Interpreter

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(Structure)
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===Client===
 
===Client===
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==Consequences==
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*Interpreter makes it easy to change and extend the grammar using inheritance.
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*Implementing the grammar is easy because the nodes in the abstract syntax tree have similar interpretations are are usually easy to write. Often, writing these classes can be automated using a compiler or parser generator.
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*It is hard to maintain complex grammar using Interpreter because it defines at least one class for every rule in the grammar. This can soon lead to a huge number of classes.
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*It is easy to add new ways to interpret an expression.
  
 
== Related Patterns ==  
 
== Related Patterns ==  

Revision as of 02:22, 25 July 2009

The interpreter pattern is specific implimentation of the composite pattern generally used for language parsing. In the interpreter pattern a class is produced for each symbol in the language. A statement can then be broken down into a syntax tree of these classes which can in turn be used to interpret the statement.

Contents

Use When

You should use Interpreter when there is a language that you need to interpret and when the statements of the language can be represented as abstract syntax trees. It works best when:

  • the grammar of the language is simple. Otherwise, the class hierarchy for the grammar becomes large and complex.
  • efficiency is not essential.

Structure

Interpreter.jpg

(from wikipedia)

Participants

Abstract Expression

Terminal Expression

Nonterminal Expression

Context

Client

Consequences

  • Interpreter makes it easy to change and extend the grammar using inheritance.
  • Implementing the grammar is easy because the nodes in the abstract syntax tree have similar interpretations are are usually easy to write. Often, writing these classes can be automated using a compiler or parser generator.
  • It is hard to maintain complex grammar using Interpreter because it defines at least one class for every rule in the grammar. This can soon lead to a huge number of classes.
  • It is easy to add new ways to interpret an expression.

Related Patterns

  • Composite: The abstract syntax tree uses a Composite pattern.
  • Flyweight: This pattern can be used to share the terminal symbols.
  • Iterator: This pattern can be used to traverse the Interpreter structure.
  • Visitor: This pattern can be used to collect the behavior for all abstract syntax tree nodes in one class.


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