Yeah, sorry. Here's the full snippet:

    instance (PrintfArg a, PrintfType r) => PrintfType (a -> r) where
    spr fmts args = \ a -> spr fmts (toUPrintf a : args)

And spr is a polymorphic function of the type:

    spr :: String -> [UPrintf] -> t

Essentially the way this works is that it recursively conses up a list of the arguments, calling toUPrintf on each.