defmodule Containers do
  @moduledoc """
  Containers are functional data structures that help provide greater runtime safety and polymorphism.

  ## Protocols

    * `Appendable` - A container that provies an interface of `append`. Safe against `nil` values.
      Namely when passing a container with the value `nil` into either the first of second argument
      to `append`, the other value is not change and there is no runtime error.
    * `Mappable` - A container that provies an interface to `map`. When `map` is called on a container that
      has a `nil` value that container just passes through with out the mapping function being called, and
      this helps prevent runtime errors.
    * `Sequenceable` - A container that provides an interface of `next`. This allows the chaining of computations.
    * `Unwrappable`  - A container that provides an interface to `safe` and `unsafe` unwrapping of inner value. Safe
      will need a default in case of `nil` value of container, helping prevent runtime errors. Unsafe will just return
      the value of the container regardless of a `nil` value potentially causing runtime errors
    * `Flattenable` - A container that provides an interface to `flatten` function. This allows for nested containers of the
       same container type to have the outter layer removed.

  Since these are protocols, and highly decoupled, a developer can implement them as needed on their own structs.
  """

  @type appendable :: Containers.Text.t | Containers.Optional.t
  @type mappable :: Containers.Text.t | Containers.Optional.t | Containers.Result.t | list
  @type sequenceable :: Containers.Text.t | Containers.Optional.t | Containers.Result.t
  @type unwrappable :: Containers.Text.t | Containers.Optional.t | Containers.Result.t
  @type flattenable :: Containers.Result.t | Containers.Optional.t


  @doc """
  Append two values of the Containers.Appendable protocol

  This is useful for chaning of appending appendable items safely. That is to say if there
  is a `nil` value being used like `nil <> " world!"` there will be a run time error. In this
  case the container for the string type will safe do concatenation.

  ## Examples

      iex> hello = Containers.Text.from_string("Hello")
      iex> world = Containers.Text.from_string(" world!")
      iex> Containers.append(hello, world)
      %Containers.Text{value: "Hello world!"}

      iex> hello = Containers.Text.from_string("Hello")
      iex> world = Containers.Text.from_string(" world!")
      iex> nil_string = Containers.Text.from_string(nil)
      iex> hello |> Containers.append(nil_string) |> Containers.append(world)
      %Containers.Text{value: "Hello world!"}
  """
  @spec append(appendable(), appendable()) :: appendable()
  def append(v1, v2), do: Containers.Appendable.append(v1, v2)

  @doc """
  Map some mapping function `f` on the innner value `n` number of mappable container structures deep. This is
  useful for when you have deeply nested containers that are mappable and you want to operator
  on the most inner value without have to unwrap.

  Note there is a provided `map2` function in this module for mapping two layers deeps, but if you needing
  mapping for a strucuture that is more deeply nested then 2 contianers, then this is the function
  you are looking for.
  """
  @spec mapn(mappable, integer, (... -> any())) :: mappable
  def mapn(mappable, 1, f), do: map(mappable, f)
  def mapn(mappable, n, f) do
    map(mappable, fn x -> mapn(x, n - 1, f) end)
  end

  @doc """
  map some function `f` of the some structure `s`. Works like the `Enum.map` but provides
  more polymorphic protocol, and does rely on the `Enumerable` protocol allowing use of
  just getting map without needing to implement the full `Enumerable` protocol.

  ## Examples

      iex> my_optional = Containers.Optional.to_optional(1)
      iex> Containers.map(my_optional, fn(i) -> i + 1 end)
      %Containers.Optional{value: 2}
  """
  @spec map(mappable, (... -> any())) :: mappable
  def map(s, f), do: Containers.Mappable.map(s, f)

  @doc """
  map some function `f` over some nested strcutre `s`. This is useful for when you have a
  mappable in another mappable and you just want to use a mapping function on the inner
  value of the nested map.
  """
  @spec map2(mappable, (... -> any())) :: mappable
  def map2(s, f), do: mapn(s, 2, f) 

  @doc """
  next is a function that will allow chaining of computations while passing the `value` of the
  last computation.
  """
  @spec next(sequenceable(), (any() -> sequenceable())) :: sequenceable()
  def next(s, f), do: Containers.Sequenceable.next(s, f)

  @doc """
  `>>>` is the infix operator for `next`

  ## Examples

      iex> import Containers
      iex> my_optional = Containers.Optional.to_optional(1)
      iex> my_optional >>> fn(i) -> Containers.Optional.to_optional(i + 1) end
      %Containers.Optional{value: 2}
  """
  def s >>> f, do: Containers.Sequenceable.next(s, f)

  @doc """
  safely unwrap the inner value of a container, proviing a default in case the value is `nil`.
  This is should help prevent runtime errors within a `|>` chain handling strings.


  ## Examples

      iex> my_string = Containers.Text.from_string("hello")
      iex> Containers.safe_unwrap(my_string, "this wont be needed")
      "hello"

      iex> my_nil_string = Containers.Text.from_string(nil)
      iex> Containers.safe_unwrap(my_nil_string, "This will be the value")
      "This will be the value"
  """
  @spec safe_unwrap(unwrappable(), any()) :: any()
  def safe_unwrap(s, default), do: Containers.Unwrappable.safe(s, default)

  @doc """
  unsafely unwrap the inner value of a continer. This may return nil so any guarantees
  against a runtime error no longer apply.

  ## Examples

      iex> my_string = Containers.Text.from_string("Hello")
      iex> Containers.unsafe_unwrap(my_string)
      "Hello"

      iex> my_nil_string = Containers.Text.from_string(nil)
      iex> Containers.unsafe_unwrap(my_nil_string)
      nil
  """
  @spec unsafe_unwrap(unwrappable()) :: any()
  def unsafe_unwrap(s), do: Containers.Unwrappable.unsafe!(s)

  @doc """
  concat a list of Containers that implement the Appendable protocol

  ```
  hello_world = "hello_world"

  hello_world |> String.split("_")
  |> Enum.map(&String.capitalize/1)
  |> Enum.map(&Containers.Text.from_string/1)
  |> Containers.concat()
  |> Containers.safe_unwrap("")

  "HelloWorld"
  ```

  ## Examples

      iex> hello = Containers.Text.from_string("hello")
      iex> world = Containers.Text.from_string(" world")
      iex> excliam = Containers.Text.from_string("!")
      iex> Containers.concat([hello, world, excliam])
      %Containers.Text{value: "hello world!"}
  """
  @spec concat(list(appendable)) :: appendable()
  def concat(appendables) do
    appendables
    |> Enum.reverse
    |> Enum.reduce(&append/2)
  end

  @doc """
  This is useful for when you have a container that inner structure of that same container,
  and you want to flatten that down to one level.

  ## Examples

      iex> nested = %Containers.Optional{value: %Containers.Optional{value: "hello"}}
      iex> Containers.flatten(nested)
      %Containers.Optional{value: "hello"}

  """
  @spec flatten(flattenable) :: flattenable
  def flatten(flat), do: Containers.Flattenable.flatten(flat)
end