INTERPRETER

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

• Its 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.

• At least thats what the book says

• Adding new ways to interpret expressions.
• Flexible - Tokenizers

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 whos 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