OCaml Error Messages                                       -*-compilation-*-

Shows different OCaml error messages and how they are rendered.

File "file.ml", line 4, characters 6-7:
Error: This expression has type int
       This is not a function; it cannot be applied.

File "file.ml", line 3, characters 6-7:
Warning 26: unused variable y.

File "file.ml", line 6, characters 15-38:
Error: Signature mismatch:
       Modules do not match: sig val x : float end is not included in X
       Values do not match: val x : float is not included in val x : int
       File "file.ml", line 3, characters 2-13: Expected declaration
       File "file.ml", line 7, characters 6-7: Actual declaration

File "file.ml", line 8, characters 6-7:
Warning 32: unused value y.


* Since OCaml 4.08, the error messages have the following form.

File "helloworld.ml", line 2, characters 36-64:
2 | module rec A: sig type t += A end = struct type t += A = B.A end
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: Cannot safely evaluate the definition of the following cycle
       of recursively-defined modules: A -> B -> A.
       There are no safe modules in this cycle (see manual section 8.2).

File "helloworld.ml", lines 4-7, characters 6-3:
4 | ......struct
5 |   module F(X:sig end) = struct end
6 |   let f () = B.value
7 | end
Error: Cannot safely evaluate the definition of the following cycle
       of recursively-defined modules: A -> B -> A.
       There are no safe modules in this cycle (see manual section 8.2).

File "robustmatch.ml", lines 33-37, characters 6-23:
 9 | ......match t1, t2, x with
10 |       | AB, AB, A -> ()
11 |       | MAB, _, A -> ()
12 |       | _,  AB, B -> ()
13 |       | _, MAB, B -> ()
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a case that is not matched:
(AB, MAB, A)

File "helloworld.ml", line 2, characters 36-64:
Error: Cannot safely evaluate the definition of the following cycle
       of recursively-defined modules: A -> B -> A.
       There are no safe modules in this cycle (see manual section 8.2).

File "helloworld.ml", line 2, characters 36-64:
Warning 3: Cannot safely evaluate the definition of the following cycle
       of recursively-defined modules: A -> B -> A.
       There are no safe modules in this cycle (see manual section 8.2).

File "helloworld.ml", line 2, characters 36-64:
2 | module rec A: sig type t += A end = struct type t += A = B.A end
                                        ^^^^^^^^^^^^^^^^^^^^^^^^
Warning: Cannot safely evaluate the definition of the following cycle
       of recursively-defined modules: A -> B -> A.
       There are no safe modules in this cycle (see manual section 8.2).

File "main.ml", line 3, characters 8-50:
Error: This expression has type float but an expression was expected of type
         int

File "main.ml", line 3, characters 8-50:
Warning 3: This expression has type float but an expression was expected of type
         int

File "main.ml", line 13, characters 34-35:
13 |   let f : M.t -> M.t = fun M.C -> y
                                       ^
Error: This expression has type M/2.t but an expression was expected of type
         M/1.t
       File "main.ml", line 10, characters 2-41:
         Definition of module M/1
       File "main.ml", line 7, characters 0-32:
         Definition of module M/2

File "main.ml", line 13, characters 34-35:
13 |   let f : M.t -> M.t = fun M.C -> y
                                       ^
Error: This expression has type M/2.t but an expression was expected of type
         M/1.t
       File "main.ml", line 10, characters 2-41:
         Definition of module M/1
       File "main.ml", line 7, characters 0-32:
         Definition of module M/2

File "main.ml", line 13, characters 34-35:
13 |   let f : M.t -> M.t = fun M.C -> y
                                       ^
Warning 3: This expression has type M/2.t but an expression was expected of type
         M/1.t
       File "main.ml", lines 10-12, characters 2-5:
         Definition of module M/1
       File "main.ml", line 7, characters 0-32:
         Definition of module M/2

File "main.ml", line 13, characters 34-35:
13 |   let f : M.t -> M.t = fun M.C -> y
                                       ^
Warning 3: This expression has type M/2.t but an expression was expected of type
         M/1.t
       File "main.ml", lines 10-12, characters 2-5:
         Definition of module M/1
       File "main.ml", line 7, characters 0-32:
         Definition of module M/2