# Regex.Spec
Generates [type specifications][1] for [Regular Expressions][2] at compile time allowing [Dialyzer][dialyzer] to detect type errors.
[](https://github.com/devstopfix/regex_spec/actions)
[](https://hex.pm/packages/regex_spec)
This library is a **work in progress** that was inspired by [Stephanie Weirich][sweirich]'s talk [Dependent Types in Haskell at Strange Loop 2017][talk]. The design is evolving based on the constraints imposed by Elixir:
1. we cannot specify a list of n elements, so we need to return a tuple
2. we use `String.t() | nil` to represent a maybe type based on a `(...)?` pattern
3. we generate a map of results with optional and required keys for named captures
4. can we pass in a list of [parse][parsei] functions that can be applied to the matches and modify the return type e.g. from `String.t() -> integer()`
This library generates the common [Regex][2] functions for your
regular expression with a type spec. Create a module for your type
and use:
```elixir
defmodule MyApp do
defmodule MMYYDDDD do
use Regex.Spec, regex: ~r/(\d{2})\-(\d{2})\-(\d{4})/
end
IO.inspect(MMYYDDDD.matches?("2020-12-31")) # false
IO.inspect(MMYYDDDD.run("12-31-2020")) # ["12-31-2020", "12", "31", "2020"]
IO.inspect(MMYYDDDD.runt("12-31-2020")) # {"12-31-2020", "12", "31", "2020"}
end
```
The specification for [run/3][run] is a list of Strings, and we cannot specify a
[dependent type such as a list of length 4][dep]. Therefore we implement the
normal `run` function and also `runt` which can be read as *run typed* or *run tuple*
which returns a typed tuple.
## Installation
This library is incomplete, but you can append to your mix.exs deps:
```elixir
{:regex_spec, "~> 0.21"}
```
## Design
This library uses [leex][leex] and [yecc][yecc] to compile your regular expression into a tree of capture groups and then into a Dialyzer tuple type specification.
The docs can be found at [https://hexdocs.pm/regex_spec](https://hexdocs.pm/regex_spec).
[1]: https://hexdocs.pm/elixir/typespecs.html
[2]: https://hexdocs.pm/elixir/Regex.html
[leex]: https://erlang.org/doc/man/leex.html
[yecc]: https://erlang.org/doc/man/yecc.html
[run]: https://hexdocs.pm/elixir/Regex.html#run/3
[dep]: https://en.wikipedia.org/wiki/Dependent_type
[talk]: https://www.youtube.com/watch?v=wNa3MMbhwS4
[dialyzer]: https://erlang.org/doc/man/dialyzer.html
[sweirich]: https://github.com/sweirich
[parsei]: https://hexdocs.pm/elixir/Integer.html#parse/2
