Compile the following three files one-by-one.
  (*a.ml*) let _ = ref () in let f = fun _ -> () in f 1; f 'a'
  (*b.ml*) let f = let _ = () in fun _ -> () in f 1; f 'a'
  (*c.ml*) let f = let _ = ref () in fun _ -> () in f 1; f 'a'
The files a.ml and b.ml compile; the file c.ml fails with
  Error: This expression has type char
    but an expression was expected of type int
Could someone explain why having a mutable field ("contents" in this
case) restricts the polymorphism of f?

PS: A few hours ago I tried to post from Google Groups, but the
message didn't seem to go thru. Apologies if this is a duplicate