Ndtyjky

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