Implement tasks condition feature

features/condition.feature

@@ -0,0 +1,15 @@
+Feature: condition
+
+  Scenario: prevents task evaluation when condition is not met
+    Given a recipe with:
+      """
+      task :hello do
+        condition { false }
+
+        puts 'evaluated'
+        exit 70
+      end
+      """
+    When I execute the recipe
+    Then the exit status must be 0
+    And the output must not contain "evaluated"

lib/producer/core/task.rb

@@ -9,7 +9,21 @@ module Producer
       end
 
       def evaluate
-        instance_eval &@block
+        DSL.new &@block
+      end
+
+
+      class DSL
+        ConditionNotMetError = Class.new(RuntimeError)
+
+        def initialize(&block)
+          instance_eval &block
+        rescue ConditionNotMetError
+        end
+
+        def condition(&block)
+          raise ConditionNotMetError unless block.call
+        end
       end
     end
   end

spec/producer/core/task_spec.rb

@@ -3,7 +3,7 @@ require 'spec_helper'
 module Producer::Core
   describe Task do
     let(:name)      { :some_task }
-    let(:block)     { }
+    let(:block)     { Proc.new { } }
     subject(:task)  { Task.new(name, &block) }
 
     describe '#name' do
@@ -12,11 +12,46 @@ module Producer::Core
       end
     end
 
-    describe 'evaluate' do
-      let(:block) { Proc.new { raise 'error from task' } }
+    describe '#evaluate' do
+      it 'builds a task DSL sandbox' do
+        expect(Task::DSL).to receive(:new).with(&block)
+        task.evaluate
+      end
+    end
 
-      it 'evaluates its block' do
-        expect { task.evaluate }.to raise_error(RuntimeError, 'error from task')
+    describe Task::DSL do
+      let(:dsl) { Task::DSL.new &block }
+
+      describe '#initialize' do
+        let(:block) { Proc.new { raise 'error from task' } }
+
+        it 'evaluates its block' do
+          expect { dsl }.to raise_error(RuntimeError, 'error from task')
+        end
+      end
+
+      describe '#condition' do
+        context 'condition is met (block evals to true)' do
+          let(:block) { Proc.new {
+            condition { true }
+            raise 'error after condition'
+          } }
+
+          it 'evaluates all the block' do
+            expect { dsl }.to raise_error(RuntimeError, 'error after condition')
+          end
+        end
+
+        context 'condition is not met (block evals to false)' do
+          let(:block) { Proc.new {
+            condition { false }
+            raise
+          } }
+
+          it 'evaluates all the block' do
+            expect { dsl }.not_to raise_error
+          end
+        end
       end
     end
   end