## ﻿12.21 Alerts

- 12.21.1 Marking a component
- 12.21.2 Controlling which alerts are enabled
- 12.21.3 Shorthand syntax for deprecation

(Introduced in 4.08)

Since OCaml 4.08, it is possible to mark components (such as value or type declarations) in signatures with “alerts” that will be reported when those components are referenced. This generalizes the notion of “deprecated” components which were previously reported as warning 3. Those alerts can be used for instance to report usage of unsafe features, or of features which are only available on some platforms, etc.

Alert categories are identified by a symbolic identifier (a lowercase identifier, following the usual lexical rules) and an optional message. The identifier is used to control which alerts are enabled, and which ones are turned into fatal errors. The message is reported to the user when the alert is triggered (i.e. when the marked component is referenced).

The ocaml.alert or alert attribute serves two purposes: (i) to mark component with an alert to be triggered when the component is referenced, and (ii) to control which alerts are enabled.

### ﻿12.21.1 Marking a component

When marking a component, the attribute takes an identifier possibly followed by a message. Here is an example of a value declaration marked with an alert:

module U: sig val fork: unit -> bool [@@alert unix "This function is only available under Unix."] end

Here unix is the identifier for the alert. If this alert category is enabled, any reference to U.fork will produce a message at compile time, which can be turned or not into a fatal error.

For other structure or signature components, the syntax is as follows:

module type Sig = sig val v: int [@@alert name] end let v [@alert name] = 1 let `A (v [@alert name]) = `A 1 external e : int -> int = "c_function" [@@alert name] type t1 = A | B [@@alert name] (* N.B.: this only alerts on use of "t1", not uses of the constructors "A" and "B". So let x = A wouldn't alert, so long as you don't write down its type. *) type t2 = A | B [@alert name] (* Alerts on use of "B", but not "A" nor "t2". Note the single "@". *) type t3 = { a : int; b : int [@alert name] } (* Alerts on use of "b", but not "a" nor "t3". Note the single "@". *) exception E of int [@alert name] (* Alerts on use of "E". Note the single "@". *) module M = struct end [@@alert name] module type S = sig end [@@alert name] class c1 = object end [@@alert name] class type c2 = object end [@@alert name]

When the typer coerces a signature with alerts to a signature with different alerts:

module M : sig val x : int val y : int [@@alert name] end = struct let x [@alert name] = 0 let y = 1 end

any alert that isn’t passed through, i.e. the alert on x here, gets triggered.

Finally, to alert when the module corresponding to a file is referenced, a floating attribute can be written at the top of the “.mli” file (i.e. before any other non-attribute item), or the top of the “.ml” file if there is no “.mli” file:

[@@@alert unsafe "This module is unsafe!"]

### ﻿12.21.2 Controlling which alerts are enabled

Controlling which alerts are enabled and whether they are turned into fatal errors is done either through the compiler’s command-line option -alert <spec> or locally in the code through the alert or ocaml.alert attribute taking a single string payload <spec>. In both cases, the syntax for <spec> is a concatenation of items of the form:

- +id enables alert id.
- -id disables alert id.
- ++id turns alert id into a fatal error.
- --id turns alert id into non-fatal mode.
- @id equivalent to ++id+id (enables id and turns it into a fatal-error)

As a special case, if id is all, it stands for all alerts.

Here are some examples:

    (* Disable all alerts, reenables just unix (as a soft alert) and window
       (as a fatal-error), for the rest of the current structure *)

    [@@@alert "-all--all+unix@window"]
     ...

    let x =
      (* Locally disable the window alert *)
      begin[@alert "-window"]
          ...
      end

### ﻿12.21.3 Shorthand syntax for deprecation

Before OCaml 4.08, there was support for a single kind of deprecation alert. It is now known as the deprecated alert, but legacy attributes to trigger it and the legacy ways to control it as warning 3 are still supported. For instance, passing -w +3 on the command-line is equivalent to -alert +deprecated, and:

val x: int [@@ocaml.deprecated "Please do something else"]

is equivalent to:

val x: int [@@ocaml.alert deprecated "Please do something else"]