Pfthb

If I load the following code in utop, after #require "mparser", it is accepted in the top level and give the signature below

open MParser

let infix p op = Infix (p |>> (fun _ a b -> (`Binop (op, a, b))), Assoc_left)

let operators =
 [
 [
 infix (char '*') `Mul;
 infix (char '/') `Div;
 ];
 [
 infix (char '+') `Add;
 infix (char '-') `Sub;
 ];
 ]

let decimal = many1_chars digit |>> int_of_string

let term = (decimal |>> fun i -> `Int i)
let expr s = expression operators term s

let rec calc = function
 | `Int i -> i
 | `Binop (op, a, b) ->
 match op with
 | `Add -> calc a + calc b
 | `Sub -> calc a - calc b
 | `Mul -> calc a * calc b
 | `Div -> calc a / calc b

accepted by utop as

val infix :
 ('a, 'b) MParser.t ->
 'c -> ([> `Binop of 'c * 'd * 'd ] as 'd, 'b) MParser.operator = <fun>
val operators :
 (_[> `Binop of _[> `Add | `Div | `Mul | `Sub ] * 'a * 'a | `Int of int ]
 as 'a, unit)
 MParser.operator list list =
 [[Infix (<fun>, Assoc_left); Infix (<fun>, Assoc_left)];
 [Infix (<fun>, Assoc_left); Infix (<fun>, Assoc_left)]]
val decimal : (int, unit) MParser.t = <fun>
val term : ([> `Int of int ], unit) MParser.t = <fun>
val expr :
 unit MParser.state ->
 (_[> `Binop of _[> `Add | `Div | `Mul | `Sub ] * 'a * 'a | `Int of int ]
 as 'a, unit)
 MParser.reply = <fun>
val calc :
 ([< `Binop of [< `Add | `Div | `Mul | `Sub ] * 'a * 'a | `Int of int ] as 'a) ->
 int = <fun>

Now, if I try to load with dune utop a library containing this code as a file/module, I received the following error :

~$ dune utop lib
 ocamlc lib/.lib.objs/lib__VariantExemple.cmi,cmo,cmt (exit 2)
(cd _build/default && /usr/local/bin/ocamlc.opt -w @a-4-29-40-41-42-44-45-48-58-59-60-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -bin-annot -I lib/.lib.objs -I lib/.lib.objs/.private -I /Users/nrolland/.opam/system/lib/bytes -I /Users/nrolland/.opam/system/lib/mparser -I /Users/nrolland/.opam/system/lib/re -I /Users/nrolland/.opam/system/lib/re/perl -I /Users/nrolland/.opam/system/lib/seq -no-alias-deps -opaque -open Lib -o lib/.lib.objs/lib__VariantExemple.cmo -c -impl lib/variantExemple.ml)
File "lib/variantExemple.ml", line 5, characters 4-13:
Error: The type of this expression,
 (_[> `Binop of _[> `Add | `Div | `Mul | `Sub ] * 'a * 'a | `Int of int ]
 as 'a, '_weak1)
 operator list list, contains type variables that cannot be generalized

It looks like there are some type annotation missing.
I am not too familiar with polymorphic variant type and , is there an obvious solution out ?

I was hoping that sticking the signature part given by utop in an interface would work, but it does not seem to be valid in a .mli file

Edit : the simple solution is to add a closed type annotation.

let operators : ([ `Binop of [ `Add | `Div | `Mul | `Sub ] * 'a * 'a | `Int of int ] as 'a, unit) operator list list =

I am not sure if there is a reason for why an interactive session and a dune utop lib one-shot loading should behave differently

You have _ in front of your types, which suggest your type is weakly polymorphic, and the compiler refuse to let such things live in a compiled object.

you can get the same result with the mwe :

let store = ref None

The toplevel is ok with that as it can be resolved to a monomorphic type later if you evaluate something like store:= Some1, which "monomorphise" the type from _a option ref to int option ref