ExercisedEvent.choice field

Questions
1

Which of the following does the ExercisedEvent.choice field hold:

  1. The qualified Daml type name including package id
  2. The qualified Daml type name sans the package id
  3. The unqualified type name
  4. Something else
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2

It is the choice name. That’s not technically related to the type name at all in Daml-LF. It’s just a single string that can have no relation to the type of the exercise arguments.

However the daml compiler generates choices such that there is always a corresponding type as well for the exercise arguments. The choice name is then the unqualified type name.

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3

Thanks. I am a little confused at this bit though: “That’s not technically related to the type name at all in Daml-LF”.

When you write choice Foo, it’s my understanding that in the compiler front end, this gets de-sugared into a record type holding the arguments and a type-class instance defining the behavior. Are you saying that the fact that the name generated for the former matches the name in the choice declaration is just a convention rather than a guarantee, because the Daml-LF spec mandates no such correspondence (and can’t because it doesn’t specify how Daml code is translated to Daml-LF)?

4

When you write choice Foo, it’s my understanding that in the compiler front end, this gets de-sugared into a record type holding the arguments and a type-class instance defining the behavior.

That’s still at the Daml level. Daml-LF has no typeclasses. In Daml-LF a template is defined (among other things) by a list of choices. Each of those choices has a name, a choice argument and the implementation (and some other stuff which doesn’t matter here). There is no relationship between the name and the choice argument in Daml-LF. You can use a completely different type, share types across choices (Archive actually does that), ….

Damlc generates a new choice argument type for each choice and sets the choice name to the unqalified name of that type.

5

Yes, that’s what’s what I was getting at when I wrote:

The part I was missing is that the choice name is part of the Daml-LF spec, so when you declare choice Foo you are specifying the Daml-LF choice name rather than the name of the associated type.

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