Brian Hulley wrote:
Hi - I have the following code:
data MState = MState -- details omitted type MonadStateMState = MonadState MState -- necessary for Haddock newtype ManagerM a = ManagerM (StateT MState IO a) deriving (Monad, MonadIO, MonadStateMState)
which means that ManagerM is an instance of Monad, MonadIO, and MonadState MState. However, the Haddock docs look like:
data ManagerM a Instances ??? a => Monad (ManagerM a) ??? a => MonadIO (ManagerM a) ??? a => MonadStateMState (ManagerM a)
which doesn't seem at all right to me. I'd have thought it should say:
data ManagerM a Instances Monad ManagerM MonadIO ManagerM MonadStateMState ManagerM
Is this just a bug in Haddock or am I misunderstanding something about Haskell?
It's a bug / missing feature in Haddock. Haddock is basically pretty dumb when it comes to understanding Haskell code; it knows about the syntax and the module system, and that's about all. It makes a half-hearted attempt to figure out what instances you get from deriving clauses, but it's not complete, and you've encountered a case it doesn't handle. One day Haddock will be built on top of the GHC API, and all this will be fixed... Cheers, Simon