TIL difference between RSpec's syntaxes for returning a mocked value.

debugger.rb

(byebug) hello = instance_double("Planet")
#<InstanceDouble(Planet) (anonymous)>

# Using RSpec block syntax to specific a return value.

(byebug) allow(hello).to receive(:world) { "hello, world" } 
#<RSpec::Mocks::VerifyingMessageExpectation #<InstanceDouble(Planet) (anonymous)>.world(any arguments)>

# We get the value we intended

(byebug) hello.world
"hello, world"

# Notice that it's a different string instance each invocation

(byebug) hello.world.object_id
70139543692820
(byebug) hello.world.object_id
70139543680920

# Now, using and_return to specify a return value.

(byebug) allow(hello).to receive(:world).and_return("hello, world") # AND_RETURN
#<RSpec::Mocks::VerifyingMessageExpectation #<InstanceDouble(Planet) (anonymous)>.world(any arguments)>
(byebug) hello.world
"hello, world"

# Notice here and_return returns the same string object.

(byebug) hello.world.object_id
70139543677620
(byebug) hello.world.object_id
70139543677620

# Blocks are re-evaluated each time. Obvious in hindsight, though not at first.