Interpreter

• Also known as: Little Language
• A way to include language elements in a program
• Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language

AbstractExpression

• Declares an interface for executing an operation

TerminalExpression

• Implements an Interpret operation associated with terminal symbols in the grammar.
• An instance is required for every terminal symbol in the sentence.

NonterminalExpression

• 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

• Contains information that is global to the interpreter.

Client

• 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.