5.7.1.1 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)))))