Add helpful error message when not returning a Dry::Monads::Result from step

lib/dry/operation.rb

@@ -2,6 +2,7 @@
 
 require "zeitwerk"
 require "dry/monads"
+require "dry/operation/errors"
 
 module Dry
   # DSL for chaining operations that can fail
@@ -136,7 +137,11 @@ def steps(&block)
     # @return [Object] wrapped value
     # @see #steps
     def step(result)
-      result.value_or { throw_failure(result) }
+      if result.is_a?(Dry::Monads::Result)
+        result.value_or { throw_failure(result) }
+      else
+        raise InvalidStepResultError.new(result: result)
+      end
     end
 
     # Invokes a callable in case of block's failure

lib/dry/operation/errors.rb

@@ -2,8 +2,10 @@
 
 module Dry
   class Operation
+    class Error < ::StandardError; end
+
     # Methods to prepend have already been defined
-    class MethodsToPrependAlreadyDefinedError < ::StandardError
+    class MethodsToPrependAlreadyDefinedError < Error
       def initialize(methods:)
         super <<~MSG
           '.operate_on' must be called before the given methods are defined.
@@ -13,7 +15,7 @@ def initialize(methods:)
     end
 
     # Configuring prepending after a method has already been prepended
-    class PrependConfigurationError < ::StandardError
+    class PrependConfigurationError < Error
       def initialize(methods:)
         super <<~MSG
           '.operate_on' and '.skip_prepending' can't be called after any methods\
@@ -24,7 +26,7 @@ def initialize(methods:)
     end
 
     # Missing dependency required by an extension
-    class MissingDependencyError < ::StandardError
+    class MissingDependencyError < Error
       def initialize(gem:, extension:)
         super <<~MSG
           To use the #{extension} extension, you first need to install the \
@@ -33,6 +35,15 @@ def initialize(gem:, extension:)
       end
     end
 
+    class InvalidStepResultError < Error
+      def initialize(result:)
+        super <<~MSG
+          Your step must return `Success(..)` or `Failure(..)`, \
+          from `Dry::Monads::Result`. Instead, it was `#{result.inspect}`.
+        MSG
+      end
+    end
+
     # An error related to an extension
     class ExtensionError < ::StandardError; end
   end

spec/unit/operation_spec.rb

@@ -54,6 +54,18 @@ def foo(value)
         described_class.new.step(failure)
       }.to throw_symbol(:halt, failure)
     end
+
+    it "raises helpful error when returning something that is not a Result" do
+      expect {
+        described_class.new.step(123)
+      }.to raise_error(Dry::Operation::InvalidStepResultError)
+        .with_message(
+          <<~MSG
+            Your step must return `Success(..)` or `Failure(..)`, \
+            from `Dry::Monads::Result`. Instead, it was `123`.
+          MSG
+        )
+    end
   end
 
   describe "#intercepting_failure" do