The Interpreter Pattern Defining the Interpreter l Intent

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The Interpreter Pattern

The Interpreter Pattern

Defining the Interpreter l Intent l Given a language, define a representation for its

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

Pieces of the Interpreter l Abstract Expression l Terminal Expressions l Non. Terminal Expression

Pieces of the Interpreter l Abstract Expression l Terminal Expressions l Non. Terminal Expression l Context l Hold global information for the interpreter l Client Builds the Abstract Syntax Tree l Invokes Interpret on the tree l

Flow of the Interpreter l Build the Abstract Syntax Tree l Interpret the tree

Flow of the Interpreter l Build the Abstract Syntax Tree l Interpret the tree in the given Context Each non-terminal node will interpret its children and return a result from that. l Terminal nodes return actual values for nonterminal nodes to use. l The context serves as a global data for interpreting the entire tree l

Example Language: Arithmetic l Grammar l Equation : : = Variable | Integer |

Example Language: Arithmetic l Grammar l Equation : : = Variable | Integer | Plus | Minus | Times | Divide | Mod | Negate | ‘(‘ Eq ‘) l Terminal equations l l l Variable: : = ‘a’ | ‘b’ | ‘c’ …. |”ab” | … Integer: : = 1 | 2 | 3 | 4 | 5 | 6 | … Non-Terminal equations l Plus: : = Eq ‘+’ Eq Minus: : = Eq ‘-’ Eq Times: : = Eq ’*’ Eq Divide: : = Eq ‘-’ Eq Mod: : = Eq ‘%’ Eq …Negate: : ‘-’ Eq

Abstract Syntax Tree Example l 5 *2+B

Abstract Syntax Tree Example l 5 *2+B

Consequences l Easily extensible grammar l Implementing grammar is easy l Complex grammar is

Consequences l Easily extensible grammar l Implementing grammar is easy l Complex grammar is difficult to manage l Can easily add new ways to interpret expressions through the Visitor.

Related Patterns l Composite l l Interator l l Can be useful for traveling

Related Patterns l Composite l l Interator l l Can be useful for traveling the AST Visitor l l The Composite pattern is essential for representing the AST Essential for implementing multiple ways of interpreting the AST Flyweight l Especially useful when terminal symbols are used repetitively

Common uses l Widely used in compilers that are implemented with an OO language

Common uses l Widely used in compilers that are implemented with an OO language l Any case where you use the Composite Pattern to represent a language

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Intent Intent Activity Activity Intent Filter Intent ResolutionIntent Intent Activity Activity Intent Filter Intent Resolution
Intent Intent Activity Activity Intent Filter Intent ResolutionIntent Intent Activity Activity Intent Filter Intent Resolution
Intent Filter Activity Notification Intent Intent Filter IntentIntent Filter Activity Notification Intent Intent Filter Intent
Ch 23 Interpreter Pattern Ch 23 Interpreter PatternCh 23 Interpreter Pattern Ch 23 Interpreter Pattern
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Bridge Design Pattern Name Bridge Design Pattern IntentBridge Design Pattern Name Bridge Design Pattern Intent
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