Implement If() macro

This commit adds an `If()` macro to conditionally execute steps in an
operation. It's implementation is based on the implementation of the
`Wrap()` macro to execute the nested sub-activity.

Co-authored-by: Roland Schwarzer <[email protected]>

lib/trailblazer/macro.rb

@@ -11,6 +11,7 @@
 require "trailblazer/macro/rescue"
 require "trailblazer/macro/wrap"
 require "trailblazer/macro/each"
+require "trailblazer/macro/if"
 
 module Trailblazer
   module Macro
@@ -83,6 +84,6 @@ module Activity::DSL::Linear::Helper
     # Extending the {Linear::Helper} namespace is the canonical way to import
     # macros into Railway, FastTrack, Operation, etc.
     extend Forwardable
-    def_delegators Trailblazer::Macro, :Model, :Nested, :Wrap, :Rescue, :Each
+    def_delegators Trailblazer::Macro, :Model, :Nested, :Wrap, :Rescue, :Each, :If
   end # Helper
 end

lib/trailblazer/macro/if.rb

@@ -0,0 +1,23 @@
+module Trailblazer
+  module Macro
+    def self.If(condition, name: :default, id: Macro.id_for(condition, macro: :If, hint: condition), &block)
+      unless block_given?
+        raise ArgumentError, "If() requires a block"
+      end
+
+      option = Trailblazer::Option(condition)
+      wrap = ->((ctx, flow_options), **circuit_args, &block) {
+        ctx["result.condition.#{name}"] = result =
+          option.call(ctx, keyword_arguments: ctx.to_hash, **circuit_args)
+
+        if result
+          block.call
+        else
+          [Trailblazer::Activity::Right, [ctx, flow_options]]
+        end
+      }
+
+      Wrap(wrap, id: id, &block)
+    end
+  end
+end

test/docs/if_test.rb

@@ -0,0 +1,180 @@
+require "test_helper"
+
+class IfMacroTest < Minitest::Spec
+  class Song
+    module Activity
+      class Upload < Trailblazer::Activity::FastTrack
+        step :model
+        step If(:condition) {
+          step :update   # this changes the database.
+          step :transfer # this might even break!
+        }
+        step :notify
+        fail :log_error
+        #~meths
+        include T.def_steps(:model, :condition, :update, :transfer, :notify, :log_error)
+        #~meths end
+      end
+    end
+  end
+
+  it do
+    assert_invoke Song::Activity::Upload, condition: true, seq: "[:model, :condition, :update, :transfer, :notify]", expected_ctx_variables: { "result.condition.default" => true }
+    assert_invoke Song::Activity::Upload, condition: false, seq: "[:model, :condition, :notify]", expected_ctx_variables: { "result.condition.default" => false }
+    assert_invoke Song::Activity::Upload, condition: true, transfer: false, seq: "[:model, :condition, :update, :transfer, :log_error]", terminus: :failure, expected_ctx_variables: { "result.condition.default" => true }
+  end
+end
+
+class IfWithCustomNameTest < Minitest::Spec
+  class Song
+    module Activity
+      class Upload < Trailblazer::Activity::FastTrack
+        step If(:condition, name: :custom_name) {
+          step :update   # this changes the database.
+        }
+        #~meths
+        include T.def_steps(:condition, :update)
+        #~meths end
+      end
+    end
+  end
+
+  it do
+    assert_invoke Song::Activity::Upload, condition: true, seq: "[:condition, :update]", expected_ctx_variables: { "result.condition.custom_name" => true }
+    assert_invoke Song::Activity::Upload, condition: false, seq: "[:condition]", expected_ctx_variables: { "result.condition.custom_name" => false }
+  end
+end
+
+class IfWithProcTest < Minitest::Spec
+  class Song
+    module Activity
+      class Upload < Trailblazer::Activity::FastTrack
+        step If(->(_ctx, condition:, **) { condition }) {
+          step :update   # this changes the database.
+        }
+        #~meths
+        include T.def_steps(:update)
+        #~meths end
+      end
+    end
+  end
+
+  it do
+    assert_invoke Song::Activity::Upload, condition: true, seq: "[:update]", expected_ctx_variables: { "result.condition.default" => true }
+    assert_invoke Song::Activity::Upload, condition: false, seq: "[]", expected_ctx_variables: { "result.condition.default" => false }
+  end
+end
+
+class IfWithCallableTest < Minitest::Spec
+  class Song
+    module Activity
+      class Upload < Trailblazer::Activity::FastTrack
+        class Callable
+          def self.call(_ctx, condition:, **)
+            condition
+          end
+        end
+
+        step If(Callable) {
+          step :update   # this changes the database.
+        }
+        #~meths
+        include T.def_steps(:update)
+        #~meths end
+      end
+    end
+  end
+
+  it do
+    assert_invoke Song::Activity::Upload, condition: true, seq: "[:update]", expected_ctx_variables: { "result.condition.default" => true }
+    assert_invoke Song::Activity::Upload, condition: false, seq: "[]", expected_ctx_variables: { "result.condition.default" => false }
+  end
+end
+
+class IfWithNestedIfTest < Minitest::Spec
+  class Song
+    module Activity
+      class Upload < Trailblazer::Activity::FastTrack
+        step If(:condition) {
+          step :update   # this changes the database.
+          step If(:nested_condition, name: :nested) {
+            step :notify
+          }
+          step :finalize
+        }
+        #~meths
+        include T.def_steps(:condition, :update, :nested_condition, :notify, :finalize)
+        #~meths end
+      end
+    end
+  end
+
+  it do
+    assert_invoke Song::Activity::Upload, condition: true, nested_condition: true, seq: "[:condition, :update, :nested_condition, :notify, :finalize]", expected_ctx_variables: { "result.condition.default" => true, "result.condition.nested" => true }
+    assert_invoke Song::Activity::Upload, condition: false, seq: "[:condition]", expected_ctx_variables: { "result.condition.default" => false }
+    assert_invoke Song::Activity::Upload, condition: true, nested_condition: false, seq: "[:condition, :update, :nested_condition, :finalize]", expected_ctx_variables: { "result.condition.default" => true, "result.condition.nested" => false }
+  end
+end
+
+class IfTracingTest < Minitest::Spec
+  class Song
+    module Activity
+      class Upload < Trailblazer::Activity::FastTrack
+        class DecideWhatToDo
+          def self.call(*); end
+        end
+
+        def self.my_condition_handler(*); end
+
+        step If(:condition) {}
+        step If(DecideWhatToDo) {}
+        step If(method(:my_condition_handler)) {}
+        step If(->(*) {}, id: "proc.my_condition_handler") {}
+
+        #~meths
+        include T.def_steps(:condition)
+        #~meths end
+      end
+    end
+  end
+
+  it do
+    [
+      "If/condition",
+      "If/IfTracingTest::Song::Activity::Upload::DecideWhatToDo",
+      "If/method(:my_condition_handler)",
+      "proc.my_condition_handler"
+    ].each do |id|
+      assert_equal Trailblazer::Developer::Introspect.find_path(Song::Activity::Upload, [id])[0].id, id
+    end
+
+    assert_equal trace(Song::Activity::Upload, { seq: [], condition: false })[0], <<~SEQ.chomp
+      TOP
+      |-- Start.default
+      |-- If/condition
+      |-- If/IfTracingTest::Song::Activity::Upload::DecideWhatToDo
+      |-- If/method(:my_condition_handler)
+      |-- proc.my_condition_handler
+      `-- End.success
+    SEQ
+  end
+end
+
+class IfWithoutBlockTest < Minitest::Spec
+  it do
+    exception = assert_raises ArgumentError do
+      class Song
+        module Activity
+          class Upload < Trailblazer::Activity::FastTrack
+            step If(:condition)
+
+            #~meths
+            include T.def_steps(:condition)
+            #~meths end
+          end
+        end
+      end
+    end
+    assert_equal "If() requires a block", exception.message
+  end
+end