY$ { TEXT$ "Yukon" } }
{ TABLEENTRY$ {TEXT$ "200 froobles"} }
. . .
}
}
A node may be opened
subEg ← {TABLEENTRY$}
{ TABLE$ heading←{TEXT$ "Frooble production by territory"}
{ TABLEROW$
subEg ← {TABLEENTRY$}
{ subEg^| {TEXT$ "Yukon" } }
{ subEg^| {TEXT$ "200 froobles"} }
. . .
}
}
Interscript Seminar November, 1983
Syntax and Semantics by Example (4)
Name scopes nest
subEg ← 7
{ TABLE$ heading←{TEXT$ "Frooble production by territory" }
{ TABLEROW$
subEg ← {TABLEENTRY$}
{ subEg^| {TEXT$ "Yukon" } }
{ subEg^| {TEXT$ "200 froobles" } }
. . .
}
}
Interscript Seminar November, 1983
Syntax and Semantics by Example (5)
Representing Structure in Documents
Kinds of structure:
"Styles" by name (indirection)
Inheritance by name (indirection)
Associating separate parts by name (indirection)
Structural binding
sb %← 5
Structural evaluation
name% instead of name^
sb^+7 and 5+7 and 12 are equivalent
sb%+7 has value 12, but it is only equivalent to sb%+7
Structural node opening
name%| instead of name^|
format.body %← {PARAGRAPH$ lineJustification←True^
font ← TimesRoman^ }
{ format.body%| . . . } has value
{ PARAGRAPH$ lineJustification←True^
font ← TimesRoman^ . . . }
Interscript Seminar November, 1983
When are Two Scripts Equivalent?
When they have the same formal semantics
Testing editors
Does the editor produce valid scripts?
Are internalization and externalization an idempotent pair?
Interscript Seminar November, 1983
Tags and Node Invariants
Every tag has a declaration
attributes: names and types of relevant attributes
contentType: type of the allowable content
nodeInvariant: quoted Boolean expression
hasMoreInv: If false then nodeInvariant tells all
requiredTags: any other tags that must also be present
reducesTo: quoted expression to express node in terms of
a more basic tag
tagOnly: If true, no invariant check is needed to determine
validity of parent node
A tag's properties can be defined in Interscript
Interscript Seminar November, 1983
Ignorance-Based Editing
An editor may display whatever it can recognize
Editing known quantities in the context of unknown quantities
the text caption of a figure
the frame for some synthetic graphics
Any operation that preserves invariants results in a valid script
perform semantically meaningful operations
satisfy invariant without understanding it
change nothing (contents, tags, relevant attributes)
Interscript Seminar November, 1983