I've rebased this on master. Looking into the segfault now.

The issue was evalStmt receives an EvalExpr ForeignHValue representing a IO [a] action.

With your code, if you make the method of the instance return an IO [a] action then evalStmt will find no problem in executing it:

module Main where

import qualified Data.Text as T

class Debug a where
  debugDisplayTree :: a -> IO [String]

instance Debug T.Text where
  debugDisplayTree txt = print ("Text: " ++ T.unpack txt) >> return []

f :: T.Text -> T.Text
f x = x

main :: IO ()
main = do
  print (f $ T.pack "Hello, Haskell!")

You should see what the result of debugDisplayTree being printed out only by stopping at f, because the method is being called on x!

If you look in GHC.Runtime.Interpreter you will find other functions which wrap "foreign" evaluation. They may be closer to what you intended to explore. Namely:

-- | Execute an action of type @IO ()@
evalIO :: Interp -> ForeignHValue -> IO ()
evalIO interp fhv =
  liftIO $ withForeignRef fhv $ \fhv ->
    interpCmd interp (EvalIO fhv) >>= fromEvalResult

-- | Execute an action of type @IO String@
evalString :: Interp -> ForeignHValue -> IO String
evalString interp fhv =
  liftIO $ withForeignRef fhv $ \fhv ->
    interpCmd interp (EvalString fhv) >>= fromEvalResult

-- | Execute an action of type @String -> IO String@
evalStringToIOString :: Interp -> ForeignHValue -> String -> IO String
evalStringToIOString interp fhv str =
  liftIO $ withForeignRef fhv $ \fhv ->
    interpCmd interp (EvalStringToString fhv str) >>= fromEvalResult

Feel free to keep exploring. I still haven't had the design discussion about the type class we'd want to have, at least to a first approximation.

@meipp what is your status regarding your work? If you'd like, I can pick this up and finish it.

The main difficulty is designing the type class, but I've got a few ideas to try. The challenging part of calling the instance dynamically you've already figured out, so I can just re-use that for a custom class.

This is now more of a priority for the project I believe and I'd be willing/looking forward to work on this now

Still no productive update here. Can't work out the calling structure: evalStmt, evalString do SigTerm/SigAbrt. You can pick up.