Interpreter Design Pattern

 UML diagram
 sample code in C#


Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

Frequency of use:   low

UML class diagram


    The classes and/or objects participating in this pattern are:

  • AbstractExpression  (Expression)
    • declares an interface for executing an operation
  • TerminalExpression  ( ThousandExpression, HundredExpression, TenExpression, OneExpression )
    • implements an Interpret operation associated with terminal symbols in the grammar.
    • an instance is required for every terminal symbol in the sentence.
  • NonterminalExpression  ( not used )
    • one such class is required for every rule R ::= R1R2...Rn in the grammar
    • maintains instance variables of type AbstractExpression for each of the symbols R1 through Rn.
    • implements an Interpret operation for nonterminal symbols in the grammar. Interpret typically calls itself recursively on the variables representing R1 through Rn.
  • Context  (Context)
    • contains information that is global to the interpreter
  • Client  (InterpreterApp)
    • builds (or is given) an abstract syntax tree representing a particular sentence in the language that the grammar defines. The abstract syntax tree is assembled from instances of the NonterminalExpression and TerminalExpression classes
    • invokes the Interpret operation

sample code in C#

This structural code demonstrates the Interpreter patterns, which using a defined grammer, provides the interpreter that processes parsed statements.

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// Interpreter pattern -- Structural example

This real-world code demonstrates the Interpreter pattern which is used to convert a Roman numeral to a decimal.

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// Interpreter pattern -- Real World example

This .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc.

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// Interpreter pattern -- .NET optimized

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Design Pattern
Framework 4.5

C# and VB.NET


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