An interpreter is a medium that changes unrecognized information into a recognizable form. The interpreter pattern describes how to define a grammar for a simple language and represent sentences in that language to interpret them. It uses classes to represent each grammar rule and creates an abstract syntax tree to represent expressions. While it makes programming and problem solving easier, complex grammars can be difficult to maintain with this pattern.

1. INTERPRETER
Main Topics
What is an Interpreter.
Why should we learn about them.

2. Simple Definition
Interpreter:
A medium through which unrecognized
information is changed into a form that
can be recognized.

3. Why Should we learn about
Interpreters
 Makes Your Life Easier
– Amount of work
– Amount of programming
– Adaptability to new situations

4. Why Should we learn about
Interpreters
 Makes Your Life Easier
– Amount of work
 If a problem occurs often enough, you might want to
express instances of the problem as a simple
language.
Example: String Matching (Regular Expressions)

5. Why Should we learn about
Interpreters
 Makes Your Life Easier
– Amount of work
 The interpreter pattern describes how to define a
grammar for a simple language, how to represent
sentences in the language and interpret those
sentences

6. Why Should we learn about
Interpreters
 Makes Your Life Easier
– Amount of programming
 The interpreter pattern uses a simple class to
represent each grammar rule.
 First, some more definitions

7. More Definitions
– Literal Expression
 This determines if there is an exact match of two
objects ( operator = )
– Alternation Expression
 Is there an alternate expression that is acceptable
(operator | )
– Repetition Expression
 Does this repeat itself (operator * )

8. More Definitions
– Sequence Expression
 Determines if both objects are present (operator &)

9. Class Diagram
Every regular expression defined by this grammar is
represented by an abstract syntax tree made up of
instances of these classes. p.244

10. Syntax Tree
 This tree represents
the regular expression
– Raining & (dog | cat)*
– Diagram p.244

11. Why Should we learn about
Interpreters
 Makes Your Life Easier
– Amount of programming
 Works best when
– The grammar is simple
• (class hierarchy)
– Efficiency is not a concern
• (space & time management)

12. Collaboration Diagram
Participants:
AbstractExpression, TerminalExpression,
NonTerminalExpression, Context, Client. (p.245)

13. Pros & Cons of Interpreters
ANY GUESSES???

14. Pros & Cons of Interpreters
 It’s easy to change and extend the grammar
 Inheritance – existing expressions can be modified,
and new expressions can be defined as variations of
old ones
Implementing the grammar is ‘easy’ too.
Adding new ways to interpret expressions.
Flexible - Tokenizers
At least that’s what the book says

15. Pros & Cons of Interpreters
 Complex Grammars are hard to maintain
 The interpreter pattern defines at least one class for
each rule.

16. Implementation
 Creating the abstract syntax tree
 The interpreter pattern does not explain how to
create an abstract syntax tree.
 Sharing terminal symbols with the
flyweight pattern
 Grammars who’s sentences contain many
occurrences of a terminal symbol might benefit from
sharing a single copy of that symbol.

17. Implementation
 Creating the abstract syntax tree
 You can (but have the option of not) define the
Interpret operation in your expression classes.
– SequenceExpression, LiteralExpression… etc.

18. Known Uses
 The interpreter pattern is widely used in
compilers implemented with object oriented
languages
– Example: SmallTalk

19. Related Patterns
– Composite (p.163): the abstract syntax tree is
an instance of the composite pattern
– FlyWeight (p.193): shows how to share
terminal symbols within the abstract syntax
tree.
– Iterator (p.257): can be used to traverse the
structure.
– Visitor (p.331): can be used to maintain the
behavior in each node in the abstract syntax tree
in one class

20. Final Questions or Comments
•Sample code
•SmallTalk can be found on pages 248 – 251
•C++ can be found on pages 251 - 255