Domo

This is an experimental library to play with for fun and joy.

To give the example app a try:
Clone this repo
Switch to the master version of the Elixir with asdf local elixir master
Change to example_app directory and follow instructions from README.md

⚠️ Preview, requires Elixir 1.11.0-dev to run

Domo is a library for defining custom composable types for fields of a struct to make these pieces of data to flow through the app consistently.

The library aims for two goals:

to model a business domain entity's possible valid states with custom types for fields of struct representing the entity
to verify that entity structs are assembled to one of the allowed valid states at compile-time

It's a library to define what piece of data is what and make a compiler and dialyzer to cover one's back, reminding about taken definitions.

The validation of the incoming data is on the author of the concrete application. The library can only ensure the consistent processing of that valid data throughout the system.

The library has the means to build structs and depends on the TypedStruct to do so.

Rationale

To model a business domain entity, one may define a named struct with several fields of primitive data types. Construction of the struct from parsed data can look like this:

%Order{
  id: "156",
  quantity: 2.5
  note: "Deliver on Tue"
}

and modification of the struct's data can be done with a function of the following signature:

@spec put_quantity(Order.t(), float()) :: Order.t()
def put_quantity(order, quantity) ...

Primitive types of binary and float are universal and have no relation to the Order struct specifically. That is, any data of these types can leak into the new struct instance by mistake. The float type defining quantity reflects no measure from the business domain. Meaning, that a new requirement - to measure quantity in Kilograms or Units makes space for misinterpretation of the quantity field's value processed in any part of the app.

How about some domain modeling?

In the context given above, it'd be great to define a contract to allow only valid states for Order struct fields, that enables:

local reasoning about the relation of value to the struct in any nested function of the app
compile-time verification of assembling/updating of the structure from values that relates only to it

One possible valid way to do this is to use Domo library like the following:

defmodule Order do
  use Domo

  deftag Id, for_type: String.t()

  deftag Quantity do
      for_type __MODULE__.Kilograms.t() | __MODULE__.Units.t()

      deftag Kilograms, for_type: float
      deftag Units, for_type: integer
  end

  deftag Note, for_type: :none | String.t()

  typedstruct do
    field :id, Id.t()
    field :quantity, Quantity.t()
    field :note, Note.t(), default: Note --- :none
  end
end

Then the construction of the struct becomes like this:

Order.new!(%{
  id: Id --- "156",
  quantity: Quantity --- Kilograms --- 2.5
  note: Note --- "Deliver on Tue"
})

And a signature of a custom function to modify the struct becomes like this:

@spec put_quantity(Order.t(), Order.Quantity.t()) :: Order.t()
def put_quantity(order, Quantity --- Units --- units) ...
def put_quantity(order, Quantity --- Kilograms --- kilos) ...

Thanks to the Domo library, every field of the structure becomes a tagged tuple consisting of a tag and a value. A tag is a module itself. Several tags can be nested, defining valid tag chains as a shape for values of primitive type. That makes it possible to do pattern matching against the shape of the struct's value. That enables the dialyzer to validate contracts for the structure itself and the structure's field values.

Usage

Setup

To use Domo in your project, add this to your Mix dependencies:

{:domo, "~> 0.0.7"}

To avoid mix format putting parentheses on tagged tuples definitions made with ---/2 operator, you can add to your .formatter.exs:

[
  ...,
  import_deps: [:typed_struct]
]

General usage

Define a tag

To define a tag on the top level of a file import Domo, then define the tag name and type associated value with deftag/2 macro.

import Domo

deftag Title, for_type: String.t()
deftag Height do
  for_type: __MODULE__.Meters.t() | __MODULE__.Foots.t()

  deftag Meters, for_type: float
  deftag Foots, for_type: float
end

Any tag is a module by itself. Type t() of the tag is a tagged tuple. When defining a tag in a block form, you can specify the associated value type through the for_type/1 macro.

To add a tag or a tag chain to a value use ---/2 macro.

alias Height.{Meters, Foots}

t = Title --- "Eiffel tower"
m = Height --- Meters --- 324.0
f = Height --- Foots --- 1062.992

Under the hood, the tag chain is a series of nested tagged tuples where the value is in the core. Because of that, you can use the ---/2 macro in pattern matching.

{Height, {Meters, 324.0}} == m

@spec to_string(Height.t()) :: String.t()
def to_string(Height --- Meters --- val), do: to_string(val) <> " m"
def to_string(Height --- Foots --- val), do: to_string(val) <> " ft"

Each tag module has type t() of tagged tuple with the name of tag itself and a value type specified with for_type. Use t() in the function spec to inform the dialyzer about the tagged argument.

Define a structure

To define a structure with field value's contracts, use Domo, then define your struct with a typedstruct/1 block.

defmodule Order do
  use Domo

  deftag Id, for_type: String.t()
  deftag Note, for_type: :none | String.t()

  @typedoc "An Order from Sales context"
  typedstruct do
    field :id, Id.t()
    field :note, Note.t(), default: Note --- :none
  end
end

Each field is defined through field/3 macro. The generated structure has all fields enforced, default values specified by default: key, and type t() constructed with field types. See TypedStruct library documentation for implementation details.

Use new!/1 and put!/3 functions that are automatically defined for the struct to create a new instance and update an existing one.

alias Order
alias Order.{Id, Note}

%{id: Id --- "o123556"}
|> Order.new!()
|> Order.put!(:note, Note --- "Deliver on Tue")

The dialyzer can check if properly tagged values are passed as parameters to these functions. The new!/1 function can be overridden to make data validations.

After the module compilation, the Domo library checks if all tags that are used with the ---/2 operator are defined and appropriately aliased. The following options can be passed with use Domo, ...

Options

* `undefined_tag_error_as_warning` - if set to true, prints warning
  instead of raising an exception for undefined tags.

* `no_field` - if set to true, skips import of typedstruct/1
  and field/3 macros, useful with the import of the Ecto.Schema
  in the same module.

Reflexion

Each struct or tag defines __tags__/0 function that returns a list of tags defined in the module. Additionally each tag module defines __tag__?/0 function that returns true.

For example:

iex.(1)> defmodule Order do
....(1)>   use Domo
....(1)>
....(1)>   deftag Id, for_type: String.t()
....(1)>
....(1)>   deftag Quantity do
....(1)>      for_type __MODULE__.Kilograms.t() | __MODULE__.Units.t()
....(1)>
....(1)>      deftag Kilograms, for_type: float
....(1)>      deftag Units, for_type: integer
....(1)>   end
....(1)>
....(1)>   deftag Note, for_type: :none | String.t()
....(1)>
....(1)>   typedstruct do
....(1)>     field :id, Id.t()
....(1)>     field :quantity, Quantity.t()
....(1)>     field :note, Note.t(), default: Note --- :none
....(1)>   end
....(1)> end
{:module, Order,
<<70, 79, 82, 49, 0, 0, 17, 156, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 1, 131,
  0, 0, 0, 41, 12, 69, 108, 105, 120, 105, 114, 46, 79, 114, 100, 101, 114, 8,
  95, 95, 105, 110, 102, 111, 95, 95, 7, ...>>,
[put!: 3, put!: 3, put!: 3, put!: 3, put!: 3]}
iex.(2)> Order.__tags__
[Order.Id, Order.Quantity, Order.Note]
iex.(3)> Order.Id.__tag__?
true

Pipeland

To add a tag or a tag chain to a value in a pipe use tag/2 macro and to remove use untag!/2 macro appropriately.

For instance:

import Domo
alias Order.Id

identifier
|> untag!(Id)
|> String.graphemes()
|> Enum.intersperse("_")
|> Enum.join()
|> tag(Id)

Contributing

Fork the repository and make a feature branch
Working on the feature, please add typespecs
After working on the feature format code with
```
mix format
```
run the tests and static analyzers to ensure that all works as expected with
```
mix test && mix dialyzer
```
and make sure that the code coverage is ~100% what can be seen with
```
mix coveralls.html
```
Make a PR to this repository

Roadmap

It's still possible to construct a struct with a new/1 function with fields of the wrong shape at runtime overlooking the dialyzer warnings. Check if the field values passed as argument to the new/1 matches the shape that is defined in typedstruct/1.
Add keyword list as a possible argument for new!/1 (probably need to update credo to support lists with required key-value pairs).

License

This project is licensed under the MIT license.