Is it possible in DAML to have a generic parameter to a choice, without the template itself being generic?

Question

In my model I'm trying to define a non-generic template that has a choice which takes a generic parameter. I can't figure out how to do that. Is it even possible? If not, why?

Answer 1

It's not possible, and deliberately so. One of DAML's design principles is that when you sign a contract you know exactly what you are agreeing to.

Imagine there was a type class Transferrable for assets and I published a package containing an empty type class Stealable and a template PermissionToSteal:

class (Template a, Transferrable a) => Stealable a where

template PermissionToSteal
  with
    owner : Party
    thief : Party
  where
    signatory owner, thief

    controller thief can
    (Stealable a) => Steal : (ContractId a)
      with
        asset : a
      do
        transfer asset thief

I may be able to convince you to sign such a contract as owner as you feel safe in the knowledge that there are no instances of Stealable.

But what if I now uploaded another package with an instance Stealable Cash?