I want to create a function names dynamically. I wrote this macro
defmacro generate_dynamic(name) do
quote do
def add_(unquote(name)) do
end
end
end
And I used it like this:
defmodule AnotherModule do
generate_dynamic :animal
end
Now, I only get AnotherModule.add_ function defined, whereas I expect AnotherModule.add_animal function.
To achieve this, you can prepend :add_ to the name before unquoting. Also, the parentheses after the method name in this case are required to prevent ambiguities. This should do the trick:
defmacro generate_dynamic(name) do
quote do
def unquote(:"add_#{name}")() do
# ...
end
end
end
Sometimes, as a useful shortcut, you can achieve the same result inline, without writing a macro using an unquote fragment.
defmodule Hello do
[:alice, :bob] |> Enum.each fn name ->
def unquote(:"hello_#{name}")() do
IO.inspect("Hello #{unquote(name)}")
end
end
end
Hello.hello_bob # => "Hello bob"
Hello.hello_alice # => "Hello alice"
unquote only worked from inside a macro. How come it's working inside an anonymous function?unquotes that are outside of a quote block.I did this same sort of thing in a gist to try and mimic Ruby's attr_accessor:
defmodule MacroExp do
defmacro attr_accessor(atom) do
getter = String.to_atom("get_#{atom}")
setter = String.to_atom("set_#{atom}")
quote do
def unquote(getter)(data) do
data |> Map.from_struct |> Map.get(unquote(atom))
end
def unquote(setter)(data, value) do
data |> Map.put(unquote(atom), value)
end
end
end
defmacro attr_reader(atom) do
getter = String.to_atom("get_#{atom}")
quote do
def unquote(getter)(data) do
data |> Map.from_struct |> Map.get(unquote(atom))
end
end
end
end
defmodule Calculation do
import MacroExp
defstruct first: nil, second: nil, operator: :plus
attr_accessor :first # defines set_first/2 and get_first/1
attr_accessor :second # defines set_second/2 and get_second/1
attr_reader :operator # defines set_operator/2 and get_operator/1
def result(%Calculation{first: first, second: second, operator: :plus}) do
first + second
end
end
