I am playing around with the extensible-effects framework and cannot figure out how to solve this problem. I am trying to define a set of operations that only work after a file has been opened. For example, reading a character from a file or detecting EOF. My problem is I cannot figure out how to define one "runner" for my Effect that can deal with operations with different result types. For example, I would like my `readFrom` and `atEOF` operations to have the following types: data OpenFile r v = ... -- my "OpenFile" effect -- read a character from the file readFrom :: Eff r Char -- detect EOF atEOF :: Eff r Bool And I would like to use this operations as follows (where I read a byte from the file, check EOF, and return the concatenation of the two values): main = do c <- runOpenFile "Setup.hs" (do a <- readFrom -- `a` has `Char` type eof <- atEOF -- `eof` has `Bool` type return $ [a] ++ show eof) putStrLn c However, I cannot define a `runOpenFile` that compiles. For example, this definition gives me "Overlapping Instance" errors: runOpenFile :: FilePath -> Eff (OpenFile v :> r) result -> IO result The best I've come up with is to define a `Result` type that holds all possible results: data Result = AtEOF Bool | ReadChar Char The `Result` type then lets me put a single type on the `OpenFile` effect in `runOpenFile`: runOpenFile :: FilePath -> Eff (OpenFile Result :> r) result -> IO result But that makes my `main` program pretty ugly, because I have to deconstruct those result types each time: main = do c <- runOpenFile "Setup.hs" (do (ReadChar a) <- readFrom (AtEOF eof) <- atEOF return $ [a] ++ show eof) putStrLn c Does anyone have suggestions? Justin p.s. The complete code (in a sort-of similar form) can be seen at https://github.com/m4dc4p/ext-eff-sample/blob/master/src/OpenFile.hs