Can We Have Form Objects in Elixir?

Holy crap. That's amazing. Thanks for sharing. Been digging into Elixir off and on for months, and hadn't come across this. Mind-blowing, indeed.

This code snippet is a little bit confusing if you aren't already familiar with Elixir, because most of the syntax you're seeing is actually from Ecto's [0] DSL. Calling "use Ecto.Schema" at the top of the module brings some extra functionality into the current scope. For example, embedded_schema is a macro exported by Ecto.Schema [1].

embeds_many is another such macro [2], that allows you embed another schema, in-line, into the current schema. This is contrast to your more common has_many relationship, which references another table entirely.

Here, :string, :map, etc are atoms that are being passed to the field macro [3], to define the schema for the table.

[0] Basically a super lightweight ActiveRecord

[1] https://github.com/elixir-ecto/ecto/blob/b030353d4b94ddd4216...

[2] https://github.com/elixir-ecto/ecto/blob/b030353d4b94ddd4216...

[3] https://github.com/elixir-ecto/ecto/blob/b030353d4b94ddd4216...

It's confusing because it's using a library called Ecto, which provides it's own DSL for describing schemas and queries.

NOTE: I haven never used Ecto, but I have used Elixir for a few hobby projects.

1. `embeds_many` is a function name, described here: https://hexdocs.pm/ecto/Ecto.Schema.html#embeds_many/3

2. Anything that begins with a colon is an Atom in Elixir, including :string, :map, :changes, :field, and :value. These atoms are used by Ecto at runtime to identify certain things. `:changes` is a unique name given to the embedded schema, `:field` and `:value` are field names on that schema, and :string and :map are informing Ecto of how to treat certain values.

3. `Change` is the name of a Module which describes the Schema we are embedding

4. `primary_key: false` is a configuration option

5. Everything after `do...` is describing what the embedded schema would look like. So it has a `:field` field that is of type string, and a `:value` field that is of type map.

Hopefully that makes some sense... I'm not totally clear on it either (having not used Ecto before) but that's what I'm grokking.

Others provided an explanation of the Ecto DSL, I just wanted to add the explanation of basic syntax. It goes like this.

In Elixir, the most basic function application looks fairly standard:

    func_name(arg1, arg2, ...)

But the parens are optional, so you can write:

    func_name arg1, arg2, ...

as well.

Next, in Erlang and Elixir function arity (number of arguments it takes) is always fixed (i.e. no varargs or Python's *args). Because of this you generally have to pack your arguments in a list; functions like `printf` work that way:

    iex(2)> :io.format("fmt ~p foo: ~p bar: ~p", [1, 3, 4])
    fmt 1 foo: 3 bar: 4

That takes care of (positional) varargs. There are some other functions, however, which would benefit from "keyword arguments", which also isn't directly supported. To get around this, you can take a list of pairs (keyword, value) as one of your arguments (usually last):

     some_func(arg1, [{:keyword1, value}, {:keyword2, value}])

Where `{}` creates a tuple and `:` creates an atom (called symbols in Lisp), which you can think of as a special[1] string value. This pattern is so common, that Elixir provides a syntactic sugar for it. The above is equivalent to:

     some_func(arg1, [keyword1: value, keyword2: value])

The brackets around keyword list are also optional, but only inside function call. The most sugar-y way to write this looks like this:

     some_func arg1, keyword1: value, keyword2: value

The last thing of note is that the `do ... end` is just a syntactic sugar for `[do: ...]`, where `...` is treated as a block of code.

Normally, the block of code would be evaluated and only last value would be passed to the function:

     some_fun arg1, kwarg1: 1, kwarg2: 3 do 1; 2; 3 end   # notice no comma before `do`

would be equivalent to (after all the sugar being resolved):

     some_fun(arg1, [{:kwarg1, 1}, {:kwargs2, 2}], [{:do, 3}])

However, when writing a macro, the macro gets the unevaluated block, which then can be modified or executed. See http://elixir-lang.org/getting-started/meta/macros.html for more info on that.

Edited to add: Capitalized names in Elixir are actually special atoms, mostly used for naming modules. It works like this:

    iex(5)> :'Elixir.Change' == Change
    true

There is more going on because of the fact you can alias module names, but mostly you can think about this as (another) special syntax for atoms.

[1] Allocated once and cached. When working with normal strings:

    a = "Asd"
    b = "Asd"

you have no guarantee that `a` and `b` point to the same thing in memory, while with atoms you do have that guarantee. This makes comparing them efficient.

You still need server side validation so a form object with validators is still useful.