What are some good resources for learning to write an interpreter?

A good way to start is with a tiny calculator language that handles expressions such as 3+5 and 6-2. Tools like Lex and Yacc, or newer alternatives such as Bison and ANTLR, can help you learn the lexer-and-parser side without trying to build everything at once. Once that works, add variables, functions, and other features gradually.

If you’re interested in how very small interpreters work internally, Threaded Interpretive Languages explores the Forth family and explains how an operating system, extensible high-level language, and low-level interpreter can fit together in a remarkably small system. Starting Forth and Thinking Forth are related resources that may also be useful, though this material is more specialized than a general beginner guide.

Writing an Interpreter in Go by Thorsten Ball is a practical, hands-on option. The classic Compilers: Principles, Techniques, and Tools—often called the Dragon Book—is also worth knowing about, although it can be fairly dense and may be better as a reference than as a first introduction.

Writing Interactive Compilers and Interpreters by Brown is another useful recommendation and is aimed at learning by building. Start with a four-function calculator using simple variables, then expand it in stages. Trying to understand parsing, interpretation, optimization, and code generation all at once can be overwhelming, so it’s fine to focus on the basics first.