Stable API for querying flags in Setup.hs

[noughtmare]

I'm a contributor to UUAGC, which is a preprocessor that generates .hs files from a custom DSL. UUAGC is self-hosted. To build it we want to be able to choose between two approaches:

1. Build completely from source using an external version of UUAGC, or
2. Build using pregenerated .hs files

To do this we used to use a CPP flag -DEXTERNAL_UUAGC and conditionally compiled our Setup.hs based on that:

-- Setup.hs
{-# LANGUAGE CPP #-}
module Main where

#ifdef EXTERNAL_UUAGC
import System.Environment (getArgs)
import Distribution.Simple (defaultMainWithHooksArgs)

main :: IO ()
main = args >>= defaultMainWithHooksArgs uuagcUserHook

args :: IO [String]
args = do
  as <- getArgs
  let addFlags | "configure" `elem` as = ("--flags=bootstrap_external" :)
               | otherwise             = id
  return (addFlags as)
#else
import Distribution.Simple (defaultMain, defaultMainWithHooksArgs)

main :: IO ()
main = defaultMain
#endif

This works when compiling with stack like this: stack build --ghc-options="-DEXTERNAL_UUAGC". Unfortunately, this does not work (any more?) with cabal cabal build --ghc-options="-DEXTERNAL_UUAGC".

Now, I think this approach was very ugly. Instead it would be much nicer if we could simply use a cabal flag to toggle this behavior. I've been able to implement that:

{-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
module Main where

import System.Environment (getArgs)
import Distribution.Simple (defaultMainWithHooksArgs, UserHooks (..), simpleUserHooks)
import Distribution.Simple.LocalBuildInfo (flagAssignment)
import Distribution.Simple.UUAGC (uuagcUserHook)
import Distribution.Types.Flag (lookupFlagAssignment, mkFlagName)
import Debug.Trace

main :: IO ()
main = do
  args <- getArgs
  defaultMainWithHooksArgs hooks args
  where
    hooks = uuagcUserHook { buildHook = myBuildHook }
    myBuildHook pd lbi uh bf
      | lookupFlagAssignment (mkFlagName "bootstrap_external") (flagAssignment lbi) == Just True
      = buildHook uuagcUserHook pd lbi uh bf
      | otherwise
      = buildHook simpleUserHooks pd lbi uh bf

However, I feel like this relies too much on unstable functions of the Cabal API. I believe this is already broken in the latest version of Cabal.

Could this functionality be provided by a stable API? Or is there a better way to do this?