allow custom param coercion failure messages

Custom param types can now report why coercion failed in their .parse
method by returning an InvalidValue initialized with an error message.
For example:

  Grape::Validations::Types::InvalidValue.new "is too short"

README.md

@@ -803,7 +803,9 @@ Aside from the default set of supported types listed above, any class can be
 used as a type so long as an explicit coercion method is supplied. If the type
 implements a class-level `parse` method, Grape will use it automatically.
 This method must take one string argument and return an instance of the correct
-type, or raise an exception to indicate the value was invalid. E.g.,
+type. An exception raised inside the `parse` method will be reported as a validation
+failure with a generic error message. To report a custom error message, return an
+`InvalidValue` initialized with the custom message. E.g.,
 
 ```ruby
 class Color
@@ -813,8 +815,11 @@ class Color
   end
 
   def self.parse(value)
-    fail 'Invalid color' unless %w(blue red green).include?(value)
-    new(value)
+    if  %w(blue red green).include?(value)
+      new(value)
+    else
+      Grape::Validations::Types::InvalidValue.new "is not a valid color"
+    end
   end
 end
 

lib/grape/validations/types.rb

@@ -24,7 +24,12 @@ module Validations
     module Types
       # Instances of this class may be used as tokens to denote that
       # a parameter value could not be coerced.
-      class InvalidValue; end
+      class InvalidValue
+        attr_reader :message
+        def initialize(message = nil)
+          @message = message
+        end
+      end
 
       # Types representing a single value, which are coerced through Virtus
       # or special logic in Grape.

lib/grape/validations/validators/coerce.rb

@@ -13,8 +13,16 @@ def initialize(*_args)
       def validate_param!(attr_name, params)
         raise Grape::Exceptions::Validation, params: [@scope.full_name(attr_name)], message: message(:coerce) unless params.is_a? Hash
         new_value = coerce_value(params[attr_name])
-        raise Grape::Exceptions::Validation, params: [@scope.full_name(attr_name)], message: message(:coerce) unless valid_type?(new_value)
-        params[attr_name] = new_value
+        if valid_type?(new_value)
+          params[attr_name] = new_value
+        else
+          bad_value = new_value
+          if bad_value.is_a?(Types::InvalidValue) && !bad_value.message.nil?
+            fail Grape::Exceptions::Validation, params: [@scope.full_name(attr_name)], message: bad_value.message.to_s
+          else
+            fail Grape::Exceptions::Validation, params: [@scope.full_name(attr_name)], message_key: :coerce
+          end
+        end
       end
 
       private

spec/grape/validations/params_scope_spec.rb

@@ -119,6 +119,54 @@ def initialize(value)
       expect(last_response.status).to eq(400)
       expect(last_response.body).to match(/foo is invalid/)
     end
+
+    context 'when the parse method returns an InvalidValue' do
+      module ParamsScopeSpec
+        class AnotherCustomType
+          attr_reader :value
+          def self.parse(value)
+            case value
+            when 'invalid with message'
+              Grape::Validations::Types::InvalidValue.new 'is not correct'
+            when 'invalid without message'
+              Grape::Validations::Types::InvalidValue.new
+            else
+              new(value)
+            end
+          end
+
+          def initialize(value)
+            @value = value
+          end
+        end
+      end
+
+      context 'with a message' do
+        it 'fails with the InvalidValue\'s error message' do
+          subject.params do
+            requires :foo, type: ParamsScopeSpec::AnotherCustomType
+          end
+          subject.get('/types') { params[:foo].value }
+
+          get '/types', foo: 'invalid with message'
+          expect(last_response.status).to eq(400)
+          expect(last_response.body).to match(/foo is not correct/)
+        end
+      end
+
+      context 'without a message' do
+        it 'fails with the default coercion failure message' do
+          subject.params do
+            requires :foo, type: ParamsScopeSpec::AnotherCustomType
+          end
+          subject.get('/types') { params[:foo].value }
+
+          get '/types', foo: 'invalid without message'
+          expect(last_response.status).to eq(400)
+          expect(last_response.body).to match(/foo is invalid/)
+        end
+      end
+    end
   end
 
   context 'array without coerce type explicitly given' do