Raise error when trying to expose an invalid key

lib/bcdd/result/context/success.rb

@@ -1,19 +1,35 @@
 # frozen_string_literal: true
 
 class BCDD::Result
+  class Context::Error < BCDD::Result::Error
+    InvalidExposure = ::Class.new(self)
+  end
+
   class Context::Success < Context
     include ::BCDD::Result::Success::Methods
 
+    FetchValues = ->(acc_values, keys) do
+      fetched_values = acc_values.fetch_values(*keys)
+
+      keys.zip(fetched_values).to_h
+    rescue ::KeyError => e
+      message = "#{e.message}. Available to expose: #{acc_values.keys.map(&:inspect).join(', ')}"
+
+      raise Context::Error::InvalidExposure, message
+    end
+
     def and_expose(type, keys, terminal: true)
       unless keys.is_a?(::Array) && !keys.empty? && keys.all?(::Symbol)
         raise ::ArgumentError, 'keys must be an Array of Symbols'
       end
 
       Transitions.tracking.reset_and_then!
 
-      exposed_value = acc.merge(value).slice(*keys)
+      acc_values = acc.merge(value)
+
+      value_to_expose = FetchValues.call(acc_values, keys)
 
-      self.class.new(type: type, value: exposed_value, source: source, terminal: terminal)
+      self.class.new(type: type, value: value_to_expose, source: source, terminal: terminal)
     end
   end
 end

sig/bcdd/result/context.rbs

@@ -28,10 +28,19 @@ class BCDD::Result::Context < BCDD::Result
   def raise_unexpected_outcome_error: (BCDD::Result::Context | untyped, Symbol) -> void
 end
 
+class BCDD::Result::Context
+  class Error < BCDD::Result::Error
+    class InvalidExposure < BCDD::Result::Context::Error
+    end
+  end
+end
+
 class BCDD::Result::Context
   class Success < BCDD::Result::Context
     include BCDD::Result::Success::Methods
 
+    FetchValues: Proc
+
     def and_expose: (Symbol, Array[Symbol], terminal: bool) -> BCDD::Result::Context::Success
   end
 

test/bcdd/result/context/and_expose/invalid_keys_test.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class BCDD::Result
+  class ContextAndExposeInvalidKeysTest < Minitest::Test
+    class Divide
+      include BCDD::Result::Context.mixin
+
+      def call(arg1, arg2)
+        validate_numbers(arg1, arg2)
+          .and_then(:divide, extra_division: 2)
+          .and_expose(:division_completed, %i[final_numbers extra_division number1 number2])
+      end
+
+      private
+
+      def validate_numbers(arg1, arg2)
+        arg1.is_a?(Numeric) or return Failure(:invalid_arg, message: 'arg1 must be numeric')
+        arg2.is_a?(Numeric) or return Failure(:invalid_arg, message: 'arg2 must be numeric')
+
+        Success(:ok, number1: arg1, number2: arg2)
+      end
+
+      def divide(number1:, number2:, extra_division:)
+        Success(:division_completed, final_number: (number1 / number2) / extra_division)
+      end
+    end
+
+    test '#and_expose receive an invalid key' do
+      err = assert_raises(BCDD::Result::Context::Error::InvalidExposure) { Divide.new.call(12, 2) }
+
+      assert err.message.start_with?('key not found: :final_numbers')
+      assert err.message.end_with?('. Available to expose: :number1, :number2, :extra_division, :final_number')
+    end
+  end
+end