#2899 Nuances of Func.retype()

SlimerDude Thu 7 Dec 2023

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(|->|#)


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

brian Thu 7 Dec 2023

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)

SlimerDude Fri 8 Dec 2023

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!).

brian Sat 9 Dec 2023

Understand. I pushed a fix

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