add docs on custom formats

Author: ezekg

README.md

@@ -68,6 +68,7 @@ Links:
   - [Non-scalar types](#non-scalar-types)
   - [Custom types](#custom-types)
   - [Formats](#formats)
+  - [Custom formats](#custom-formats)
 - [Contributing](#contributing)
 - [License](#license)
 
@@ -944,6 +945,39 @@ class UsersController < ApplicationController
 end
 ```
 
+### Custom formats
+
+You may register custom formatters that can be utilized in your schemas.
+
+```rb
+TypedParams::Formatters.register(:strong_params,
+  transform: -> (params, controller:) {
+    wrapper   = controller.controller_name.singularize.to_sym
+    unwrapped = params[wrapper]
+
+    ActionController::Parameters.new(unwrapped).permit!
+  },
+)
+```
+
+```rb
+typed_params {
+  format :strong_params
+
+  param :user, type: :hash do
+    param :email, type: :string, format: { with: /@/ }
+    param :password, type: :string
+  end
+}
+def create
+  puts user_params
+  # => #<ActionController::Parameters
+  #       {"email"=>"[email protected]","password"=>"7c84241a1102"}
+  #       permitted: true
+  #     >
+end
+```
+
 ## Is it any good?
 
 [Yes.](https://news.ycombinator.com/item?id=3067434)

spec/typed_params/formatters_spec.rb

@@ -3,30 +3,30 @@
 require 'spec_helper'
 
 RSpec.describe TypedParams::Formatters do
-  describe '.register' do
-    after { TypedParams::Formatters.unregister(:test) }
+  after { TypedParams::Formatters.unregister(:test) }
 
+  describe '.register' do
     it 'should register format' do
       format = TypedParams::Formatters.register(:test,
-        transform: -> k, v { [k, v] },
+        transform: -> params { params },
       )
 
       expect(TypedParams::Formatters.formats[:test]).to eq format
     end
 
     it 'should not register a duplicate format' do
       format = TypedParams::Formatters.register(:test,
-        transform: -> k, v { [k, v] },
+        transform: -> params { params },
       )
 
-      expect { TypedParams::Formatters.register(:test, transform: -> k, v { [k, v] }) }
+      expect { TypedParams::Formatters.register(:test, transform: -> params { params }) }
         .to raise_error ArgumentError
     end
   end
 
   describe '.unregister' do
     it 'should unregister format' do
-      TypedParams::Formatters.register(:test, transform: -> k, v { [k, v] },)
+      TypedParams::Formatters.register(:test, transform: -> params { params })
       TypedParams::Formatters.unregister(:test)
 
       expect(TypedParams::Formatters.formats[:test]).to be_nil
@@ -40,4 +40,22 @@
       expect(format.format).to eq :jsonapi
     end
   end
-end
+
+  it 'should use format' do
+    TypedParams::Formatters.register(:test,
+      transform: -> params { params.deep_transform_keys { _1.to_s.camelize(:lower).to_sym } },
+    )
+
+    schema = TypedParams::Schema.new(type: :hash) do
+      format :test
+
+      param :foo_bar, type: :string
+    end
+
+    params = TypedParams::Parameterizer.new(schema:).call(
+      value: { foo_bar: 'baz' },
+    )
+
+    expect(params.unwrap).to eq(fooBar: 'baz')
+  end
+end