Values::ArgumentError - exception class containing missing/unexpected constructor arguments

lib/values.rb

@@ -10,6 +10,17 @@
 #   p.y
 #   #=> 0
 #
+module Values
+  class FieldError < ::ArgumentError
+    attr_reader :missing_fields, :unexpected_fields
+
+    def initialize(message, missing_fields=[], unexpected_fields=[])
+      @missing_fields, @unexpected_fields = missing_fields, unexpected_fields
+      super(message)
+    end
+  end
+end
+
 class Value
   # Create a new value class.
   #
@@ -24,7 +35,9 @@ def self.new(*fields, &block)
       attr_reader(:hash, *fields)
 
       define_method(:initialize) do |*values|
-        raise ArgumentError.new("wrong number of arguments, #{values.size} for #{fields.size}") if fields.size != values.size
+        if (fields.size != values.size)
+          raise Values::FieldError.new("wrong number of arguments, #{values.size} for #{fields.size}", fields[values.size..-1])
+        end
 
         fields.zip(values) do |field, value|
           instance_variable_set(:"@#{field}", value)
@@ -38,14 +51,13 @@ def self.new(*fields, &block)
       const_set :VALUE_ATTRS, fields
 
       def self.with(hash)
-        unexpected_keys = hash.keys - self::VALUE_ATTRS
-        if unexpected_keys.any?
-          raise ArgumentError.new("Unexpected hash keys: #{unexpected_keys}")
-        end
+        unexpected_fields = hash.keys - self::VALUE_ATTRS
+        missing_fields = self::VALUE_ATTRS - hash.keys
 
-        missing_keys = self::VALUE_ATTRS - hash.keys
-        if missing_keys.any?
-          raise ArgumentError.new("Missing hash keys: #{missing_keys} (got keys #{hash.keys})")
+        if unexpected_fields.any?
+          raise Values::FieldError.new("Unexpected hash keys: #{unexpected_fields}", missing_fields, unexpected_fields)
+        elsif missing_fields.any?
+          raise Values::FieldError.new("Missing hash keys: #{missing_fields} (got keys #{hash.keys})", missing_fields, unexpected_fields)
         end
 
         new(*hash.values_at(*self::VALUE_ATTRS))

spec/values_spec.rb

@@ -42,7 +42,12 @@
     end
 
     it 'raises argument errors if not given the right number of arguments' do
-      expect { Point.new }.to raise_error(ArgumentError, 'wrong number of arguments, 0 for 2')
+      expect { Point.new }.to raise_error do |error|
+        expect(error).to be_a(Values::FieldError)
+        expect(error.message).to eq('wrong number of arguments, 0 for 2')
+        expect(error.missing_fields).to contain_exactly(:x, :y)
+        expect(error.unexpected_fields).to be_empty
+      end
     end
   end
 
@@ -100,11 +105,19 @@ def change_color(new_color)
   end
 
   it 'errors if you instantiate it from a hash with unrecognised fields' do
-    expect { Money.with(:unrecognized_field => 1, :amount => 2, :denomination => 'USD') }.to raise_error(ArgumentError)
+    expect { Money.with(:unrecognized_field => 1, :amount => 2, :denomination => 'USD') }.to raise_error do |error|
+      expect(error).to be_a(Values::FieldError)
+      expect(error.missing_fields).to be_empty
+      expect(error.unexpected_fields).to contain_exactly(:unrecognized_field)
+    end
   end
 
   it 'errors if you instantiate it from a hash with missing fields' do
-    expect { Money.with({}) }.to raise_error(ArgumentError)
+    expect { Money.with({}) }.to raise_error do |error|
+      expect(error).to be_a(Values::FieldError)
+      expect(error.missing_fields).to contain_exactly(:amount, :denomination)
+      expect(error.unexpected_fields).to be_empty
+    end
   end
 
   it 'does not error when fields are explicitly nil' do
@@ -200,7 +213,11 @@ def change_color(new_color)
       end
 
       it 'raises argument error if unknown field' do
-        expect { p.with({ :foo => 3 }) }.to raise_error(ArgumentError)
+        expect { p.with({ :foo => 3 , :bar => "baz" }) }.to raise_error do |error|
+          expect(error).to be_a(Values::FieldError)
+          expect(error.unexpected_fields).to contain_exactly(:foo, :bar)
+          expect(error.missing_fields).to be_empty
+        end
       end
     end
   end