sadjow
Reading the guides at this point http://elixir-lang.org/getting-started/keywords-and-maps.html#nested-data-structures We have an example that we can modify a specific data in a map using the put_in/2.
I’m curious how Elixir makes this work:
iex> users = [
john: %{name: "John", age: 27, languages: ["Erlang", "Ruby", "Elixir"]},
mary: %{name: "Mary", age: 29, languages: ["Elixir", "F#", "Clojure"]}
]
[john: %{age: 27, languages: ["Erlang", "Ruby", "Elixir"], name: "John"},
mary: %{age: 29, languages: ["Elixir", "F#", "Clojure"], name: "Mary"}]
iex> users = put_in users[:john].age, 31
[john: %{age: 31, languages: ["Erlang", "Ruby", "Elixir"], name: "John"},
mary: %{age: 29, languages: ["Elixir", "F#", "Clojure"], name: "Mary"}]
How Elixir returns the whole map? If we do not pass it to the function?
Thanks in advance.
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jmitchell
put_in/2is a macro, not a function. The arguments are passed unevaluated, and theput_in/2implementation operates on their syntactic representations instead. Since the first argument isusers[:john].age, the macro actually does have access tousers.NobbZ
Well,
put_in/2is not a function but a macro, so it can do any magic it wants with the AST of your first parameter before expanding itself.You can find the definiton on github: elixir/lib/elixir/lib/kernel.ex at main · elixir-lang/elixir · GitHub
sadjow
Thanks @jmitchell and @NobbZ ! Awesome. I’ll read more about the macros to understand how it works!
jmitchell
A good place to start is the Quote and unquote guide, and then the following section on macros. You may also find it helpful to study how the short-circuiting or AST macro works before trying to tackle
put_in/2. In particular, notice that the right side is only evaluated when the left side is falsy, whereas if||were a function all of its arguments would be evaluated before the function body.