I'm trying to write a mutable hash table library that would work both in the IO and ST monads, so I'm using the PrimMonad typeclass [see JmoyHash.hs in the attached archive]. For efficiency, I would like the functions to be specialized to the concrete monad at the call site. According to Section 9.31.9.2 of the GHC User's Guide https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts... The optimiser *also* considers each *imported* INLINABLE overloaded function, and specialises it for the different types at which it is called in M. So I marked all the functions in my library as INLINABLE. Yet adding a SPECIALIZE pragma in Main.hs (currently commented out) for an imported function improves runtime performance by 3x, which should not be happening since, if I understand the manual right, the function should have been specialized anyway since it is marked INLINABLE. I am writing to this list rather than filing a bug since I'm not sure if I'm reading the manual right and I have not explicitly verified that the specialization is not happening. I would greatly appreciate any help on both counts. I'm using GHC 8.0.1 with the -O2 flag. Marking the function as INLINE might solve the problem but that's something I don't want to do as it seems to me that specialization and not inlining is what's appropriate here. Regards, Jyotirmoy Bhattacharya