Btw, there are a number of ways to write that expression depending on the complexity of the function

Short syntax:

   Enum.map [1,2,3], &(&1 * &1)

Long syntax:

   Enum.map [1,2,3], (fn num ->
     num * num
   end)

Or even:

   square_me = &(&1 * &1)
   Enum.map [1,2,3], square_me

   square_me_too = fn(num) -> num * num end
   Enum.map [1,2,3], square_me_too

Defining a function separately (e.g. even in another module) and referencing it:

   def square(num) do
      num * num
   end

   def doing_something() do
      ...

      Enum.map [1,2,3], &square/1
      Enum.map [1,2,3], &SomeOtherModule.square/1

      ...
   end