TypeCheck: Fast and flexible runtime type-checking for your Elixir projects.

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Core ideas

  • Type- and function specifications are constructed using (essentially) the same syntax as built-in Elixir Typespecs.
  • When a value does not match a type check, the user is shown human-friendly error messages.
  • Types and type-checks are generated at compiletime.
    • This means type-checking code is optimized rigorously by the compiler.
  • Property-checking generators can be extracted from type specifications without extra work.
    • Automatically create a spectest which checks for each function if it adheres to its spec.
  • Flexibility to add custom checks: Subparts of a type can be named, and 'type guards' can be specified to restrict what values are allowed to match that refer to these types.

Usage Example

We add use TypeCheck to a module and wherever we want to add runtime type-checks we replace the normal calls to @type and @spec with @type! and @spec! respectively.

defmodule User do
  use TypeCheck
  defstruct [:name, :age]

  @type! t :: %User{name: binary, age: integer}
end

defmodule AgeCheck do
  use TypeCheck

  @spec! user_older_than?(User.t, integer) :: boolean
  def user_older_than?(user, age) do
    user.age >= age
  end
end

Now we can try the following:

iex> AgeCheck.user_older_than?(%User{name: "Qqwy", age: 11}, 10)
true
iex> AgeCheck.user_older_than?(%User{name: "Qqwy", age: 9}, 10)
false

So far so good. Now let's see what happens when we pass values that are incorrect:

iex> AgeCheck.user_older_than?("foobar", 42)
** (TypeCheck.TypeError) At lib/type_check_example.ex:28:
The call to `user_older_than?/2` failed,
because parameter no. 1 does not adhere to the spec `%User{age: integer(), name: binary()}`.
Rather, its value is: `"foobar"`.
Details:
  The call `user_older_than?("foobar", 42)` 
  does not adhere to spec `user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()`. Reason:
    parameter no. 1:
      `"foobar"` does not check against `%User{age: integer(), name: binary()}`. Reason:
        `"foobar"` is not a map.
    (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2
iex> AgeCheck.user_older_than?(%User{name: nil, age: 11}, 10)
** (TypeCheck.TypeError) At lib/type_check_example.ex:28:
The call to `user_older_than?/2` failed,
because parameter no. 1 does not adhere to the spec `%User{age: integer(), name: binary()}`.
Rather, its value is: `%User{age: 11, name: nil}`.
Details:
  The call `user_older_than?(%User{age: 11, name: nil}, 10)` 
  does not adhere to spec `user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()`. Reason:
    parameter no. 1:
      `%User{age: 11, name: nil}` does not check against `%User{age: integer(), name: binary()}`. Reason:
        under key `:name`:
          `nil` is not a binary.
    (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2
iex> AgeCheck.user_older_than?(%User{name: "Aaron", age: nil}, 10) 
** (TypeCheck.TypeError) At lib/type_check_example.ex:28:
The call to `user_older_than?/2` failed,
because parameter no. 1 does not adhere to the spec `%User{age: integer(), name: binary()}`.
Rather, its value is: `%User{age: nil, name: "Aaron"}`.
Details:
  The call `user_older_than?(%User{age: nil, name: "Aaron"}, 10)` 
  does not adhere to spec `user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()`. Reason:
    parameter no. 1:
      `%User{age: nil, name: "Aaron"}` does not check against `%User{age: integer(), name: binary()}`. Reason:
        under key `:age`:
          `nil` is not an integer.
    (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2
    
iex> AgeCheck.user_older_than?(%User{name: "José", age: 11}, 10.0) 
** (TypeCheck.TypeError) At lib/type_check_example.ex:28:
The call to `user_older_than?/2` failed,
because parameter no. 2 does not adhere to the spec `integer()`.
Rather, its value is: `10.0`.
Details:
  The call `user_older_than?(%User{age: 11, name: "José"}, 10.0)` 
  does not adhere to spec `user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()`. Reason:
    parameter no. 2:
      `10.0` is not an integer.
    (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2

And if we were to introduce an error in the function definition:

defmodule AgeCheck do
  use TypeCheck

  @spec! user_older_than?(User.t, integer) :: boolean
  def user_older_than?(user, age) do
    user.age
  end
end

Then we get a nice error message explaining that problem as well:

** (TypeCheck.TypeError) The call to `user_older_than?/2` failed,
because the returned result does not adhere to the spec `boolean()`.
Rather, its value is: `26`.
Details:
  The result of calling `user_older_than?(%User{age: 26, name: "Marten"}, 10)` 
  does not adhere to spec `user_older_than?(%User{age: integer(), name: binary()},  integer()) :: boolean()`. Reason:
    Returned result:
      `26` is not a boolean.
    (type_check_example 0.1.0) lib/type_check_example.ex:28: AgeCheck.user_older_than?/2

Features & Roadmap

Implemented

  • [x] Proof and implementation of the basic concept
  • [x] Custom type definitions (type, typep, opaque)
    • [x] Basic
    • [x] Parameterized
  • [x] Hide implementation of opaque from documentation
  • [x] Spec argument types checking
  • [x] Spec return type checking
  • [x] Spec possibly named arguments
  • [x] Implementation of Elixir's builtin types
    • [x] Primitive types
    • [x] More primitive types
    • [x] Compound types
    • [x] special forms like |, a..b etc.
    • [x] Literal lists
    • [x] Maps with keys => types
    • [x] Structs with keys => types
    • [x] More map/list-based structures.
    • [x] Bitstring type syntax (<<>>, <<_ :: size>>, <<_ :: _ * unit>>, <<_ :: size, _ :: _ * unit>>)
  • [x] A when to add guards to typedefs for more power.
  • [x] Make errors raised when types do not match humanly readable
    • [x] Improve readability of spec-errors by repeating spec and which parameter did not match.
  • [x] Creating generators from types
  • [x] Don't warn on zero-arity types used without parentheses.
  • [x] Hide structure of opaque and typep from documentation
  • [x] Make sure to handle recursive (and mutually recursive) types without hanging.
    • [x] A compile-error is raised when a type is expanded more than a million times
    • [x] A macro called lazy is introduced to allow to defer type expansion to runtime (to within the check).
  • [x] the Elixir formatter likes the way types+specs are constructed
  • [x] A type impl(ProtocolName) to work with 'any type implementing protocol Protocolname'.
    • [x] Type checks.
    • [x] StreamData generator.
  • [x] High code-coverage to ensure stability of implementation.
  • [x] Make sure we handle most (if not all) of Typespec's primitive types and syntax. (With the exception of functions and binary pattern matching)
  • [x] Option to turn @type/@opaque/@typep-injection off for the cases in which it generates improper results.
  • [x] Manually overriding generators for user-specified types if so desired.
  • [x] Creating generators from specs
    • [x] Wrap spec-generators so you have a single statement to call in the test suite which will prop-test your function against all allowed inputs/outputs.
  • [x] Option to turn the generation of runtime checks off for a given module in a particular environment (enable_runtime_checks).
  • [x] Support for function-types (for typechecks as well as property-testing generators):
    • (-> result_type)
    • (...-> result_type)
    • (param_type, param2_type -> result_type)
  • [ ] Overrides for builtin remote types (String.t,Enum.t, Range.t, MapSet.t etc.) (75% done) Details

Pre-stable

  • [ ] Overrides for more builtin remote types
  • [ ] Support for maps with mixed required(type) and optional(type) syntaxes.
  • [ ] Hide named types from opaque types.
  • [ ] Configurable setting to turn on/off at compile-time, and maybe dynamically at run-time (with slight performance penalty).
  • [ ] Finalize formatter specification and make a generator for this so that people can easily test their own formatters.

Longer-term future ideas

  • [ ] Per-module or even per-spec settings to turn on/off, configure formatter, etc.

Changelog

  • 0.10.1 -
    • Fixes:
      • Swaps Murmur out for Erlang's builtin :erlang.phash2/2 to generate data for function-types, allowing the removal of the optional dependency on the :murmur library.
  • 0.10.0 -
    • Additions
      • Support for function-types (for typechecks as well as property-testing generators):
        • (-> result_type)
        • (...-> result_type)
        • (param_type, param2_type -> result_type)
    • Fixes:
      • Wrapping private functions no longer make the function public. (c.f. #64)
      • Wrapping macros now works correctly. (also related to #64)
      • Using __MODULE__ inside a struct inside a type now expands correctly. (c.f. #66)
  • 0.9.0 -
    • Support for bitstring type syntax: <<>>, <<_ :: size>>, <<_ :: _ * unit>>, <<_ :: size, _ :: _ * unit>> (both as types and as generators)
  • 0.8.2 -
    • Fixed compiler warnings when optional dependency StreamData is not installed.
    • Fixed pretty-printing of typedoc of opaque types.
  • 0.8.1 -
    • Improved documentation with a page comparing TypeCheck with Elixir's plain typespecs. (Thank you very much, @baldwindavid )
    • Addition of missing override for the type t Range.t/0. (Thank you very much, @baldwindavid )
  • 0.8.0 -
    • Fixes prettyprinting of TypeCheck.Builtin.Range.
    • Addition of require TypeCheck.Type to use TypeCheck so there no longer is a need to call this manually if you want to e.g. use TypeCheck.Type.build/1.
    • Pretty-printing of types and TypeError output in multiple colors.
    • Nicer indentation of errors.
    • named types are now printed in abbreviated fashion if they are repeated multiple times in an error message. This makes a nested error message much easier to read, especially for larger specs.
    • [type] no longer creates a fixed_list(type) but instead a list(type) (just as Elixir's own typespecs.)
    • Support for [...] and [type, ...]as alias for nonempty_list() and nonempty_list(type) respectively.
    • Remove support for list literals with multiple elements.
    • Improved documentation. Thank you, @0ourobor0s!
  • 0.7.0 - Addition of the option enable_runtime_checks. When false, all runtime checks in the given module are completely disabled.
    • Adding DateTime.t to the default overrides, as it was still missing.
  • 0.6.0 - Addition of spectest & 'default overrides' Elixir's standard library types:
    • Adding TypeCheck.ExUnit, with the function spectest to test function-specifications.
      • Possibility to use options :except, :only, :initial_seed.
      • Possibility to pass custom options to StreamData.
    • Adding TypeCheck.DefaultOverrides with many sub-modules containing checked typespecs for the types in Elixir's standard library (75% done).
      • Ensure that these types are correct also on older Elixir versions (1.9, 1.10, 1.11)
    • By default load these 'DefaultOverrides', but have the option to turn this behaviour off in TypeCheck.Option.
    • Nice generators for Enum.t, Collectable.t, String.t.
    • Support for the builtin types:
      • pid()
      • nonempty_list(), nonempty_list(type).
    • Allow use TypeCheck in IEx or other non-module contexts, to require TypeCheck and import TypeCheck.Builtin in the current scope (without importing/using the macros that only work at the module level.)
    • The introspection function __type_check__/1 is now added to any module that contains a use TypeCheck.
    • Fixes the Inspect implementation of custom structs, by falling back to Any, which is more useful than attempting to use a customized implementation that would try to read the values in the struct and failing because the struct-type containing types in the fields.
    • Fixes conditional compilation warnings when optional dependency :stream_data was not included in your project.
  • 0.5.0 - Stability improvements:
    • Adding Typecheck.Option debug: true, which will (at compile-time) print the checks that TypeCheck is generating.
    • Actually autogenerate a @spec, which did not yet happen before.
    • When writing @autogen_typespec false, no typespec is exported for the next @type!/@opaque/@spec! encountered in a module.
    • Code coverage increased to 85%
    • Bugfixes w.r.t. generating typespecs
    • Fixes compiler-warnings on unused named types when using a type guard. (c.f. #25)
    • Fixes any warnings that were triggered during the test suite before.
  • 0.4.0 - Support for impl(ProtocolName) to accept any type implementing a particular protocol.
    • Also adds rudimentary support for overriding remote types.
    • Bugfix when inspecting lazy( ...)-types.
  • 0.3.2 - Support for unquote fragments inside types and specs. (c.f. #39)
  • 0.3.1 - Fixed link in the documentation.
  • 0.3.0 - Improve DefaultFormatter output when used with long function- or type-signatures (c.f. #32). Also, bugfix for Builtin.tuple/1.
  • 0.2.3 - Bugfix release: Ensure TypeCheck compiles on Elixir v1.11 (#30), Ensure StreamData truly is an optional dependency (#27).
  • 0.2.2 - Support for literal strings should no longer break in Elixir's builtin typespecs.
  • 0.2.1 - Improved parsing of types that have a type-guard at the root level. (c.f. #24), support for custom generators.
  • 0.2.0 - Improved (and changed) API that works better with the Elixir formatter: Use @type!/@spec! instead, support named types in specs.
  • 0.1.2 - Added missing keyword type to TypeCheck.Builtin (#20)
  • 0.1.1 - Fixing some documentation typos
  • 0.1.0 - Initial Release

Installation

TypeCheck is available in Hex. The package can be installed by adding type_check to your list of dependencies in mix.exs:

def deps do
  [
    {:type_check, "~> 0.10.0"},
    # To allow spectesting and property-testing data generators (optional):
    {:stream_data, "~> 0.5.0", only: :test}, 
  ]
end

The documentation can be found at https://hexdocs.pm/type_check.

Formatter

TypeCheck exports a couple of macros that you might want to use without parentheses. To make mix format respect this setting, add import_deps: [:type_check] to your .formatter.exs file.

TypeCheck compared to other tools

TypeCheck is by no means the other solution out there to reduce the number of bugs in your code.

Elixir's builtin typespecs and Dialyzer

Elixir's builtin type-specifications use the same syntax as TypeCheck. They are however not used by the compiler or the runtime, and therefore mainly exist to improve your documentation.

Besides documentation, extra external tools like Dialyzer can be used to perform static analysis of the types used in your application.

Dialyzer is an opt-in static analysis tool. This means that it can point out some inconsistencies or bugs, but because of its opt-in nature, there are also many problems it cannot detect, and it requires your dependencies to have written all of their typespecs correctly.

Dialyzer is also (unfortunately) infamous for its at times difficult-to-understand error messages.

An advantage that Dialyzer has over TypeCheck is that its checking is done without having to execute your program code (thus not having any effect on the runtime behaviour or efficiency of your projects).

Because TypeCheck adds @type, @typep, @opaque and @spec-attributes based on the types that are defined, it is possible to use Dialyzer together with TypeCheck.

Norm

Norm is an Elixir library for specifying the structure of data that can be used for both validation and data-generation.

On a superficial level, Norm and TypeCheck seem similar. However, there are important differences in their design considerations.

Is it any good?

yes