You can get pretty far with the same trick oleg mentions at [1].
If you use local type signature then you can do things like this:
{-
ghci infers this type:
*Main> :t f
f :: (Ord a) => Int -> a -> t -> String
-}
f i j x | False = (undefined (i::Int) (isOrd j)) :: String
f i j x = error "not filled in"
isOrd :: Ord a => a -> ()
isOrd = undefined
Cheers,
Chris
[1] http://okmij.org/ftp/Haskell/types.html#partial-sigs
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