The slides from a presentation I did on the Interpreter design pattern at the Melbourne Patterns group.

1. Interpreter
Abhijit Hiremagalur
Cogent Consulting

2. Given a language, define a representation for its
grammar along with an interpreter that uses the
representation to interpret sentences in the
language [GOF, 243]

3. Domain
Language
Grammar
Object Hierarchy

4. Composite + interpret()

6. Reverse Polish Notation

7. 3+4
is expressed as
34+

8. 3-4+5

9. (3 - 4) + 5 or 3 - (4 +5)
Which one is it?

10. 543-+
5 (4 3 -) +
5 (1) +
6

11. The code

12. ‘41 2 1 - +’ equals 42

13. Stack what?
[]
(41) => [41]
(2) => [41, 2]
(1) => [41, 2, 1]
(-) => [41, 1]
(+) => [42]

14. Easy to implement,
change & extend?

15. Doesn’t address parsing

16. Use something else if
grammar is complex

17. Refactor to an
Interpreter when...

18. “Numerous methods on a
class combine elements of an
implicit language”

19. Replace Implicit Language
with Interpreter [Kerievsky]

22. “Define classes for the
elements of the implicit
language so that instances
may be combined to form
interpretable expressions.”

23. A.K.A ?
• Little language
• Macro
• Domain specific language

24. Domain Specific
Language (DSL)

25. Internal vs External DSL

26. Spec productSpec = parse(“price below 9.00f and not coloured white”);
Spec productSpec = both(priceIsBelow(9.00f)).and(not(coloured(WHITE)));

27. Share symbols with
Flyweight

28. Move operations to
Visitor

29. The end.