ABSTRACT
The focus of chapter 18 chapter are the data structures that underlie expressions. Mastering this knowledge will allow you to inspect and modify captured code, and to generate code with code. We’ll come back to expr() in Chapter 19 (“Quasiquotation”), and to eval() in Chapter 20 (“Evaluation”).
“Abstract syntax trees” introduces the idea of the abstract syntax tree (AST), and reveals the tree like structure that underlies all R code. “Expressions” dives into the details of the data structures that underpin the AST: constants, symbols, and calls, which are collectively known as expressions. “Parsing and grammar” covers parsing, the act of converting the linear sequence of character in code into the AST, and uses that idea to explore some details of R’s grammar. “Walking AST with recursive functions” shows you how you can use recursive functions to compute on the language, writing functions that compute with expressions. “Specialised data structures” circles back to three more specialised data structures: pairlists, missing arguments, and expression vectors.
