[Haskell-cafe] How to add Haddock comment for standalone derived instances?

Hiromi ISHII konn.jinro at gmail.com
Sat Aug 23 10:04:56 UTC 2014

Hi cafe,

Is there any way to add the documentation comment for the instances defined with StandaloneDeriving?

I'm currently defining data-type using GADTs and its Typeable instance.
Normally, this can be done only StandaloneDeriving and DeriveDataTypeable extensions:

> {-# LANGUAGE StandaloneDeriving, GADTs, DeriveDataTypeable #-}
> data Ordinal n where
>   OZ :: Ordinal (S n)
>   OS :: Ordinal n -> Ordinal (S n)
> deriving instance Typeable Ordinal

I added `Typeable` instance for Ordinal recently, so I want to add some comments like "Since".
But any of the following doesn't work or, even worse, haddock won't compile:

* Just before `deriving instance` line using  `-- | `
* Just after `deriving` keyword but before `instance` using `-- | `
* Just after the `Typeable Ordinal`, but no newline in-between, with `-- ^ `
* The next line of `deriving` clause with `-- ^ `

Is there any way to add documentation for instances with standalone deriving, or it's just not supported yet?

-- Hiromi ISHII
konn.jinro at gmail.com

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