First, the thousands characters (the leading “M” characters) are read. For instance, the BooleanExpression (X AND NOT Y) OR Z in the context of (X,Y,Z) = (true, false, false) evaluates to eval(X AND NOT Y) OR eval(Z), which is (eval(X) AND eval(NOT Y)) OR false, which evaluates to (true AND NOT eval(Y)), which is NOT(false), resulting in an evaluation of true.Įxample: Roman Numerals Chapter 5 – Reading left to right, a Roman Numeral can be interpreted via four “sub-interpreters”. A BooleanExpression is interpreted (or “interpreted) in the context of its variable names and values.

One CompoundExpression class is required for every rule in the grammar, each one implementing an Interpret operation for nonterminal symbols in the grammar.Įxample: Boolean Expressions Chapter 5 – A BooleanExpression is either a terminal expression (a Constant) or a compound expression (an AND, OR, or NOT Expression).

The Client invokes the interpret operation and builds (or is given) an abstract syntax tree, assembled from CompoundExpressions and TerminalExpressions) representing a particular sentence in the language that the grammar defines.

The Context contains information that is global to the interpreter.

The TerminalExpression implements an Interpret operation associated with terminal symbols in the grammar.

The Interpreter Pattern Chapter 5 – The AbstractExpression defines an interface for executing an operation. The Interpreter Pattern generally describes defining a grammar for that language and using that grammar to interpret statements in that language. Behavioral Pattern: Interpreter Chapter 5 – Some programs benefit from having a language to describe operations that they can perform.

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