Proposal: Add an AnyTemplate to DAML and DAML-LF

This proposal describes an AnyTemplate type that I would like to see added to DAML and DAML-LF and the surrounding API. This is motivated by DAML Triggers where the trigger is passed a Create event with a field of type AnyTemplate that they can then safely convert to the original template type.

Changes to DAML

• Add a new type AnyTemplate which corresponds to an existential template type.

Add a new type AnyTemplate to the standard library and extend the Template typeclass with the following 3 functions:

• toAnyTemplate : Template t => t -> AnyTemplate
• fromAnyTemplate : Template t => AnyTemplate -> Maybe t

Changes to DAML-LF

• Add a new builtin type AnyTemplate type corresponding to the AnyTemplate type in DAML.
• Add a toAnyTemplate builtin of type, a -> AnyTemplate. The typechecker checks that a is a template. We already have other primitives like create that restrict a type to being a template.
• Add a family of fromAnyTemplate tyCon builtins of type AnyTemplate -> Optional a that are parametrized by a TypeConName. This primitive will check that the template wrapped in AnyTemplate has the corresponding TypeConName and if so unwrap it. During typechecking, we check that the TypeConName corresponds to a.

The builtins here will not technically be part of BuiltinFunction but will be separate AST nodes since BuiltinFunction is an Enum so we cannot have the TypeCon parameter here.

Questions/Design Decisions

Should the new types be serializable?

I wouldn’t make any of those types serializable at least initially. If we see usecases, we can relax it later. Relaxing restrictions is always easier than imposing them.

Implementation Plan

Given that this is a big missing piece for an MVP of DAML triggers and there are other efforts in DAML-LF land (e.g. Numeric and Map), I think it makes sense for the language team (primarily myself) to take over all of the implementation work here.

Pinging @hurryabit @associahedron @remyhaemmerle-da @gerolf-da as the LF maintainers and @bame-da for the DAML UX.

[cocreature]

We get a family of builtins indexed by the template id that you convert to. So in the ast there is just a single case class but it has the template id as a parameter. This is similar to how create is represented.

Gerolf Seitz [e-mail] schrieb am Fr., 13. Sep. 2019, 14:28:

Add a family of fromAnyTemplate tyCon builtins of type AnyTemplate -> Optional a that are parametrized by a TypeConName.

@cocreature https://github.com/cocreature: What does this “family of builtins” refer to? Is this more than one function?

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[gerolf-da]

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