Basic support for the "with" template helper

lib/ruby-handlebars/helpers/register_default_helpers.rb

@@ -2,6 +2,7 @@
 require_relative 'helper_missing_helper'
 require_relative 'if_helper'
 require_relative 'unless_helper'
+require_relative 'with_helper'
 
 module Handlebars
   module Helpers
@@ -10,6 +11,7 @@ def self.register_default_helpers(hbs)
       HelperMissingHelper.register(hbs)
       IfHelper.register(hbs)
       UnlessHelper.register(hbs)
+      WithHelper.register(hbs)
     end
   end
 end

lib/ruby-handlebars/helpers/with_helper.rb

@@ -0,0 +1,25 @@
+require_relative 'default_helper'
+
+module Handlebars
+  module Helpers
+    class WithHelper < DefaultHelper
+      def self.registry_name
+        'with'
+      end
+
+      def self.apply(context, data, block, else_block)
+        if data
+          context.with_temporary_context(data) do
+            block.fn(context)
+          end
+        else
+          else_block.fn(context)
+        end
+      end
+
+      def self.apply_as(context, data, name, block, else_block)
+        self.apply(context, { name.to_sym => data }, block, else_block)
+      end
+    end
+  end
+end

spec/ruby-handlebars/helpers/with_helper_spec.rb

@@ -0,0 +1,80 @@
+require_relative '../../spec_helper'
+require_relative './shared'
+
+require_relative '../../../lib/ruby-handlebars'
+require_relative '../../../lib/ruby-handlebars/tree'
+require_relative '../../../lib/ruby-handlebars/helpers/with_helper'
+
+describe Handlebars::Helpers::WithHelper do
+  let(:subject) { Handlebars::Helpers::WithHelper }
+  let(:hbs) { Handlebars::Handlebars.new }
+
+  it_behaves_like "a registerable helper", "with"
+
+  context '.apply' do
+    include_context "shared apply helper"
+  end
+
+  context "integration" do
+    include_context "shared helpers integration tests"
+
+    let(:person_data) { { person: { firstname: "Yehuda", lastname: "Katz" }} }
+    let(:city_data) { {
+      city: {
+        name: "San Francisco",
+        summary: "San Francisco is the <b>cultural center</b> of <b>Northern California</b>",
+        location: {
+          north: "37.73,",
+          east: -122.44,
+        },
+        population: 883305,
+      },
+    } }
+
+    it "changes the evaluation context" do
+      template = <<~HANDLEBARS
+        {{#with person}}
+        {{firstname}} {{lastname}}
+        {{/with}}
+      HANDLEBARS
+
+      expect(evaluate(template, person_data).strip).to eq("Yehuda Katz")
+    end
+
+    it "supports block parameters" do
+      template = <<~HANDLEBARS
+        {{#with city as | city |}}
+          {{#with city.location as | loc |}}
+            {{city.name}}: {{loc.north}} {{loc.east}}
+          {{/with}}
+        {{/with}}
+      HANDLEBARS
+
+      expect(evaluate(template, city_data).strip).to eq("San Francisco: 37.73, -122.44")
+    end
+
+    it "supports else blocks" do
+      template = <<~HANDLEBARS
+        {{#with city}}
+        {{city.name}} (not shown because there is no city)
+        {{else}}
+        No city found
+        {{/with}}
+      HANDLEBARS
+
+      expect(evaluate(template, person_data).strip).to eq("No city found")
+    end
+
+    it "supports relative paths", skip: "Relative paths are not yet supported" do
+      template = <<~HANDLEBARS
+        {{#with city as | city |}}
+          {{#with city.location as | loc |}}
+            {{city.name}}: {{../population}}
+          {{/with}}
+        {{/with}}
+      HANDLEBARS
+
+      expect(evaluate(template, city_data).strip).to eq("San Francisco: 883305")
+    end
+  end
+end