Interpreter Design Pattern

Download the working example demo code in java from my GIT repository –
https://github.com/premaseem/designPatterns/tree/master/ZipDownloadableProjects

Example Code Description :
1. Date interpreter example : The main cause of Y2K virus was interpretaiton of date format, in this example code I tried to address the same so that system can intrepret any date given in any formats. we have a interpreter for DD, MM and YYYY (rule to pick the date from context) so you provide the format and system gives you the conversion. Straight forward. 
2. And Or Expression : This example taks a string where in it should match the expression based on rule which accomplies And / Or expression. Code is simple and small but a bit triky to get hold of. Feel free to downlaod and play with it.

Direct link :
https://drive.google.com/file/d/0B-9WtmpPhI4ZREEwNWpRYWVzSGM/view?usp=sharing

Intent
  • Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
  • Map a domain to a language, the language to a grammar, and the grammar to a hierarchical object-oriented design.

interpreter

Problem Area

This interpreter design pattern does not give solution for building a whole large interpreter for a language. It can be applicable for smaller chunks where grammar and interpretation is applicable. We can consider scenarios like regular expressions and interpreting mathematical expression.

Discussion

The Interpreter pattern discusses: defining a domain language (i.e. problem characterization) as a simple language grammar, representing domain rules as language sentences, and interpreting these sentences to solve the problem. The pattern uses a class to represent each grammar rule. And since grammars are usually hierarchical in structure, an inheritance hierarchy of rule classes maps nicely.

 

An abstract base class specifies the method interpret(). Each concrete subclass implements interpret() by accepting (as an argument) the current state of the language stream, and adding its contribution to the problem solving process.

Example

The Intepreter pattern defines a grammatical representation for a language and an interpreter to interpret the grammar. Musicians are examples of Interpreters. The pitch of a sound and its duration can be represented in musical notation on a staff. This notation provides the language of music. Musicians playing the music from the score are able to reproduce the original pitch and duration of each sound represented.

Example of Interpreter

Check list
  1. Decide if a “little language” offers a justifiable return on investment.
  2. Define a grammar for the language.
  3. Map each production in the grammar to a class.
  4. Organize the suite of classes into the structure of the Composite pattern.
  5. Define an interpret(Context) method in the Composite hierarchy.
  6. The Context object encapsulates the current state of the input and output as the former is parsed and the latter is accumulated. It is manipulated by each grammar class as the “interpreting” process transforms the input into the output.

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