Interpreter

The Interpreter design pattern defines how to implement an interpretation of a language using the object-oriented programming.

The Interpreter design pattern defines how to implement an interpretation of a language using the object-oriented programming. It can be an interpretation of our own programming language or e.g. evaluation of a mathematical expression. The pattern solves how to represent the language code in memory as a set of objects and how to execute it later. On the contrary, it does not solve the problem of parsing the source code into this object model, it only describes how the model should look like and how to evaluate it later. We'll show a working example of parsing mathematical expressions.

Motivation

Although there might be already too many programming languages out there today, sometimes we can still get into a situation where it's useful to implement our own meta-language. E.g. a Spanish SQL dialect for company scripts, which even people without English knowledge will be able to use, and will have a simpler grammar. The code in such a language might look like Actualizar personas establece nombre "John" donde nombre es "Johnny". Then we'd like to process the code in this language and interpret it to the code of another language, e. g. to execute real SQL statements based on this Spanish database language. This code would be translated into UPDATE personas SET nombre="John" WHERE nombre="Johnny". For the ideal implementation of our interpreter, we don't have to go far, it's one of the GOF patterns.

Pattern

Interpreter introduces 2 basic language expression types:

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