Add base classes to evaluate variables expressions

lib/gitlab/ci/pipeline/expression/equality.rb

+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Equality
+          PATTERN = /==/.freeze
+
+          def initialize(left, right)
+          end
+
+          def evaluate(**variables)
+            @left.evaluate(variables) == @right.evaluate(variables)
+          end
+
+          def self.build(string)
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/ci/pipeline/expression/lexer.rb

+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Lexer
+          def initialize(statement)
+            @statement = statement
+          end
+
+          def tokenize
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/ci/pipeline/expression/statement.rb

@@ -8,10 +8,10 @@ module Gitlab
             @statement = statement
           end
 
-          def errors
+          def variables
           end
 
-          def matches?
+          def evaluate
           end
         end
       end

lib/gitlab/ci/pipeline/expression/string.rb

+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class String
+          PATTERN = /("|')(?<value>.+)('|")/.freeze
+
+          def initialize(value)
+            @value = value
+          end
+
+          def evaluate(**_)
+            @value.to_s
+          end
+
+          def self.build(string)
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/ci/pipeline/expression/variable.rb

+module Gitlab
+  module Ci
+    module Pipeline
+      module Expression
+        class Equality
+          PATTERN = /$(?<name>\w+)/.freeze
+
+          def initialize(value)
+            @value = value
+          end
+
+          def evaluate(**variables)
+          end
+
+          def self.build(string)
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/pipeline/expression/statement_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Pipeline::Expression::Statement do
+end