1. Presentation by Julie Betlach 7/02/2009
GoF Interpreter (pp )
Presentation by Julie Betlach
7/02/2009

2. Interpreter Pattern - Intent / Applicability
Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
Applicability – Use Interpreter Pattern…
The grammar is simple
Parser generators are a better alternative.
Efficiency is not a critical concern
Not usually efficient to parse trees directly.
Translate them into another form. Consider transforming regular expressions into state machines first.
But even then, the translator can be implemented by the Interpreter Pattern.

3. Motivation
The following Reverse Polish notation example illustrates the interpreter pattern.
The grammar below:
expression ::= plus | minus | variable plus ::= expression expression '+' minus ::= expression expression '-' variable  ::= 'a' | 'b' | 'c' | ... | 'z'
defines a language which contains reverse polish expressions like:
a b + a b c + - a b + c a - -
Picture & Example from:

4. Interpreter Structure
Picture & Example from:

5. Composite Structure Looks a lot like Composite Structure…
Picture & Example from:

6. Structure A class is used to represent each grammar rule.
expression ::= plus | minus | variable plus ::= expression expression '+' minus ::= expression expression '-' variable  ::= 'a' | 'b' | 'c' | ... | 'z'

8. It’s easy to change and extend the grammar
Consequences
Good
It’s easy to change and extend the grammar
Implementing the grammar is easy
Easy to evaluate an expression in a new way
Examples: pretty printing or type checking
If you keep adding new ways to evaluate an expression, consider using Visitor
Bad
Complex grammars are hard to maintain
One class for each rule

9. Composite – Abstract syntax tree is an instance of Composite Pattern
Related Patterns
Composite – Abstract syntax tree is an instance of Composite Pattern
Flyweight – Shows how to share terminal symbols within the abstract syntax tree
Iterator – The interpreter can use an iterator to traverse the structure
Visitor – Can be used to maintain the behavior in each node in the abstract syntax tree in one class