.ml

type ('s, 't) parser = 's -> ('t, 't) outcome
and ('s, 't) outcome = Fail | Success of { result: 't; state: 's; }

let fail s = Fail

let mapOutcome (f : 'a -> 'b) (o : ('s, 'a) outcome) : ('s, 'b) outcome = match o with
        | Fail -> Fail
        | Success { result = r; state; } -> Success { result = f r; state; }

let map
  (f: 'x -> 'b)
  (p: ('s, 'x) parser)
  : (('s, 'b) parser) =
  fun state0 -> mapOutcome f (p state0) (* with
        | Fail -> Fail
        | Success { result = (r : 'x); state; } -> Success { result = (f r : 'b); state; }
        *)

.mli

type ('s, 't) parser = 's -> ('t, 't) outcome
and ('s, 't) outcome = Fail | Success of { result: 't; state: 's; }

val fail : ('s, 't) parser

val mapOutcome : ('a -> 'b) -> ('s, 'a) outcome -> ('s, 'b) outcome

val map : ('a -> 'b) -> ('s, 'a) parser -> ('s, 'b) parser

gistfile1.txt

Error: The implementation parse.ml does not match the interface parse.cmi:
       Values do not match:
         val map : ('b -> 'b) -> ('s, 'b) parser -> ('s, 'b) parser
       is not included in
         val map : ('a -> 'b) -> ('s, 'a) parser -> ('s, 'b) parser
       File "parse.ml", line 10, characters 4-7: Actual declaration