The `[@error_message]` attribute

You can put the [@error_message] attribute on type or kind constraints to add custom text to error messages, useful when writing a ppx.

For example, suppose we have a ppx that translates [%i_have_a x] to Some x. This makes sense only for values. So the ppx might actually translate

let f x = [%i_have_a x]

let f x = Some (x : ((_ : value)[@error_message "only works with values"]))

which will say only works with values to the user if the type of x does not have kind value.

This also works on type constraints like (x : string), though the implementation on type constraints is more fragile, producing the custom message only if we already know the type of the expression before seeing the type constraint. For example, this produces a custom message:

let f (x : bool) = (x : int)[@error_message "custom"]

But this does not:

let f x = (x : bool)[@error_message "custom"] + 1

Topics

The [@error_message] attribute

The `[@error_message]` attribute