How do I use TemplateHaskell's addDependentFile on a file relative to the file being compiled?

haskell, template-haskell

Solution

You can use `location` from `Language.Haskell.TH.Syntax` to extract the file name of the file being compiled, and use this to assemble the correct path:

-- | Uses 'addDependentFile' on a file relative to the current file
-- to mark it as being checked for changes when compiled with TemplateHaskell.
--
-- Returns an empty list of declarations so that it can be used with:
--
-- >$(addDependentFileRelative "MyDependency.txt")
addDependentFileRelative :: FilePath -> Q [Dec]
addDependentFileRelative relativeFile = do
    currentFilename <- loc_filename <$> location
    pwd             <- runIO getCurrentDirectory

    let invocationRelativePath = takeDirectory (pwd </> currentFilename) </> relativeFile

    addDependentFile invocationRelativePath

    return []

(public domain code)

Problem

I want that my TemplateHaskell expression, which uses IO and depends on the file `MyDependency.txt`, is recomputed when that file is being changed. Therefore I am using `addDependentFile "MyDependency.txt"` to tell ghc to check that file for modification when compiling my code. Unfortunately, this does not work because `addDependentFile` only works relative to the directory from which ghc is called. How can I use it to depend on a file that is just next to (in the same directory as) the file I am compiling?

Original source