[Haskell-cafe] Re: Haddock seems to generate wrong types in newtype deriving

[Simon Marlow]

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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[Haskell-cafe] Re: Haddock seems to generate wrong types in newtype deriving

[Brian Hulley]

Simon Marlow wrote:

Brian Hulley wrote:

Hi -
I have the following code:


[snip]

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...


Thanks - I'm glad it's not just me!
In the meantime, I found a better workaround (since the type decl using a 
class name is not legal Haskell) is just to use -cpp to preprocess for both 
ghc and haddock so that haddock sees explicitly defined dummy instances 
instead of a newtype deriving clause, then perfect results are obtained... 
:-)


Regards, Brian. 


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