Allow second parameter for the instance in type coercers

Author: nepalez

CHANGELOG.md

@@ -5,6 +5,35 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [2.3.0] [WIP]
+
+### Added
+- Type coercer can take second argument for the initialized instance (nepalez)
+  This allows to wrap assigned value to the object that refers back
+  to the initializer instance. More verbose example:
+
+  ```ruby
+  class Location < String
+    attr_reader :parameter # refers back to its parameter
+  
+    def initialize(name, parameter)
+      super(name)
+      @parameter = parameter
+    end
+  end
+
+  class Parameter
+    extend Dry::Initializer
+    param :name
+    param :location, ->(value, param) { Location.new(value, param) }
+  end
+
+  offset = Parameter.new "offset", location: "query"
+  offset.name     # => "offset"
+  offset.location # => "query"
+  offset.location.parameter == offset # true
+  ```
+
 ## [2.2.0] [2017-09-13]
 
 ### Added
@@ -711,3 +740,4 @@ First public release
 [2.0.0]: https://github.com/dry-rb/dry-initializer/compare/v1.4.1...v2.0.0
 [2.1.0]: https://github.com/dry-rb/dry-initializer/compare/v2.0.0...v2.1.0
 [2.2.0]: https://github.com/dry-rb/dry-initializer/compare/v2.1.0...v2.2.0
+[2.3.0]: https://github.com/dry-rb/dry-initializer/compare/v2.2.0...v2.3.0

lib/dry/initializer/builders/attribute.rb

@@ -65,7 +65,12 @@ def default_line
 
     def coercion_line
       return unless @type
-      "#{@val} = #{@item}.type.call(#{@val}) unless #{@val} == #{@null}"
+      arity = @type.is_a?(Proc) ? @type.arity : @type.method(:call).arity
+      if arity.abs == 1
+        "#{@val} = #{@item}.type.call(#{@val}) unless #{@val} == #{@null}"
+      else
+        "#{@val} = #{@item}.type.call(#{@val}, self) unless #{@val} == #{@null}"
+      end
     end
 
     def assignment_line

lib/dry/initializer/definition.rb

@@ -81,10 +81,11 @@ def check_target(value)
 
     def check_type(value)
       return if value.nil?
-      arity = value.respond_to?(:call) ? value.method(:call).arity : 0
-      return value unless arity.zero? || arity > 1
+      arity =   value.arity               if value.is_a? Proc
+      arity ||= value.method(:call).arity if value.respond_to? :call
+      return value if [1, 2].include? arity.to_i.abs
       raise TypeError,
-            "type of #{inspect} should respond to #call with one argument"
+            "type of #{inspect} should respond to #call with 1..2 arguments"
     end
 
     def check_default(value)

spec/type_constraint_spec.rb

@@ -1,6 +1,36 @@
 require "dry-types"
 
 describe "type constraint" do
+  context "by a proc with 1 argument" do
+    before do
+      class Test::Foo
+        extend Dry::Initializer
+        param :foo, proc(&:to_s), optional: true
+      end
+    end
+
+    subject { Test::Foo.new :foo }
+
+    it "coerces a value" do
+      expect(subject.foo).to eq "foo"
+    end
+  end
+
+  context "by a proc with 2 arguments" do
+    before do
+      class Test::Foo
+        extend Dry::Initializer
+        param :foo, proc { |val, obj| "#{obj.hash}:#{val}" }, optional: true
+      end
+    end
+
+    subject { Test::Foo.new :foo }
+
+    it "coerces a value with self as a second argument" do
+      expect(subject.foo).to eq "#{subject.hash}:foo"
+    end
+  end
+
   context "by dry-type" do
     before do
       class Test::Foo