Is it possible to generate comments to functions in Template Haskell?

Question

In our project we have a lot of TH-generated functions. It'd make sense to add generic comments to them so that they are visible in Haddock/Hoogle. At the very least, something like "This has been generated by TH." Is something like that possible?

Answer 1

Yes it is.

Here is a basic example that declares a copy of some function, adds hello_ in front of its name, and adds Hello ! in front of its documentation.

{-# LANGUAGE TemplateHaskell #-}

module MyTH where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

helloCopy :: Name -> Q [Dec]
helloCopy name = do
  doc  <- getDoc (DeclDoc name)
  let helloName = mkName ("hello_" ++ nameBase name)
  let helloDoc = case doc of
        Nothing -> "Hello !"
        Just old -> "Hello !" ++ old
  addModFinalizer $ putDoc (DeclDoc helloName) helloDoc
  [d| $(varP helloName)= $(varE name) |]

module MyModule where

import MyTH

-- | Interesting stuff about blub
blub :: Int
blub = 4

$(helloCopy 'blub)

Now if you run haddock you will get a documentation for hello_blub that says "Hello ! Interesting stuff about blub"