Elixir's anonymous function written in external .ex file can't run with interactive shell

Question

I can write an anonymous function in the interactive shell directly like below.

iex> total_bottles_milk = fn total -> total * 2 end  
iex> total_bottles_milk.(2)

However, if I write in an external file and run in the interactive shell, it shows a Compile Error.

My file name and directory path is lib/expense.ex

Below is my code

defmodule Expense do

    total_bread_slices = fn total -> (total * 10) / 100 end
    total_bottles_milk = fn total -> total * 2 end
    total_cakes = fn total -> total * 15 end

    def total_expense(bread_slices, bottles_of_milk, cakes) do
        total_bread_slices.(bread_slices) + total_bottles_milk.(bottles_of_milk) + total_cakes.(cakes)
    end

end

When I go into the folder path and run iex -S mix to run my Expense module, terminal shows Compilation error.
I'm wondering only I can run anonymous function directly into the interactive shell and not from external sources and compile it. I want to write my function as a first-class citizen. And if there is a way, how can I do it?

Answer 1

You cannot create "variables" like this in elixir (see EDIT 1 and Edit 2 below). The error you're seeing is normal.

You can put your anonymous functions into named functions and call them from there which would give you the same result:

defmodule Expense do

  def total_expense(bread_slices, bottles_of_milk, cakes) do
    total_bread_slices().(bread_slices) + total_bottles_milk().(bottles_of_milk) + total_cakes().(cakes)
  end

  defp total_bread_slices, do: fn total -> (total * 10) / 100 end
  defp total_bottles_milk, do: fn total -> total * 2 end
  defp total_cakes, do: fn total -> total * 15 end
end

This way you're calling the named function which will return the anonymous function which then you pass the arguments to.

EDIT 1

You cannot create variables like that INSIDE modules. This works in iex because it's an interactive environment. However, the x = y syntax in invalid outside of a function in an elixir module.

EDIT 2 Thanks to a correction from @Dogbert. You can actually create variables inside modules and out of functions, but you cannot use them inside def.

Answer 2

Thanks to @Abdullah Esmail, I can write my function like this

defmodule Expense do

    def total_bread_slices do
        fn total -> (total * 10) / 100 end
    end 

    def total_bottles_milk do
        fn total -> total * 2 end
    end 

    def total_cakes do
        fn total -> total * 15 end
    end 

    def total_expense(bread_slices, bottles_of_milk, no_of_cakes) do

        bread = total_bread_slices()
        milk = total_bottles_milk()
        cakes = total_cakes()

        bread.(bread_slices) + milk.(bottles_of_milk) + cakes.(no_of_cakes)
    end

end

Answer 3

Thanks to @Dogbert, here is another method if I want to use the function as a value.

Firstly, I define a named function and then using Elixir's function capturing operator &, I can more easily use a named function as a value.

In this way, using the & operator to capture a reference to the function and the = operator to bind to the variable, I can bind the named function to a variable.

defmodule Expense do

    defp total_bread_slices(total) do
        (total * 10) / 100
    end 

    defp total_bottles_milk(total) do
        total * 2 
    end 

    defp total_cakes(total) do
        total * 15 
    end 

    def total_expense(bread_slices, bottles_of_milk, no_of_cakes) do

        bread = &total_bread_slices/1
        milk = &total_bottles_milk/1
        cakes = &total_cakes/1

        bread.(bread_slices) + milk.(bottles_of_milk) + cakes.(no_of_cakes)

    end

end

Answer 4

I had the same problem during the running examples from Elixir In Action book, second edition. To avoid retyping lambda function examples in iex, I saved them in a file. I tried to load them in iex, e.g:

iex solutions/ch05.ex

but when I called lambda function:

a_lambda.("something")

I got a compilation error in iex.

Now I copy/paste lambda code from the file directly to Iex, and it works.