Template Haskell: support for Haddock comments

I would like Template Haskell to be aware of Haddock comments.

Here's a concrete example. The data-accessor-template package (http://hackage.haskell.org/package/data-accessor-template) can be used as follows:

{-# LANGUAGE TemplateHaskell #-}
module Example where
import Data.Accessor
import Data.Accessor.Template

data MyRecord = MyRecord 
 { field1_ :: Int -- ^ a field
 , field2_ :: Int -- ^ another field
 }

deriveAccessors ''MyRecord
-- produces these:
-- field1, field2 :: Accessor MyRecord Int

We would like the values field1 and field2 to inherit the documentation from field1_ and field2_, but there is no way to automate this in Template Haskell -- or even to document field1 and field2 by hand! I would like Template Haskell to support this.

There are two related features required to make this work: firstly, the Dec datatype needs to be extended to support Haddock comments; secondly, reify needs to return these comments.

I suspect the second feature may require significant plumbing to work in the general case, because I believe that .hi files don't store Haddock comments. A tolerable compromise in this particular example would be to only reify comments for things defined in the current module.

A possible API would be

data Dec =
  ...
  | DocD Name String -- attach the given docstring to the given name
  | SigD Name Type (Maybe [String]) -- a type signature, with a possibly-empty docstring for each argument and for the result type
  ...

The DocD constructor is new, but the SigD constructor is unfortunately a modification of the current SigD Name Type constructor. We make this modification to support comments of this form:

f :: Int   -- ^ The 'Int' argument
  -> Float -- ^ The 'Float' argument
  -> IO () -- ^ The return value

A possible API for reifying comments would be

reifyDocs :: Name -> (Maybe String, Maybe [String]) -- documentation string; documentation for individual arguments

Reiner

Trac metadata

Trac field	Value
Version	7.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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