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.
Wikipedia: Interpreter Pattern
Example
abstract class Expression {
Number number;
void interpret(int value);
}
class Add implements Expression {
Number number;
Add(this.number);
void interpret(int value) {
print("Adding $value...");
number.value += value;
}
}
class Subtract implements Expression {
Number number;
Subtract(this.number);
void interpret(int value) {
print("Subtracting $value...");
number.value -= value;
}
}
class Number {
int value;
Number(int value) {
print("Starting with $value...");
this.value = value;
}
}
void main() {
var number = Number(0);
var adder = Add(number);
var subtracter = Subtract(number);
adder.interpret(100);
subtracter.interpret(60);
adder.interpret(2);
assert(number.value == 42);
print("And the result is ${number.value}!");
}