Support for custom types

[solnic]

This adds support for defining custom types

defmodule FilledString do
  use Drops.Type, string(:filled?)
end

defmodule UserContract do
  use Drops.Contract

  schema do
    %{
      required(:name) => FilledString
    }
  end
end

UserContract.conform(%{name: "Jane Doe"})
# {:ok, %{name: "Jane Doe"}}

{:error, errors} = UserContract.conform(%{name: 1})
Enum.map(errors, &to_string/1)
# ["name must be a string"]

{:error, errors} = UserContract.conform(%{name: ""})
Enum.map(errors, &to_string/1)
# ["name must be filled"]

Types are regular structs:

[%{type: type}]= UserContract.schema().keys
# %FilledString{
#   primitive: :string,
#   constraints: {:and, [predicate: {:type?, :string}, predicate: {:filled?, []}]}
# }

[Sorc96]

Hi! As I mentioned on the Elixir forum, I'm a big fan of your work, and I'd like to get some insight into your thoughts when starting a new library like this one.

Specifically, I'm interested in your reasons for making each custom type into its own module instead of defining them as functions like this:

defmodule UserContract do
  use Drops.Contract

  def filled_string do
    Drops.Type.define({:string, [:filled?]})
  end

  schema do
    %{
      required(:name) => filled_string()
    }
  end
end

EDIT: I guess the function syntax could be improved a bit Drops.Type.new(:string, [:filled?])

[solnic]

@Sorc96 thank you. We need explicit type modules so that it's easier to implement PM-based logic as well as various protocol implementations. If a type was a single struct type, things would get hairy quickly. ie there's already an issue with handling maps because they are too complex, and I clearly see a need for Types.Schema type so that it's handling logic can be better encapsulated.

In general, types are infinitely complex and they must be composable too 😅 I'm modeling things here in a similar way like in dry-types in Ruby, where it works really well. Luckily here, we can just use structs, functions and PM, which is so much simpler and better than complex inheritance hierarchies and various decoration techniques!

Does this makes sense to you?

[Sorc96]

@solnic Thanks for the explanation!