E4151

E4151#

Values of type FuncRef[T] must be closed function -- it should not capture any local variables.

MoonBit provides a builtin type FuncRef[T], which represents closed function of type T. In MoonBit's native backend, FuncRef[T] corresponds to function pointer type, so FuncRef[T] can be passed to C FFI functions requiring a callback directly, for example setting UNIX signal handler. Values of type FuncRef[T] must be closed function type, if a function which captures free variables or a non-function is supplied, an error will be reported.

Erroneous example#

test {
  let x : Int = 42
  let _ : FuncRef[() -> Unit] = fn () { println(x) }
}

Suggestion#

If your callback function really needs to have captures, consider adding the captures as extra parameters to the function.

Otherwise, use a closed function that does not capture local variables:

pub fn callback() -> FuncRef[() -> Unit] {
  fn() {
    println(42)
  }
}
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