Interpreter Pattern Behavioural

Transcription

1 Interpreter Pattern Behavioural Intent» Given a language, define a representation for tis grammar along with an interpreter that uses the representation to interpret sentences in the language Interpreter-1

2 Motivation If problem type occurs often enough, then it is worthwhile to express instances of the problem as sentences in a language You build an interpreter that solves the problem by interpreting those sentences Interpreter-2

3 Example Searching for Substrings Searching for patterns within a string is a common problem Rather than construct a custom algorithm for each pattern» Express a pattern as a regular expression» Interpret the regular expression against the string to search The Interpreter pattern describes» How to define a grammar for languages» Represent sentences in the language» Interpret those sentences Interpreter-3

4 Example Grammar Expression ::= ( Literal, Alternation, Sequence, Repetition, ʻ(ʼ Expression ʻ)ʼ ) ; Alternation ::= Expression ʻ ʼ Expression ; Sequence ::= Expression ʻ&ʼ Expression ; Repetition ::= Expression ʻ*ʼ ; Literal ::= +( ʻaʼ, ʻbʼ, ʻcʼ,, ʻzʼ ) ; Legend Space means ʻandʼ, (comma) means ʻorʼ ( ) (must choose) means choose one +( ) means choose one or more [ ] (optional) means choose one or none +[ ] means choose zero or more Interpreter-4

5 Example Architecture Interpreter-5

6 Example Abstract Syntax Tree Interpreter-6

7 Abstract Architecture Interpreter-7

8 Scenario Scenario: Interpret 1 Build / get abstract syntax tree 2 Interpret the tree CLIENT 1 2 ABSTRACT_SYNTAX_TREE INTERPRET Interpreter-8

9 Participants AbstractExpression» Declares an abstract Intepret operation that is common to all nodes in the abstract syntax tree TerminalExpression» Implements an Interpret operation associated with terminal symbols in the grammar Context» Contains information thatʼs global to the interpreter Interpreter-9

10 Participants 2 NonTerminalExpression» One class required for each non-terminal in the grammar» Maintains instance variables for sub-expressions» Implements an Interpret operation for nonterminal symbols in the grammar Client» Builds / gets an abstract syntax tree for a sentence in the language» Invokes the interpret operation Interpreter-10

11 Applicability Use when there is a language to interpret and you can build an abstract syntax tree to represent a sentence in the language Works best when» Grammar is simple > For complex grammars use parser generators» Efficiency is not a critical concern > Efficient interpretation is best done by translating AST to another form E.g. a state machine > Can use the pattern to do the translation Interpreter-11

12 Consequences It is easy to change and extend the grammar» Use inheritance» Can build incrementally Implementing the grammar is easy» Classes representing nodes in ASTs have similar implementations, making them easy to define Interpreter-12

13 Consequences 2 Complex grammars are hard to maintain» Interpret needs at least one class for every rule in the grammar > Rules in BNF may require more than one class per rule» The number of classes and variation make it difficult to use the pattern Easy to add new way to interpret expressions» Support pretty printing and type checking by adding new operations to the classes > If the number of new operations is likely to keep growing, then consider use of the Visitor pattern Interpreter-13

14 Related Patterns Composite is used to represent an abstract syntax tree Flyweight shows how to share terminal symbols within the abstract syntax tree The interpreter can use an Iterator to traverse the structure Visitor can be used to maintain the behaviour in each node in the abstract syntax tree in one class Interpreter-14

15 Interpreter in Java API The Container class in java.awt aggregates Component objects. Interpretation is applied in Container by its validatetree method, which recursively descends the container tree and recomputes the layout for any subtrees Interpreter-15