Macros Are Hygienic
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 ... |
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