小编jla*_*tan的帖子

let rec 

map2 (f : 'a * 'b -> 'c) (l1 : 'a list) (l2 : 'b list) : 'c list =
  match (l1,l2) with
    | ([], []) -> []
    | (x::l1),(y::l2) -> f (x, y)::(map2 f (l1, l2))

它正在回归:

Error: This expression has type 'a list * 'a list
   but an expression was expected of type 'a list

我在这做错了什么？