feature(ruby-grape#2290): Use a param value as the default value of…

… other param

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 #### Features
 
+* [#2371](https://github.com/ruby-grape/grape/pull/2371): Use a param value as the `default` value of other param - [@jcagarcia](https://github.com/jcagarcia).
 * Your contribution here.
 
 #### Fixes

README.md

@@ -1242,6 +1242,17 @@ params do
 end
 ```
 
+You can use the value of one parameter as the default value of some other parameter. In this
+case, if the `primary_color` parameter is not provided, it will have the same value as the `color`
+one. If both of them not provided, both of them will have `blue` value.
+
+```ruby
+params do
+  optional :color, type: String, default: 'blue'
+  optional :primary_color, type: String, default: -> (params) { params[:color] }
+end
+```
+
 ### Supported Parameter Types
 
 The following are all valid types, supported out of the box by Grape:

lib/grape/validations/validators/default_validator.rb

@@ -11,7 +11,11 @@ def initialize(attrs, options, required, scope, **opts)
 
         def validate_param!(attr_name, params)
           params[attr_name] = if @default.is_a? Proc
-                                @default.call
+                                if @default.parameters.empty?
+                                  @default.call
+                                else
+                                  @default.call(params)
+                                end
                               elsif @default.frozen? || !@default.duplicable?
                                 @default
                               else

spec/grape/validations/validators/default_spec.rb

@@ -89,6 +89,19 @@
       get '/another_nested_optional_array' do
         { root: params[:root] }
       end
+
+      params do
+        requires :foo
+        optional :bar, default: ->(params) { params[:foo] }
+        optional :qux, default: ->(params) { params[:bar] }
+      end
+      get '/default_values_from_other_params' do
+        {
+          foo: params[:foo],
+          bar: params[:bar],
+          qux: params[:qux]
+        }
+      end
     end
   end
 
@@ -460,4 +473,17 @@ def app
       expect(JSON.parse(last_response.body)).to eq(expected)
     end
   end
+
+  it 'sets default value for optional params using other params values' do
+    expected_foo_value = 'foo-value'
+
+    get("/default_values_from_other_params?foo=#{expected_foo_value}")
+
+    expect(last_response.status).to eq(200)
+    expect(last_response.body).to eq({
+      foo: expected_foo_value,
+      bar: expected_foo_value,
+      qux: expected_foo_value
+    }.to_json)
+  end
 end