OCaml - Error: Module `Unix' is unavailable [closed] - unix

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I have been working through Harvard's CS51 class using materials available online. I'm trying to start the final project and downloaded the necessary files, but when I try to compile them I get the following error:
Error: Module `Unix' is unavailable (required by `Thread')
Command exited with code 2.
Compilation unsuccessful after building 18 targets (15 cached) in 00:00:00.
I have not made any changes to the code I downloaded yet and supposedly I should be able to compile it successfully in its current state. Any ideas why I might be getting this error?
EDIT: Below is the code from the file I downloaded and am trying to compile.
module Ev = Evaluation ;;
module MP = Miniml_parse ;;
module ML = Miniml_lex ;;
module Ex = Expr ;;
open Printf ;;
(* str_to_exp str -- Returns the expression specified by `str` using
the MiniML parser. *)
let str_to_exp (str: string) : Ex.expr =
let lexbuf = Lexing.from_string str in
let exp = MP.input ML.token lexbuf in
exp ;;
(* repl () -- Read-eval-print loop for MiniML, which prompts for and
evaluates MiniML expressions, printing the resulting value. Exits
the loop and terminates upon reading an end-of-file
(control-d). *)
let repl () =
(* lexical analyzer buffer from stdin *)
let lexbuf = Lexing.from_channel stdin in
(* set up the initial environment *)
let env = Ev.Env.empty () in
(* the main LOOP *)
while true do
(try
(* prompt *)
printf "<== %!";
(* READ and parse an expression from the input *)
let exp = MP.input ML.token lexbuf in
(* EVALuate it *)
let res = Ev.evaluate exp env in
(* PRINT the result; in this initial version, the trivial
evaluator just returns the expression unchanged as an
element of the `Env.value` type (found in `expr.ml`), so we
just extract the expression back out and print it *)
match res with
| Val resexp ->
printf "==> %s\n" (Ex.exp_to_abstract_string resexp)
| _ -> failwith "not handling other cases yet"
with
| MP.Error -> printf "xx> parse error\n"
| Ev.EvalError msg -> printf "xx> evaluation error: %s\n" msg
| Ev.EvalException -> printf "xx> evaluation exception\n"
| End_of_file -> printf "Goodbye.\n"; exit 0
);
flush stdout
done
;;
(* Run REPL if called from command line *)
try
let _ = Str.search_forward (Str.regexp "miniml\\.\\(byte\\|native\\|bc\\|exe\\)")
(Sys.argv.(0)) 0 in
repl ()
with Not_found -> () ;;
If I add open Unix it does take care of the error above, but I then get a different error:
26 | let lexbuf = Lexing.from_channel stdin in
^^^^^
Error: This expression has type Unix.file_descr
but an expression was expected of type in_channel
Command exited with code 2.

Answer 1

Generally, you have to explicitly ask to be linked to the Unix module.
The following program:
$ cat main.ml
Unix.gethostname () |> print_endline
would need to be built like this:
$ ocamlfind opt -linkpkg -package unix -o main main.ml; echo $?
0
whereas the bare minimum would fail with a similar error as yours:
$ ocamlopt -o main main.ml; echo $?
File "main.ml", line 1:
Error: No implementations provided for the following modules:
Unix referenced from main.cmx
2
That said, it looks like you're using Core, in which case (as well as most
other cases, actually) you're probably better off with dune:
$ cat dune
(executable
(name main)
(libraries unix))
$ dune build main.exe
$ ./_build/default/main.exe
amam-oy
However, if you ask Dune to link you to Core, Unix is already included
automatically, so the following dune file would also work for the above
program:
$ cat dune
(executable
(name main)
(libraries core))

Answer 2

Just add
open Unix;;
at the very start of your .ml file