Declare loses input state

[dylex]

I would expect this:

> execDeclare (declare "World") "Hello"
"World"

to produce "HelloWorld". To make this more interesting, the initial state is visible during execution, but is lost afterwards:

> runDeclareT (declare "World" >> look) "Hello"
("World","HelloWorld")

Is this intentional? If not, I think the fix is just to have the bind operators include d (mappend (mappend d d') d'' combining the input and output of the second one), and I can make a PR.

[dylex]

Actually that's not the right fix because it would keep doubling the input...

[maksbotan]

Looking at the code, you'd get expected answer even with execDeclare (declare "World" >> return ()) "Hello"

That's because declare explicitly drops previous state:

openapi3/src/Data/OpenApi/Declare.hs

Lines 87 to 89 in 8285df8

	instance (Applicative m, Monad m, Monoid d) => MonadDeclare d (DeclareT d m) where
	declare d = DeclareT (\_ -> return (d, ()))
	look = DeclareT (\d -> return (mempty, d))

It is then combined between different declare calls in >>=:

openapi3/src/Data/OpenApi/Declare.hs

Lines 56 to 59 in 8285df8

	DeclareT dx >>= f = DeclareT $ \d -> do
	~(d', x) <- dx d
	~(d'', y) <- runDeclareT (f x) (mappend d d')
	return (mappend d' d'', y)

I'm not sure how to fix this at the moment 🤔