Interpreter Pattern

In computer programming, the interpreter pattern is a design pattern that specifies how to evaluate sentences in a language.
The basic idea is to have a class for each symbol ( terminal or nonterminal) in a specialized computer language. The syntax tree of a sentence in the language is an instance of the composite pattern and is used to evaluate (interpret) the sentence for a client. See also Composite pattern.

Overview

The Interpreter

design pattern is one of the twenty-three well-known
'' GoF design patterns''
that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

What problems can the Interpreter design pattern solve?

* A grammar for a simple language should be defined
* so that sentences in the language can be interpreted.

When a problem occurs very often, it could be considered to represent it as a sentence in a simple language
(Domain Specific Languages) so that an interpreter can solve the problem
by interpreting the sentence.

For example, when many different or complex search expressions must be specified.
Implementing (hard-wiring) them directly into a class is inflexible
because it commits the class to particular expressions and makes it impossible to specify new expressions or change existing ones independently from (without having to change) the class.

What solution does the Interpreter design pattern describe?

* Define a grammar for a simple language by defining an Expression class hierarchy and implementing an interpret() operation.
* Represent a sentence in the language by an abstract syntax tree (AST) made up of Expression instances.
* Interpret a sentence by calling interpret() on the AST.

The expression objects are composed recursively into a composite/tree structure that is called
''abstract syntax tree'' (see Composite pattern).

The Interpreter pattern doesn't describe how
to build an abstract syntax tree. This can
be done either manually by a client or automatically by a parser.

See also the UML class and object diagram below.

Uses

* Specialized database query languages such as SQL.
* Specialized computer languages that are often used to describe communication protocols.
* Most general-purpose computer languages actually incorporate several specialized languages.

Structure

UML class and object diagram

In the above UML class diagram, the Client class refers to the common AbstractExpression interface for interpreting an expression
interpret(context).


The TerminalExpression class has no children and interprets an expression directly.


The NonTerminalExpression class maintains a container of child expressions
(expressions) and forwards interpret requests
to these expressions.

The object collaboration diagram
shows the run-time interactions: The Client object sends an interpret request to the abstract syntax tree.
The request is forwarded to (performed on) all objects downwards the tree structure.

The NonTerminalExpression objects (ntExpr1,ntExpr2) forward the request to their child expressions.

The TerminalExpression objects (tExpr1,tExpr2,…) perform the interpretation directly.

UML class diagram

Examples

BNF

The following Backus–Naur form example illustrates the interpreter pattern. The grammar

expression ::= plus , minus , variable , number
plus ::= expression expression '+'
minus ::= expression expression '-'
variable ::= 'a' , 'b' , 'c' , ... , 'z'
digit = '0' , '1' , ... , '9'
number ::= digit , digit number

defines a language that contains Reverse Polish Notation expressions like:

a b +
a b c + -
a b + c a - -

C#

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

using System;
using System.Collections.Generic;

namespace OOP;

class Program

class Context

interface IExpression

abstract class OperatorExpression : IExpression

class EqualsExpression : OperatorExpression

class OrExpression : OperatorExpression

class MyExpression : IExpression

Java

public class Interpreter

While the interpreter pattern does not address parsing, a parser is provided for completeness.

private static Expr parseToken(String token, ArrayDeque stack)
public static Expr parse(String expression)

Finally evaluating the expression "w x z - +" with w = 5, x = 10, and z = 42.

public static void main(final String[] args)
}

PHP (Example 1)

/**
* AbstractExpression
*/
interface Expression

/**
* TerminalExpression
*/
class TerminalExpression implements Expression

/**
* NonTerminalExpression
*/
abstract class NonTerminalExpression implements Expression

/**
* NonTerminalExpression - PlusExpression
*/
class PlusExpression extends NonTerminalExpression

/**
* NonTerminalExpression - MinusExpression
*/
class MinusExpression extends NonTerminalExpression

/**
* Client
*/
class InterpreterClient

// test.php

function loadClass($className)

spl_autoload_register('loadClass');

(new InterpreterClient())->main();
//result: -16

PHP (Example 2)

Based on the example above with another realization of the client.

/**
* Client
*/
class InterpreterClient

JavaScript

As JavaScript is dynamically typed, we do not implement an interface.

// Nonterminal expression
class Plus
// Nonterminal expression
class Minus
// Nonterminal expression
class Times
// Nonterminal expression
class Divide
// Terminal expression
class Number
// Terminal expression
class Variable
// Client
class Parse
var res = new Parse().parse("16 v * 76 22 - -");
console.log(res)
//666

See also

* Backus–Naur form
* Combinatory logic in computing
* '' Design Patterns''
* Domain-specific language
* Interpreter (computing)

References

External links

Interpreter implementation
in Ruby

Interpreter implementation
in C++

SourceMaking tutorial

Interpreter pattern description from the Portland Pattern Repository

{{Design Patterns Patterns

Articles with example C Sharp code
Articles with example Java code
Software design patterns