Emitting warnings from Template Haskell splices

Question

I know that I can cause a compile-time error by calling fail from a splice, but is it possible to only generate a warning? In particular I would like it to be possible to turn this warning into an error when compiling with -Werror.

Essentially what I'm trying to do is this:

todo :: Q Exp
todo = do
    -- emit warning somehow

    loc <- location
    let message = ... -- generate message based on loc
    [| error $(litE (stringL message)) |]

The idea is to use this instead of undefined while coding, but make sure it doesn't sneak its way into production code by compiling with -Werror.

myFunc x | isSimpleCase x = 42
         | otherwise = $todo

Answer 1

Turns out the function I was after was the Template Haskell function report. Its type signature was in the documentation, but I had to read the source code to figure out what it did. The TH documentation sure could use some improvements.

Anyway, my todo placeholder is now working perfectly, and I'll put something up on Hackage soon if anyone is interested.