An abstract syntax tree (AST) is a tree representation of the abstract syntactic structure of the source code written in a programming language. When Julia code is parsed by its LLVM JIT compiler, it is internally represented as an abstract syntax tree. The nodes of this tree are simple data structures of type expression Expr.

(For more information on abstract syntax trees, refer to http://en.wikipedia.org/wiki/Abstract_syntax_tree).

An expression is simply an object that represents Julia code. For example, 2 + 3 is a piece of code, which is an expression of type Int64 (follow along with the code in Chapter 7\expressions.jl). Its syntax tree can be visualized as follows:

To make Julia see this as an expression and block its evaluation, we have to quote it, that is, precede it by a colon (:) as in :(2 + 3). When you evaluate :(2 + 3) in the REPL, it just returns :(2 + 3), which is of type Expr: typeof(:(2 + 3)) returns Expr. In fact, the : operator (also called the quote...