Expose schema DSL to `value` and `maybe` with `:hash`

Closes #215

docsite/source/nested-data.html.md

@@ -38,20 +38,28 @@ puts errors.to_h.inspect
 # }
 ```
 
+It is equivalent to call `value` macro with `:hash` predicate and a block:
+
+```ruby
+schema = Dry::Schema.Params do
+  required(:address).value(:hash) do
+    # ...
+  end
+end
+```
+
 ### Nested Maybe `Hash`
 
 If a nested hash could be `nil`, simply use `maybe` macro with a block:
 
 ```ruby
 schema = Dry::Schema.Params do
-  required(:address).maybe do
-    hash do
-      required(:city).filled(:string, min_size?: 3)
-      required(:street).filled(:string)
-      required(:country).hash do
-        required(:name).filled(:string)
-        required(:code).filled(:string)
-      end
+  required(:address).maybe(:hash) do
+    required(:city).filled(:string, min_size?: 3)
+    required(:street).filled(:string)
+    required(:country).hash do
+      required(:name).filled(:string)
+      required(:code).filled(:string)
     end
   end
 end

lib/dry/schema/macros/value.rb

@@ -27,13 +27,18 @@ module Dry
           end
 
           trace.evaluate(*predicates, **opts)
-          trace.append(new(chain: false).instance_exec(&block)) if block
+
+          type_spec = opts[:type_spec]
+          if block && type_spec.equal?(:hash)
+            hash(&block)
+          elsif block
+            trace.append(new(chain: false).instance_exec(&block))
+          end
 
           if trace.captures.empty?
             raise ArgumentError, 'wrong number of arguments (given 0, expected at least 1)'
           end
 
-          type_spec = opts[:type_spec]
           each(type_spec.type.member) if type_spec.respond_to?(:member)
 
           self

spec/integration/schema/macros/maybe_spec.rb

@@ -110,5 +110,33 @@ RSpec.describe 'Macros #maybe' do
       )
     end
   end
-end
 
+  context 'with a nested hash' do
+    subject(:schema) do
+      Dry::Schema.define do
+        required(:song).maybe(:hash) do
+          required(:title).filled
+          required(:author).filled
+        end
+      end
+    end
+
+    it 'passes when valid' do
+      song = { title: 'World', author: 'Joe' }
+
+      expect(schema.(song: song)).to be_success
+    end
+
+    it 'fails when not valid' do
+      song = { title: nil, author: 'Jane' }
+
+      expect(schema.(song: song).messages).to eql(
+        song: { title: ['must be filled'] }
+      )
+    end
+
+    it 'passes when nil' do
+      expect(schema.(song: nil)).to be_success
+    end
+  end
+end

spec/unit/dry/schema/macros/value_spec.rb

@@ -31,4 +31,29 @@ RSpec.describe Dry::Schema::Macros::Value do
       expect { macro.not_here }.to raise_error(NoMethodError, /not_here/)
     end
   end
+
+  context 'with a nested hash' do
+    subject(:schema) do
+      Dry::Schema.define do
+        required(:song).value(:hash) do
+          required(:title).filled
+          required(:author).filled
+        end
+      end
+    end
+
+    it 'passes when valid' do
+      song = { title: 'World', author: 'Joe' }
+
+      expect(schema.(song: song)).to be_success
+    end
+
+    it 'fails when not valid' do
+      song = { title: nil, author: 'Jane' }
+
+      expect(schema.(song: song).messages).to eql(
+        song: { title: ['must be filled'] }
+      )
+    end
+  end
 end