Raise a helpful ArgumentError for non-hash context

lib/interactor/context.rb

@@ -52,11 +52,18 @@ class Context < OpenStruct
     #   # => 2170969340
     #
     # Returns the Interactor::Context.
+    # Raises ArgumentError if the argument is not nil, an Interactor::Context
+    #   or an object responding to each_pair (like a Hash).
     def self.build(context = {})
       if self === context
         context
-      else
+      elsif context.nil? || context.respond_to?(:each_pair)
         new(context)
+      else
+        raise ArgumentError,
+          "Expected a Hash or an object responding to #each_pair, " \
+          "got #{context.class}. Did you mean to pass keyword arguments? " \
+          "(e.g. MyInteractor.call(foo: \"bar\"))"
       end
     end
 

spec/interactor/context_spec.rb

@@ -27,6 +27,31 @@ module Interactor
         }
       end
 
+      it "builds an empty context if nil is given" do
+        context = Context.build(nil)
+
+        expect(context).to be_a(Context)
+        expect(context.send(:table)).to eq({})
+      end
+
+      it "accepts any object responding to #each_pair" do
+        hash_like = double(:hash_like)
+        allow(hash_like).to receive(:each_pair) do |&block|
+          {foo: "bar"}.each_pair(&block)
+        end
+
+        context = Context.build(hash_like)
+
+        expect(context).to be_a(Context)
+        expect(context.foo).to eq("bar")
+      end
+
+      it "raises a helpful error when given a non-hash argument" do
+        expect {
+          Context.build("baz")
+        }.to raise_error(ArgumentError, /String/)
+      end
+
       it "preserves an already built context" do
         context1 = Context.build(foo: "bar")
         context2 = Context.build(context1)