File "pr3968_bad.ml", lines 20-29, characters 0-3:
20 | object
21 |   val l = e1
22 |   val r = e2
23 |   method eval env =
24 |       match l with
25 |     | `Abs(var,body) ->
26 |         Hashtbl.add env var r;
27 |         body
28 |     | _ -> `App(l,r);
29 | end
Error: The class type
         object
           val l :
             [ `Abs of string * ([> `App of 'a * exp ] as 'b)
             | `App of expr * expr ] as 'a
           val r : exp
           method eval : (string, exp) Hashtbl.t -> 'b
         end
       is not matched by the class type exp
       The class type
         object
           val l :
             [ `Abs of string * ([> `App of 'a * exp ] as 'b)
             | `App of expr * expr ] as 'a
           val r : exp
           method eval : (string, exp) Hashtbl.t -> 'b
         end
       is not matched by the class type
         object method eval : (string, exp) Hashtbl.t -> expr end
       The method eval has type
         (string, exp) Hashtbl.t ->
         ([> `App of [ `Abs of string * 'a | `App of expr * expr ] * exp ]
          as 'a)
       but is expected to have type (string, exp) Hashtbl.t -> expr
       Type
         [> `App of [ `Abs of string * 'a | `App of expr * expr ] * exp ]
         as 'a
       is not compatible with type
         expr = [ `Abs of string * expr | `App of expr * expr ]
       Type exp = < eval : (string, exp) Hashtbl.t -> expr >
       is not compatible with type
         expr = [ `Abs of string * expr | `App of expr * expr ]
       Types for tag `App are incompatible