Croma

Elixir macro utilities to make type-based programming easier.

• API Documentation
• Hex package information

Usage

• Add :croma as a mix dependency.
• Run $ mix deps.get.
• Add use Croma in your source file to import/require macros defined in croma.
• Hack!

Croma.Result

• Croma.Result.t(a) is defined as @type t(a) :: {:ok, a} | {:error, any}, representing a result of computation that can fail.

• This data type is prevalent in Erlang and Elixir world. Croma makes it easier to work with Croma.Result.t(a) by providing utilities such as get/2, get!/1, map/2, map_error/2, bind/2 and sequence/1.

• You can also use Haskell-like do-notation to combine results of multiple computations by m/1 macro. For example,

    Croma.Result.m do
      x <- {:ok, 1}
      y <- {:ok, 2}
      pure x + y
    end

  is converted to

    Croma.Result.bind(mx, fn x ->
      Croma.Result.bind(my, fn y ->
        Croma.Result.pure(x + y)
      end)
    end)

  and is evaluated to {:ok, 3}. (The do-notation is implemented by Croma.Monad.)

Croma.Defun : Typespec-oriented function definition

• Annotating functions with type specifications is good but sometimes it’s a bit tedious since one has to repeat some tokens (names of function and arguments, etc.) in @spec and def.

• defun/2 macro provides shorthand syntax for defining function with its typespec at once.

  • Example 1

      use Croma
      defun f(a :: integer, b :: String.t) :: String.t do
        "#{a} #{b}"
      end

    is expanded to

      @spec f(integer, String.t) :: String.t
      def f(a, b) do
        "#{a} #{b}"
      end

  • Example 2 (multi-clause syntax)

      use Croma
      defun dumbmap(as :: [a], f :: (a -> b)) :: [b] when a: term, b: term do
        ([]     , _) -> []
        ([h | t], f) -> [f.(h) | dumbmap(t, f)]
      end

    is expanded to

      @spec dumbmap([a], (a -> b)) :: [b] when a: term, b: term
      def dumbmap(as, f)
      def dumbmap([], _) do
        []
      end
      def dumbmap([h | t], f) do
        [f.(h) | dumbmap(t, f)]
      end

• In addition to the shorthand syntax explained above, defun is able to generate code for runtime type checking:

  • guard: soma_arg :: g[integer]
  • validation with valid?/1 of a type module (see below): some_arg :: v[SomeType.t]

• There are also defunp and defunpt macros for private functions.

Type modules

• Sometimes you may want to have more fine-grained control of data types than is allowed by Elixir’s typespec. For example you may want to distinguish “arbitrary String.t“ with “String.t that matches a specific regex”. Croma introduces “type module”s in order to express fine-grained types and enforce type contracts at runtime, with minimal effort.

• Leveraging Elixir’s lightweight syntax for defining modules (i.e. you can easily make multiple modules within a single source file), croma encourages you to define lots of small modules to organize code, especially types, in your Elixir projects. Croma expects that a type is defined in its dedicated module, which we call a “type module”. This way a type can have associated functions within its type module.

• The following definitions in type modules are used by croma:

  • @type t
    • The type represented in Elixir’s typespec.
  • valid?(any) :: boolean
    • Runtime check of whether a given value belongs to the type. Used by validation of arguments and return values in defun-family of macros.
  • new(any) :: {:ok, t} | {:error, any}
    • Tries to convert a given value to a value that belongs to this type. Useful e.g. when converting a JSON-parsed value into an Elixir value.
  • default() :: t
    • Default value of the type. Must be a constant value. Used as default values of struct fields.

  @type t and valid?/1 are mandatory as they are the raison d’etre of a type module, but the others can be omitted. And of course you can define any other functions in your type modules as you like.

• You can always define your type modules by directly implementing above functions. For simple type modules croma prepares some helpers for you:

  • type modules of built-in types such as Croma.String, Croma.Integer, etc.
  • helper modules such as Croma.SubtypeOfString to define “subtype”s of existing types
  • Croma.Struct for structs
  • ad-hoc module generator macros defined in Croma.TypeGen

Croma.SubtypeOf*

• You can define your type module for “String.t that matches ~r/foo|bar/“ as follows (we use defun here but you can of course use @spec and def instead):

    defmodule MyString1 do
      @type t :: String.t
      defun valid?(t :: term) :: boolean do
        s when is_binary(s) -> s =~ ~r/foo|bar/
        _                   -> false
      end
    end

• However, as this is a common pattern, croma provides a shortcut:

    defmodule MyString2 do
      use Croma.SubtypeOfString, pattern: ~r/foo|bar/
    end

• There are also SubtypeOfInt, SubtypeOfFloat and so on.

Croma.Struct

• Defining a type module for a struct can be tedious since you have to check all fields in the struct.

• Using type modules for struct fields, Croma.Struct generates definition of type module for a struct.

    defmodule I do
      use Croma.SubtypeOfInt, min: 1, max: 5, default: 1
    end
  
    defmodule S do
      use Croma.Struct, fields: [
        i: I,
        f: Croma.Float,
      ]
    end
  
    S.valid?(%S{i: 5, f: 1.5})         # => true
    S.valid?(%S{i: "not_int", f: 1.5}) # => false
  
    {:ok, s} = S.new(%{f: 1.5})        # => {:ok, %S{i: 1, f: 1.5}}
  
    # `update/2` is also generated for convenience
    S.update(s, [i: 5])                # => {:ok, %S{i: 5, f: 1.5}}
    S.update(s, %{i: 6})               # => {:error, {:invalid_value, [S, I]}}

Croma.TypeGen

• Suppose you have a type module I, and suppose you want to define a struct that have a field with type nil | I.t. As nilable fields are common, defining type modules for all nilable fields introduces too much boilerplate code.

• Croma has a set of macros to define this kind of trivial type modules in-line. For example you can write as follows using nilable/1:

    defmodule S do
      use Croma.Struct, fields: [
        i: Croma.TypeGen.nilable(I),
      ]
    end

Notes on backward compatibility

• In 0.7.0 we separated responsibility of validate/1 into valid?/1 and new/1.
  • Although older type module implementations that define validate/1 should work as before, please migrate to the newer interface by replacing validate/1 with valid?/1 and optionally new/1.
  • In 0.8.0 we removed support of validate/1.