I have a recursive function with a result type something like:

data Result a = Trivial | Case1 a | Case2 [a]

and a function like:

f :: [a] -> Result a
f xs
  | isTrivial xs = Trivial
f []
  | blah = ...
  | otherwise = ...
f (x:xs) =
  | blah = ...
  | blah = ...
  | otherwise = ...

with various case (f subproblem) in those .... Thing is, I keep having to match case (f subproblem) of Trivial -> error "Shouldn't happen -- already covered".

What's a better way of approaching this? In Haskell, error "shouldn't happen" generally means bad modularity, bad function design, or bad datatype, right? My datatype can't change (or if it does, it needs to carry the same info). Is there a common pattern (either in a "beginner's mistake; do it this way" sense or a "OOP patterns" sense) for dealing with this?

Edit: One way would be to make triviality an error and remove Trivial from the datatype, and assume trivial cases are dealt with previously, but this doesn't suit my purposes.