I have this

intersect1 :: ([a],[a]) -> [a]

intersect1 (s,[]) = []
interesct1 (s,t:ts) | elem t s = t : intersect1 (s,ts)
| otherwise = intersect1 (s,ts)

and when I try this

intersect1 ([1,2],[1,2,3])

I get the error

Non-exhaustive patterns in function intersect1

Not sure what's wrong with this.