3.7. Derived instances

The deriving keyword in Haskell presents a small problem for Haddock. Lacking a full type system, Haddock has to figure out what the shape of the derived instance is. There is currently a partial implementation of this, but for some data types Haddock can't determine the shape of the derived instance, and you see something like “???” in the documentation.

One workaround for this is to use CPP (Section 2.1, “Using literate or pre-processed source”) to give Haddock explicit instance headers. Here is an example from the OpenGL library:

data VertexArrayDescriptor a =
   VertexArrayDescriptor !NumComponents !DataType !Stride !(Ptr a)
#ifdef __HADDOCK__
-- Help Haddock a bit, because it doesn't do any instance inference.
instance Eq (VertexArrayDescriptor a)
instance Ord (VertexArrayDescriptor a)
instance Show (VertexArrayDescriptor a)
   deriving ( Eq, Ord, Show )

As a rule of thumb, try deriving first, and if that doesn't work then use the above workaround.