OCaml Hashtbl/0.t and Hashtbl/-1.t - ocaml

Question

I am quite new to OCaml, so I am not sure what the following error message means (specifically the /0 and the /-1):
Error: This expression has type (string, string) Hashtbl/0.t
but an expression was expected of type ('a, 'b) Hashtbl/-1.t
I am passing a Hashtbl.t into Hashtbl.find and this error shows up. I am unclear as to how the /0 and /-1 came in, and what they actually mean.
Here's a minimal working example to demonstrate my issue:
open Core_kernel.Std
let file_to_hashtbl filename =
let sexp_to_hashtbl_str = Sexplib.Conv.hashtbl_of_sexp
string_of_sexp string_of_sexp
in In_channel.with_file
filename ~f:(fun ch -> (Sexp.input_sexp ch |> sexp_to_hashtbl_str))
let ht = file_to_hashtbl "test"
let t1_val = match Hashtbl.find ht "t1" with
| Some v -> v
| None -> assert false
let () = print_endline t1_val

Answer 1

Let's show you an example :
If I write
type t = A;;
let x = A;;
type t = B;;
let y = B;;
x = y;;
Error: This expression has type t/1561 but an expression was expected of type
t/1558
This is because in the interpreter you can declare multiple types with the same name and associate values to these types. But here, as you can see, x and y are not of the same type but both the types are named t so the interpreter tries to tell you the types are both named t but are not the same.
[Compilation]
If I wanted to compile this, I would have to declare
typea.ml
type t = A
let x = A
typeb.ml
type t = B
let y = B
main.ml
open Typea
open Typeb
x = y
If I compile this I will have
Error: This expression has type Typeb.t
but an expression was expected of type Typea.t
What lesson should you learn from this ? Stop interpreting, compile !
Now that I managed to compile your file, I got an error too but much more explicit :
Error: This expression has type (string, string) Hashtbl.t
but an expression was expected of type
('a, 'b) Core_kernel.Std.Hashtbl.t =
('a, 'b) Core_kernel.Core_hashtbl.t
[Explanation and correction]
Since I'm too nice, here is your file corrected :
let file_to_hashtbl filename =
(* open the namespace only where needed *)
let open Core_kernel.Std in
let sexp_to_hashtbl_str = Sexplib.Conv.hashtbl_of_sexp
string_of_sexp string_of_sexp
in In_channel.with_file
filename ~f:(fun ch -> (Sexp.input_sexp ch |> sexp_to_hashtbl_str));;
let ht = file_to_hashtbl "test"
let t1_val =
try
Hashtbl.find ht "t1"
with Not_found -> assert false
let () = print_endline t1_val
Your error was that you opened Core_kernel.Std as a global namespace so when you wrote Hashtbl.find it looked first in Core_kernel.Std and not in the standard library.
What I did is open Core_kernel.Std in the function that needs it, not in the whole file (so it's a local namespace) (a good habit to take).
So, as you can see, the problem was that you had two definition of the type Hashtbl.t (one in Core_kernel.Std and one in the standard library) and OCaml ain't no fool, boy, he knows when you're wrong but he is hard to understand since he only speak for those who can hear. :-D
P.S. : You had an error in your Hashtbl.find because it doesn't return an option but the found value or raise a Not_found exception if no value was found. I corrected it too. ;-)

Answer 2

Apparently, it is just a matter of missing semi-columns, the foloowing code compiles :
open Core_kernel.Std;;
let file_to_hashtbl filename =
let sexp_to_hashtbl_str = Sexplib.Conv.hashtbl_of_sexp
string_of_sexp string_of_sexp
in In_channel.with_file
filename ~f:(fun ch -> (Sexp.input_sexp ch |> sexp_to_hashtbl_str));;
let ht = file_to_hashtbl "test"
let t1_val = match Hashtbl.find ht "t1" with
| Some v -> v
| None -> assert false
let () = print_endline t1_val
But, I do not know how to interpret the error message neither.