(guile.info.gz) Dia Objective

 
 17.1.1 Deciding Why You Want to Add Guile
 -----------------------------------------
 
 First off, you should understand why you want to add Guile to Dia at
 all, and that means forming a picture of what Dia does and how it does
 it.  So, what are the constituents of the Dia application?
 
    * Most importantly, the "application domain objects" -- in other
      words, the concepts that differentiate Dia from another
      application such as a word processor or spreadsheet: shapes,
      templates, connectors, pages, plus the properties of all these
      things.
 
    * The code that manages the graphical face of the application,
      including the layout and display of the objects above.
 
    * The code that handles input events, which indicate that the
      application user is wanting to do something.
 
 (In other words, a textbook example of the "model - view - controller"
 paradigm.)
 
    Next question: how will Dia benefit once the Guile integration is
 complete?  Several (positive!) answers are possible here, and the choice
 is obviously up to the application developers.  Still, one answer is
 that the main benefit will be the ability to manipulate Dia's
 application domain objects from Scheme.
 
    Suppose that Dia made a set of procedures available in Scheme,
 representing the most basic operations on objects such as shapes,
 connectors, and so on.  Using Scheme, the application user could then
 write code that builds upon these basic operations to create more
 complex procedures.  For example, given basic procedures to enumerate
 the objects on a page, to determine whether an object is a square, and
 to change the fill pattern of a single shape, the user can write a
 Scheme procedure to change the fill pattern of all squares on the
 current page:
 
      (define (change-squares'-fill-pattern new-pattern)
        (for-each-shape current-page
          (lambda (shape)
            (if (square? shape)
                (change-fill-pattern shape new-pattern)))))
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