custom types can set a message to be used in the response when invalid

Just return an instance of `Grape::Types::InvalidValue` with the message:

    class Color
      def self.parse(value)
	return value if %w[blue red green].include?(value)

        Grape::Types::InvalidValue.new('Invalid color')
      end
    end

Any raised exception will be treated as an invalid value as it was before.

CHANGELOG.md

@@ -3,6 +3,7 @@
 #### Features
 
 * Your contribution here.
+* [#2157](https://github.com/ruby-grape/grape/pull/2157): Custom types can set a message to be used in the response when invalid - [@dnesteryuk](https://github.com/dnesteryuk).
 * [#2145](https://github.com/ruby-grape/grape/pull/2145): Ruby 3.0 compatibility - [@ericproulx](https://github.com/ericproulx).
 * [#2143](https://github.com/ruby-grape/grape/pull/2143): Enable GitHub Actions with updated RuboCop and Danger - [@anakinj](https://github.com/anakinj).
 

Gemfile

@@ -35,3 +35,7 @@ group :test do
   gem 'rspec', '~> 3.0'
   gem 'ruby-grape-danger', '~> 0.2.0', require: false
 end
+
+platforms :jruby do
+  gem 'racc'
+end

README.md

@@ -1183,7 +1183,8 @@ Aside from the default set of supported types listed above, any class can be
 used as a type as 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, or return an instance of `Grape::Types::InvalidValue` which optionally
+accepts a message to be returned in the response.
 
 ```ruby
 class Color
@@ -1193,8 +1194,9 @@ class Color
   end
 
   def self.parse(value)
-    fail 'Invalid color' unless %w(blue red green).include?(value)
-    new(value)
+    return new(value) if %w[blue red green]).include?(value)
+
+    Grape::Types::InvalidValue.new('Unsupported color')
   end
 end
 

lib/grape/validations/types.rb

@@ -7,6 +7,7 @@
 require_relative 'types/variant_collection_coercer'
 require_relative 'types/json'
 require_relative 'types/file'
+require_relative 'types/invalid_value'
 
 module Grape
   module Validations
@@ -21,10 +22,6 @@ module Validations
     # and {Grape::Dsl::Parameters#optional}. The main
     # entry point for this process is {Types.build_coercer}.
     module Types
-      # Instances of this class may be used as tokens to denote that
-      # a parameter value could not be coerced.
-      class InvalidValue; end
-
       # Types representing a single value, which are coerced.
       PRIMITIVES = [
         # Numerical

lib/grape/validations/types/custom_type_coercer.rb

@@ -55,6 +55,8 @@ def call(val)
           return if val.nil?
 
           coerced_val = @method.call(val)
+
+          return coerced_val if coerced_val.is_a?(InvalidValue)
           return InvalidValue.new unless coerced?(coerced_val)
           coerced_val
         end

lib/grape/validations/types/invalid_value.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module Grape
+  module Validations
+    module Types
+      # Instances of this class may be used as tokens to denote that a parameter value could not be
+      # coerced. The given message will be used as a validation error.
+      class InvalidValue
+        attr_reader :message
+
+        def initialize(message = nil)
+          @message = message
+        end
+      end
+    end
+  end
+end
+
+# only exists to make it shorter for external use
+module Grape
+  module Types
+    InvalidValue = Class.new(Grape::Validations::Types::InvalidValue)
+  end
+end

lib/grape/validations/validators/coerce.rb

@@ -36,7 +36,7 @@ def validate_param!(attr_name, params)
 
         new_value = coerce_value(params[attr_name])
 
-        raise validation_exception(attr_name) unless valid_type?(new_value)
+        raise validation_exception(attr_name, new_value.message) unless valid_type?(new_value)
 
         # Don't assign a value if it is identical. It fixes a problem with Hashie::Mash
         # which looses wrappers for hashes and arrays after reassigning values
@@ -80,8 +80,11 @@ def type
         @option[:type].is_a?(Hash) ? @option[:type][:value] : @option[:type]
       end
 
-      def validation_exception(attr_name)
-        Grape::Exceptions::Validation.new(params: [@scope.full_name(attr_name)], message: message(:coerce))
+      def validation_exception(attr_name, custom_msg = nil)
+        Grape::Exceptions::Validation.new(
+          params: [@scope.full_name(attr_name)],
+          message: custom_msg || message(:coerce)
+        )
       end
     end
   end

spec/grape/validations/validators/coerce_spec.rb

@@ -227,23 +227,51 @@ def self.parsed?(value)
         expect(last_response.body).to eq('NilClass')
       end
 
-      it 'is a custom type' do
-        subject.params do
-          requires :uri, coerce: SecureURIOnly
-        end
-        subject.get '/secure_uri' do
-          params[:uri].class
+      context 'a custom type' do
+        it 'coerces the given value' do
+          subject.params do
+            requires :uri, coerce: SecureURIOnly
+          end
+          subject.get '/secure_uri' do
+            params[:uri].class
+          end
+
+          get 'secure_uri', uri: 'https://www.example.com'
+
+          expect(last_response.status).to eq(200)
+          expect(last_response.body).to eq('URI::HTTPS')
+
+          get 'secure_uri', uri: 'http://www.example.com'
+
+          expect(last_response.status).to eq(400)
+          expect(last_response.body).to eq('uri is invalid')
         end
 
-        get 'secure_uri', uri: 'https://www.example.com'
+        context 'returning the InvalidValue instance when invalid' do
+          let(:custom_type) do
+            Class.new do
+              def self.parse(_val)
+                Grape::Types::InvalidValue.new('must be unique')
+              end
+            end
+          end
 
-        expect(last_response.status).to eq(200)
-        expect(last_response.body).to eq('URI::HTTPS')
+          it 'uses a custom message added to the invalid value' do
+            type = custom_type
+
+            subject.params do
+              requires :name, type: type
+            end
+            subject.get '/whatever' do
+              params[:name].class
+            end
 
-        get 'secure_uri', uri: 'http://www.example.com'
+            get 'whatever', name: 'Bob'
 
-        expect(last_response.status).to eq(400)
-        expect(last_response.body).to eq('uri is invalid')
+            expect(last_response.status).to eq(400)
+            expect(last_response.body).to eq('name must be unique')
+          end
+        end
       end
 
       context 'Array' do