Question about specialization

[Harendra Kumar]

Hi GHC devs,

I have a simple program using the streamly library, as follows, the whole
code is in the same module:

{-# INLINE iterateState #-}
{-# SPECIALIZE iterateState :: Int -> SerialT (StateT Int IO) Int #-}
iterateState :: MonadState Int m => Int -> SerialT m Int
iterateState n = do
    x <- get
    if x > n
    then do
        put (x - 1)
        iterateState n
    else return x

main :: IO ()
main = do
    State.evalStateT (S.drain (iterateState 0)) 100000

Earlier the SPECIALIZE pragma was not required on iterateState, but after
some refactoring in the library (the monad bind of SerialT changed a bit),
this program now requires a SPECIALIZE on iterateState to trigger
specialization, just INLINE also does not help.

My question is whether this may be expected in some conditions or is this
something which can be considered a bug in the compiler? I am also curious
what specifically could have made the compiler not specialize this anymore,
is it the size of the function or some other threshold?

Thanks,
Harendra
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