nimcity/doc/part03/ch34.md

Chapter 34. AST creation

An AST can be created in different ways and these ways can all be combined freely. But one of the easiest ways is to use quote do. For example, an operator ==~ that checks if two floating point values almost equal can be written as a template: template ==~(x, y: float): bool = abs(x - y) < 1e-9 Or it can be written as a macro that uses quote do: import std / macros macro ==~(x, y: float): bool =

result = quote do:
  abs(`x` - `y`) < 1e-9

quote do turns a pattern of code into a NimNode. Inside the pattern backticks can be used to access symbols from the macro’s scope. The ==~ macro can also be written as: import std / macros macro ==~(x, y: float): bool =

result = newCall(bindSym"<",
  newCall(bindSym"abs", newCall(bindSym"-", x, y)),
  newLit(1e-9))

In my opinion this more imperative style of AST creation is easier to understand for beginners and so it is what is used in the following more complex examples.