I.e. something like parseExpr :: String -> Either String Exp It seems that a lot of trivial extensions could be made nicely if this was so. An example might be: [ne| 1, 2, 3, ... |] for a non-empty list statically checked to be non-empty. In there I could put any Haskell code. But presently I need to use HSE which isn't quite GHC Haskell and has a large foot-print. The alternative splice $(ne [| [1, 2, 3, ... ] |]) is rather unwieldy.