E4168

Compiler diagnostic name: error_poly_unsupported.

Error polymorphism is not supported in this position.

The raise? annotation means that a function is error-polymorphic: it can propagate the error behavior of a callback or another polymorphic operation. MoonBit only supports this form in specific function positions. It is not a general-purpose type annotation that can be used anywhere a function type is written.

Erroneous example

The following toplevel value uses raise? in a function type annotation:

///|

///|
let _callback : () -> Unit raise? = fn() { () }

MoonBit will report an error.

Suggestion

Use a non-raising function type when the callback cannot raise, or use an ordinary concrete raise annotation in a supported function declaration when the function may raise:

///|

///|
let _callback : () -> Unit = fn() { () }