Add Step#call_operation to support calling operations with no args

Some steps in a transaction may not accept any input. Make it easy for step adapters to support calling these operations by adding a `Step#call_operation` method. This only calls the operation with args if it accepts any.

lib/dry/transaction/step.rb

@@ -55,6 +55,14 @@ def call(input)
         }
       end
 
+      def call_operation(*input)
+        if arity >= 1
+          operation.call(*input)
+        else
+          operation.call
+        end
+      end
+
       def arity
         operation.is_a?(Proc) ? operation.arity : operation.method(:call).arity
       end

lib/dry/transaction/step_adapters/map.rb

@@ -6,7 +6,7 @@ class Map
         include Dry::Monads::Either::Mixin
 
         def call(step, input, *args)
-          Right(step.operation.call(input, *args))
+          Right(step.call_operation(input, *args))
         end
       end
 

lib/dry/transaction/step_adapters/raw.rb

@@ -8,7 +8,7 @@ class Raw
         include Dry::Monads::Either::Mixin
 
         def call(step, input, *args)
-          result = step.operation.call(input, *args)
+          result = step.call_operation(input, *args)
 
           unless result.is_a?(Dry::Monads::Either)
             raise ArgumentError, "step +#{step.step_name}+ must return an Either object"

lib/dry/transaction/step_adapters/tee.rb

@@ -6,7 +6,7 @@ class Tee
         include Dry::Monads::Either::Mixin
 
         def call(step, input, *args)
-          step.operation.call(input, *args)
+          step.call_operation(input, *args)
           Right(input)
         end
       end

lib/dry/transaction/step_adapters/try.rb

@@ -10,7 +10,8 @@ def call(step, input, *args)
             raise ArgumentError, "+try+ steps require one or more exception classes provided via +catch:+"
           end
 
-          Right(step.operation.call(input, *args))
+          result = step.call_operation(input, *args)
+          Right(result)
         rescue *Array(step.options[:catch]) => e
           e = step.options[:raise].new(e.message) if step.options[:raise]
           Left(e)