DataSpecs

Typespec based data cast and validator (inspired by forma).

DataSpecs cast elixir data into a more structured form by trying to map it to conform to a typespec.

It support the following typespec specifications:

• basic types
• literal types
• built-in types
• range types
• union types
• parametrized types
• map (and elixir struct) types
• remote types
• user defined types

The main use cases are about elixir data validatation against a typespec or interfacing with external data sources that provide you data as JSON or MessagePack, but that you wish to validate and transform into either proper structs or richer data types without a native JSON representation (such as dates or sets).

Usage

Given the following Person struct specification

defmodule Person do
  use DataSpecs

  @enforce_keys [:name, :surname]
  defstruct @enforce_keys ++ [:gender, :address, :birth_date]

  @type t :: %__MODULE__{
               name: String.t(),
               surname: String.t(),
               gender: option(:male | :female | :other),
               address: option([Address.t(), ...]),
               birth_date: option(Date.t())
             }

  @type option(x) :: nil | x
end

defmodule Address do
  @enforce_keys [:streetname, :streenumber, :postcode, :town]
  defstruct @enforce_keys

  @type t :: %__MODULE__{
               streetname: String.t(),
               streenumber: String.t(),
               postcode: String.t(),
               town: String.t()
             }
end

we can cast a JSON object encoding an instance of a Person with:

~s/{
  "name": "Joe",
  "surname": "Smith",
  "gender": "male",
  "birth_date": "1980-12-31",
  "address": [{
    "streetname": "High Street",
    "streenumber": "3a",
    "postcode": "SO31 4NG",
    "town": "Hedge End, Southampton"
  }]
}/
|> Jason.decode!()
|> Person.cast()

# => %Person{
#      address: [
#        %Address{
#          postcode: "SO31 4NG",
#          streenumber: "3a",
#          streetname: "High Street",
#          town: "Hedge End, Southampton"
#        }
#      ],
#      birth_date: ~D[1980-12-31],
#      gender: :male,
#      name: "Joe",
#      surname: "Smith"
#    }

DataSpecs tries to figure out how to translate its input to an elixir datatype using the typespec as "type schema".

Scalar types (such as booleans, integers, etc.) and some composite types (such as lists, plain maps), can be simply mapped one to one after validation without any additional transformation.

However, not all Elixir types have natural representations in JSON-like data (for example atoms and dates) or don't want to expose their internals (opaque types).

Refer to the library test suite for more examples.

Installation

def deps do
  [
    {:dataspecs, "~> xxx"}
  ]
end

Typespecs should be included in the compiled ebin.

Set the :strip_beams option to false when compiling your project with mix release or mix escript.

def project do
  [
    app: :my_app,
    deps: deps(),
    releases: [
      my_app: [strip_beams: false]
    ],
    ...
  ]
end

def project do
  [
    app: :my_script,
    deps: deps(),
    escript: [
      strip_beams: false,
      ...
    ],
    ...
  ]
end

Type cast

Builtin

For reference, check the cast available under DataSpecs.Cast.Builtin and DataSpecs.Cast.Extra.

The modules DataSpecs.Cast.Extra provides pre-defined custom type cast for:

• t:DateTime.t/0: cast iso datetime strings (ie: 2001-12-31 06:54:02Z -> ~U[2001-12-31 06:54:02Z])
• t:Date.t/0: cast iso date strings (ie: 2001-12-31 -> ~D[2022-06-03])
• t:MapSet.t/1: cast lists of T into a MapSet.t(T) (ie: [1, 2] -> #MapSet<[1, 2]>)

Custom

You can pass custom type casts along as an optional argument to the DataSpecs.cast/4 function.

The type of the custom cast function is

@type custom_type_cast_fun :: (value(), custom_type_casts(), [type_cast_fun()] -> value())

for example a custom t:MapSet.t/1 cast could be implement as:

def custom_mapset_cast(value, custom_type_casts, [type_cast_fun]) do
  case Enumerable.impl_for(value) do
    nil ->
      {:error, ["can't convert #{inspect(value)} to a MapSet.t/1, value not enumerable"]}

    _ ->
      value
      |> Enum.to_list()
      |> DataSpecs.Cast.Builtin.list(custom_type_casts, [type_cast_fun])
      |> case do
        {:ok, casted_value} ->
          {:ok, MapSet.new(casted_value)}

        {:error, errors} ->
          {:error, ["can't convert #{inspect(value)} to a MapSet.t/1", errors]}
      end
  end
end

The custom cast take the input value, check it's enumerable and then builds a MapSet over the items of the input value. It takes as argument a list of t:DataSpecs.Types.type_cast_fun/0 associated with the type parameters.

For example, let's say we have:

@type my_set_of_integer :: MapSet.t(integer())

and an input value:

1..10

then the custom type cast function will be called with

custom_mapset_cast(1..10, custom_type_casts, [&DataSpecs.Cast.Builtin.integer/3])

Validators

Custom validation rules can be defined with a custom type cast.

For example let's say than we want to validate a field of type string to be in upcase form:

defmodule AStruct do
  use DataSpecs

  @enforce_keys [:field]
  defstruct @enforce_keys

  @type t :: %__MODULE__{
               field: upcase_string()
             }

  @type upcase_string :: String.t()

  def custom_field_cast(value, custom_type_casts, type_params_casts) do
    with {:ok, value} <- DataSpecs.Cast.Builtin.binary(value, custom_type_casts, type_params_casts)
         ^name <- String.upcase(name) do
      {:ok, name}
    else
      {:error, errors} ->
        {:error, ["#{inspect(value)} is not an upcase string", errors]}

      false ->
        {:error, ["#{inspect(value)} is not an upcase string"]}
    end
  end
end

custom_type_casts = %{{AStruct, :upcase_string, 0} => &AStruct.custom_field_cast/3}
AStruct.cast(%{field: "AAA"}, custom_type_casts)
# => %AStruct{field: "AAA"}

Type coercion

The following types coercion are implicitly applied:

float()

integer() -> float()

Example:

# ---
1
# to
1.0

tuple()

list() -> tuple()

@type t :: {atom(), integer()}

# ---
[:a, 1]
# to
{:a, 1}

struct()

map() -> struct()

Example:

defmodule AStruct do
  defstruct [:a, :b]

  @type t :: %__MODULE__{
               a: nil | binary(),
               b: nil | binary()
             }

# ---
%{a: "1", b: "2"}
# to
%AStruct{a: "1", b: "2"}

map / struct keys

map / struct binary() keys -> existing atom()

Example:

@type a_map :: %{
        a: binary,
        b: binary
      }

# ---
%{"a" => "1", "b" => "2"}
# to
%{a: "1", b: "2"}

atom()

binary() -> existing atom()

Example:

# ---
"bin"
# to
:bin

Plug

DataSpecs.Plug.Cast provides a plug to "Jason.decode! -> DataSpecs.cast" in your routes:

defmodule Api.Router.Something do
  use Plug.Router
  import DataSpecs.Plug.Cast, only: [typeref: 2, value: 1]

  plug :match
  plug DataSpecs.Plug.Cast
  plug :dispatch

  post "/foo", typeref(Api.Model.Foo, :t) do
    %Api.Model.Foo{...} = value(conn)
    ...
  end
end

Ecto embedded schema validation comparison

https://github.com/visciang/example_validation