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
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