# Interpreter
### Introduction
* Interpreter is a design pattern that belongs to the Behavioral Pattern group.
* The Interpreter Pattern helps programmers "build" "dynamic" objects by reading a description of the object and then "building" the object according to that description.
* For example, write a program that allows users to enter the command line according to a pre-specified structure. The program will recognize the Command based on its structure and return appropriate results.
### Architecture
Components in the model:
* **AbstractionExpression:** Declare an interface for performing an operation.
* **TerminalExpression:** Implements an interpretation operation associated with terminal symbols, acting as a required expression for all terminal symbols in a sentence.
* **NonterminalExpression:** Can contain an internal TerminalExpression and can also contain another NonterminalExpression. It serves as the “grammar” of the specification language.
* **Context** : Is the information object to perform interpretation. This object is global to the interpretation process (shared between nodes).
### Advantages & disadvantages
**Advantage**
* Reduce dependency between abstraction and implementation (loose coupling).
* Reduce the number of unnecessary subclasses.
* The code will be cleaner and the application size will be smaller.
* Easier to maintain.
* Easy to expand later.
* Allows you to hide implementation details from the client.
**Disadvantage**
* The constructed specification language requires a simple grammatical structure.
* Performance is not guaranteed
### When to use it
Use Interpreter Pattern when we want:
* Simple grammar set. This pattern needs to define at least one class for each rule in the grammar. Therefore grammars containing many rules can be difficult to manage and maintain.
* Don't care much about performance. Because the grammar is analyzed in a hierarchical structure (tree), performance is not guaranteed.
* The Interpreter Pattern is often used in compilers, defining grammars, rules, SQL, XML parsers, etc.
### Related Design Patterns
* The abstract syntax tree is an implementation of the **Composite** pattern .
* Interpreters often use an **Iterator** to traverse the structure.
* Visitor can be used to maintain behavior on each node in the class's abstract syntax tree.
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