Hi, I was expecting the following to work:

Fantom Shell v1.0.79

fansh> |->|{}.retype(|->|?#)

sys::ArgErr: Not a Func type: |->sys::Void|?
  fan.sys.Func.retype (Func.java:300)

I'm aware a similar nuance has been discussed previously on the forum, and that the underlying cause is that a nullable Type is technically not a pure func type; because nullability is a feature of where the Type is applied and not of the Type itself.

Hence the following works okay:

fansh> |->|{}.retype(|->|#)

|->sys::Void|

These runtime mistakes are easy to overlook and difficult to guard against; could Func.retype() be updated to cater for nullable Types?

What do you mean by cater for nullable types? It seems like the current behavior is correct that you must pass in a function type (which is for reflection)

Hi, I would ask for retype() to be lenient and automatically call .toNonNullable() on the given Type.

For instance, This code looks perfectly fine as it retypes a func to the Field type, but it actually throws "Not a func type".

class Thingy {

  |->|?  fooFn

  Void setFn(Func newFn) {
    fooFn = newFn.retype(#fooFn.type)
  }
}

It would take a very special kind of Fantom developer to understand they just need to write newFn.retype(#fooFn.type.toNonNullable).

If not, then I would ask for clearer documentation and a different err msg for this specific nullable case (which arguably, is more work!).

Understand. I pushed a fix