I am writing an interpreter for a very simple untyped language and I want to provide a built-in function to format a list of values in a printf like fashion. There is the beautiful Text.Printf module with its multi-variadic printf function, and I would like to use that to do all the heavy lifting. I can easily implement an instance PrintfArg Value for my Value type, but what I need in addition to that is an "untyped" version of printf i.e. something like format :: String -> [Value] -> String with the property that format fmt [] = printf fmt format fmt [x] = printf fmt x format fmt [x,y] = printf fmt x y ... I am pretty sure this is possible using a bit of class hackery and perhaps GADTs or continuation passing thrown in, but I can't seem to get it working. Any hint that helps to untangle the knots in my brain would be appreciated! Cheers Ben