In the type proofs demo of the Twitter username, the casts returned nullables to model that a String was not a valid Twitter username. Wouldn't it be better if it returned a

Validated

, so it's possible to get the reason why it failed?

Refinement types can check the refined implications of your type at compile time and are in a sense more powerful than runtime checks which you get with data types such as

Validated

. With the Ide support we ship in Meta, Users can see in real time, why their input fails, given the set of refinements that are set.

I see the point for compile-time, but I thought this could be used as well for tests at runtime with

as?

to validate user data?

Each refined type has this function

from(a : A): B?

, which is what you maybe implying. So you can do:

TwitterHandle.from("@SiempreJuan")

Sorry, I don't think I explained my question well. When using the

from

function at runtime, is there a way to get the message of why it failed? I get that at compile-time it will show up in the IDE, but it could be interesting to know why something failed at runtime to display a message to the user or something similar

Yes, there is access to the validation map that contains all the entries already validated