It is tempting to think of macros as some kind of textual substitution—a macro’s body is expanded as text and then compiled at the point of call. But that’s not the case. Consider this example:
| | defmodule Scope do |
| | defmacro update_local(val) do |
| | local = "some value" |
| | result = quote do |
| | local = unquote(val) |
| | IO.puts "End of macro body, local = #{local}" |
| | end |
| | IO.puts "In macro definition, local = #{local}" |
| | result |
| | end |
| | end |
| | defmodule Test do |
| | require Scope |
| | |
| | local = 123 |
| | Scope.update_local("cat") |
| | IO.puts "On return, local = #{local}" |
| | end |
Here’s the result of running that code:
| | In macro definition, local = some value |
| | End of macro body, local = cat |
| | On return, local = 123 |
If the macro body were just substituted in at the point of call, both it and the module Test would share the same scope, and the macro would overwrite the variable local, so we’d see
| | In macro definition, local = some value |
| | End of macro body, local = cat |
| | On return, local = cat |
But that isn’t what happens. Instead, the macro definition has both its own scope and a scope during execution of the quoted macro body. Both are distinct from the scope within the Test module. The upshot is that macros will not clobber each other’s variables or the variables of modules and functions that use them.
The import and alias functions are also locally scoped. See the documentation for quote for a full description. This also describes how to turn off hygiene for variables and how to control the stack trace’s format if things go wrong while executing a macro.