I have a rope data type with the invariant that one of its data constructors can never appear as a leaf. module Data.Rope where import Data.Word (Word8) data Rope = Empty | Leaf | Node !Rope !Rope index :: Rope -> Int -> Word8 index Empty _ = error "empty" index Leaf _ = error "leaf" index (Node l r) n = index' l n where index' Leaf _ = error "leaf" index' (Node l r) n = index' l n I removed some of the actual details (Leafs have a ByteString member and Nodes have a length and a depth field). The point is that Empty can only appear at the top by construction (i.e. it's not possible to construct a rope that breaks this invariant using the exported API). If I understand compilation of case statements correctly they will compile into some if like construct that checks the constructor tag. This means that I would like that once we have established that the constructor is not Empty we could use a specialized function to traverse the tree without checking for Empty on each iteration. The above doesn't achieve this as GHC inserts an automatic error branch for Empty in index'. Perhaps one solution is to reorder the cases and remove index'? -- Johan