How can I make sure the pattern
func (2:xs) = expression where 2:xs is a length 2 list doesn't match with the pattern
func (2:x:xs) = expression2 where 2:x:xs is a length 3 list?
2 Answers
Adapt this as needed:
func [] = ... -- empty case func [x] = ... -- length=1 case func [x,y] = ... -- length=2 case func (x:y:z:zs) = ... -- length>=3 case 
Comments
End the list pattern with empty brackets:
func (2:x:[]) = expression This will ensure x is a single element from the list.
2:xsis a list of length at least 1, not of length 2.