docs: Add initial README (#70)

README.md

@@ -1,6 +1,6 @@
 # Sorbet Schema
 
-
+Extendable serialization and deserialization to various formats for Sorbet `T::Struct`s.
 
 ## Installation
 
@@ -14,29 +14,242 @@ If bundler is not being used to manage dependencies, install the gem by executin
 
 ## Usage
 
+Sorbet Schema is designed to be compatible with Sorbet's `T::Struct` class, and seeks to update many of the common pitfalls developers encountering when deserializing to and serializing from a `T::Struct`.
+
 ### Getting Started
 
-### Chaining
+While you can directly define a `Typed::Schema` to be used for your serialization needs, you'll typically use the provider helper class method to generate a `Schema` from an existing `T::Struct`.
+
+```ruby
+class Person < T::Struct
+  const :name, String
+  const :age, Integer
+end
+
+schema = Person.schema # => <Typed::Schema
+#                              fields=[....]
+#                              target=Person>
+```
+
+Once you have a schema, you can use the built-in serializers (or a [custom one](#implementing-custom-serializers) that inherits from the [Typed::Serializer](https://github.com/maxveldink/sorbet-schema/blob/main/lib/typed/serializer.rb) abstract base class) to create new instances of the struct or convert an instance of the struct to the target format.
+
+```ruby
+json_serializer = Typed::JSONSerializer.new(schema: Person.schema)
+
+# Deserialize from target format
+result = json_serializer.deserialize('{"name":"Max","age":29}')
+max = result.payload # == Person.new(name: "Max", age: 29)
+
+result = json_serializer.serialize(max)
+result.payload # == '{"name":"Max","age":29}'
+```
+
+Notice that both `deserialize` and `serialize` return `Typed::Result`s (from the [sorbet-result gem](https://github.com/maxveldink/sorbet-result)) that need to be checked for success or failure before being used. Check out that gem's README for more information on how to interact with `Result`s.
+
+One benefit of using `Result`s is we can add much more details information about why a format is unsuccessfully deserialized or serialized, to provide call sites with more information for error handling, messaging and formatting.
+
+```ruby
+# Unparsable JSON
+result = json_serializer.deserialize('{"name""Max","age":29}')
+result.error # == Typed::ParseError: json could not be parsed. Check for typos.
+
+# Missing required field
+result = json_serializer.deserialize('{"age": 29}')
+result.error # == Typed::Validations::RequiredFieldError: name is required.
+
+result = json_serializer.deserialize('{"age":"29-0"}')
+result.error # == Typed::Validations::MultipleValidationError: Multiple validation errors found: name is required. | '29-0' cannot be coerced into Integer.
+```
+
+Finally, there are built-in coercers that do their best effort to convert common types from the source format to the required schema type.
+
+```ruby
+# Deserialize from target format, with integer coercion
+result = json_serializer.deserialize('{"name":"Max","age":"29"}')
+max = result.payload # == Person.new(name: "Max", age: 29)
+```
+
+### Rails Example
+
+Here's an extended example of how Sorbet Schema can be combined with a normal Rails request to easily convert between formats.
+
+```ruby
+def verify
+  Typed::HashSerializer
+    .new(schema: Address.schema) # Generate schema from the `Address` Struct
+    .deserialize(address_params.to_h) # Use Rails' strong parameters to deserialize into the struct
+    .and_then { |address| VerifyAddress.new.call(address: T.cast(address, Address)) } # Use sorbet-result's chaining
+    .and_then do |address|
+        return render json: Typed::JSONSerializer.new(schema: Address.schema).serialize(address).payload # return a JSON response from the Address struct instance
+    end
+    .on_error do |failure| # Use sorbet-result's error handling
+      case failure
+      when AddressNotFoundError
+        head :not_found
+      when GeoNotSupportedError
+        head :not_implemented
+      else
+        render json: failure, status: :bad_request # use `Typed::Failure`s built-in `to_json` behavior
+      end
+    end
+end
+```
+
+### Available Serializers
+
+These are the currently available serializers. For more information about implementing a custom one (or contributing one back!), see [Custom Coercers](#custom-coercers).
+
+#### JSONSerializer
+
+See [Getting Started](#getting-started) for more information on how to use the JSONSerializer.
+
+#### HashSerializer
+
+While not strictly serialization, converting `T::Struct`s to and from Ruby `Hash`es has traditionally had many pitfalls ([well-documented](https://sorbet.org/docs/tstruct#legacy-code-and-historical-context) in the Sorbet docs). The `Typed::HashSerializer` aims to address several common issues, while providing the same `Result` handling for invalid or missing data and coercion behavior.
+
+To use it, simply instantiate and use it like the `JSONSerializer`:
+
+```ruby
+hash_serializer = Typed::HashSerializer.new(schema: Person.schema)
+
+# Deserialize from target format
+result = hash_serializer.deserialize({"name" => "Max", age: 29})
+max = result.payload # == Person.new(name: "Max", age: 29)
+```
+
+By default, the `HashSerializer` will _not_ serialize values when converting to a Hash. For instance, if a field is an `T::Enum` type, when it is serialized to a `Hash` the value will be the `Enum` and not the `String` representation. The `should_serialize_values` option can be passed during initialization to serialize the values when converting to the `Hash`.
+
+### Customization
+
+From the get-go, Sorbet Schema is designed to be extensible to model more complex data validation requirements and many serialization formats. We try out best to include built-in, battle-tested coercers and serializers from real world use cases and would love to see/upstream any customizations that the community have found useful!
+
+#### Custom Coercers
+
+At their simplest forms, coercers are any class that inherit from the [Typed::Coercion::Coercer](https://github.com/maxveldink/sorbet-schema/blob/main/lib/typed/coercion/coercer.rb) abstract base class. The list of default coercers that are applied can be found in the [CoercerRegistry](https://github.com/maxveldink/sorbet-schema/blob/main/lib/typed/coercion/coercer_registry.rb). Let's look at the [DateCoercer's implementation](https://github.com/maxveldink/sorbet-schema/blob/main/lib/typed/coercion/date_coercer.rb):
+
+```ruby
+require "date"
+
+class DateCoercer < Coercer
+  extend T::Generic
+
+  Target = type_member { {fixed: Date} }
+
+  sig { override.params(type: T::Types::Base).returns(T::Boolean) }
+  def used_for_type?(type)
+    T::Utils.coerce(type) == T::Utils.coerce(Date)
+  end
+
+  sig { override.params(type: T::Types::Base, value: Value).returns(Result[Target, CoercionError]) }
+  def coerce(type:, value:)
+    return Failure.new(CoercionError.new("Type must be a Date.")) unless used_for_type?(type)
+
+    return Success.new(value) if value.is_a?(Date)
+
+    Success.new(Date.parse(value))
+  rescue Date::Error, TypeError
+    Failure.new(CoercionError.new("'#{value}' cannot be coerced into Date."))
+  end
+end
+```
+
+Notice that this utilizes sorbet generic, so the target type must be defined using `type_member`. For dates, this is the built-in std lib type `Date`.
+
+From there, implement the `used_for_type?` method which receives a type and returns `true` if the coercer can be used to coerce to that type or `false` if it should not be used. Notice that we use the `T::Types` module directly from Sorbet, which allows us to model the built-in Sorbet types, such as `T::Boolean` and `T::Array`. Typically, `T::Utils.coerce(TargetType)` is used to match the target type. For dates, this is a very simple type check for a `Date`.
+
+Finally, implement the `coerce` method. If a coercion is successful, return a `Success.new(coerced_value)`. If not, return a Failure with a coercion error `Failure.new(CoercionError.new("I can't coerce to the type"))`. Take care to handle any exceptions that could arise from the attempted coercion. For dates, first it checks and make sure the type given matches the target type. This is a common check and is largely an edge case check for completeness. Next, if the value is already a Date we simply return a `Success` with it. Finally, we use the built-in `Date.parse` method to actually attempt a coercion. Since this can throw a `Date::Error` and a `TypeError`, rescue from those with a `Failure`.
+
+Once a custom coercer is defined, the last step is to register it with Sorbet Schema during initialization. Typically, this is after `sorbet-schema` has been required or during the bootstrapping step of a framework, such as Rails' initializers. Call `register_coercer` like so:
+
+```ruby
+Typed::Coercion.register_coercer(MyCoercer) # make sure `MyCoercer` is loaded by this point
+```
+
+**Note** Custom coercers are prepended to the list of available coercers so that they are checked during deserialization before the built-in coercers. This allows consuming projects to override default behavior by creating a coercer that re-implements the `coerce` method for that type.
+
+#### Inline Serializers
+
+Sometimes, there is custom behavior that needs to be added to how a field is serialized (represented as a `String`), such as when you need to use a different `strftime` format for `Date`s and `Time`s. This can be accomplished with an `InlineSerializer` (defined in [Typed::Field](https://github.com/maxveldink/sorbet-schema/blob/main/lib/typed/field.rb)), which is a `Proc` that takes the value and returns a different representation. At present, these are both very loose `T.untyped` types to allow for flexibility. Typically, a `String` is returned.
+
+The serializer can be used when creating a `Schema` and defining its `fields`, or with the `add_serializer` helper on `Schema`s.
+
+```ruby
+my_date_serializer = ->(date) { date.strftime("%Y/%m") }
+
+# use directly on a Schema
+Typed::Schema.new(
+  target: SchemaWithDateField,
+  fields: [
+    Typed::Field.new(name: :date, type: Date, serializer: my_date_serializer)
+  ]
+)
+
+# use `add_serializer` helper
+SchemaWithDateField.schema.add_serializer(:date, my_date_serializer)
+```
+
+#### Implementing Custom Serializers
+
+While Sorbet Schema ships with popular serializers, you can define your own by inheriting from [Typed::Serializer](https://github.com/maxveldink/sorbet-schema/blob/main/lib/typed/serializer.rb). Let's look at the `JSONSerializer`:
+
+```ruby
+require "json"
+
+class JSONSerializer < Serializer
+  Input = type_member { {fixed: String} }
+  Output = type_member { {fixed: String} }
+
+  sig { override.params(source: Input).returns(Result[T::Struct, DeserializeError]) }
+  def deserialize(source)
+    parsed_json = JSON.parse(source)
+
+    creation_params = schema.fields.each_with_object(T.let({}, Params)) do |field, hsh|
+      hsh[field.name] = parsed_json[field.name.to_s]
+    end
+
+    deserialize_from_creation_params(creation_params)
+  rescue JSON::ParserError
+    Failure.new(ParseError.new(format: :json))
+  end
+
+  sig { override.params(struct: T::Struct).returns(Result[Output, SerializeError]) }
+  def serialize(struct)
+    return Failure.new(SerializeError.new("'#{struct.class}' cannot be serialized to target type of '#{schema.target}'.")) if struct.class != schema.target
+
+    Success.new(JSON.generate(serialize_from_struct(struct: struct, should_serialize_values: true)))
+  end
+end
+```
+
+Since `Serializer` is a generic class, we need to define our `Input` and `Output` types. For JSON, deserialization and serialization both use JSON strings, so these are both strings.
+
+Next, the `deserialize` and `serialize` methods must be implemented. Notice that both of these return `Result`s.
+
+For deserialization, the JSON is parsed (and a parse error is handled). Then we build up a creation params hash from the parsed json to pass to the `deserialize_from_creation_params` helper, defined on `Serializer`.
+
+For serialization, the passed struct is checked to make sure it matches the `Schema`. Then it uses the `serialize_from_struct` helper and passes the resulting `Hash` to generate JSON.
+
+## Inspirations
 
-## Why use Options?
+This project is heavily inspired by [serde](https://serde.rs/) from the Rust community and the [dry-rb](https://dry-rb.org/) family of gems.
 
 ## Development
 
-After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake` to run Rubocop and the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.
+After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake` to run Standard and the tests. `bin/console` for an interactive prompt that aids with experimentation.
 
-To install this gem onto your local machine, run `bundle exec rake install`.
+To install this gem onto a local machine, run `bundle exec rake install`.
 
 ## Contributing
 
-Bug reports and pull requests are welcome on GitHub at https://github.com/maxveldink/sorbet-option. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [code of conduct](https://github.com/maxveldink/sorbet-option/blob/master/CODE_OF_CONDUCT.md).
+Bug reports and pull requests are welcome on GitHub at https://github.com/maxveldink/sorbet-schema. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [code of conduct](https://github.com/maxveldink/sorbet-schema/blob/master/CODE_OF_CONDUCT.md).
 
 ## License
 
 The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).
 
 ## Code of Conduct
 
-Everyone interacting in this project's codebase, issue trackers, chat rooms and mailing lists is expected to follow the [code of conduct](https://github.com/maxveldink/sorbet-option/blob/master/CODE_OF_CONDUCT.md).
+Everyone interacting in this project's codebase, issue trackers, chat rooms and mailing lists is expected to follow the [code of conduct](https://github.com/maxveldink/sorbet-schema/blob/master/CODE_OF_CONDUCT.md).
 
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