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Haskell allows for promoting types to the kind level with the `DataKinds` extension. Since DAML-LF has no similar concept (and frankly speaking should't have one), this does not work well with data-dependencies. Unfortunately, we need to enable the `DataKinds` extension by default in `damlc` since our record system and the `Numeric` type depend on it. Thus, issuing a warning that the `DataKinds` is not supported with data-dependencies is not an option and we need more refined warnings. Most uses of promoted kinds won't even compile to DAML-LF and there's no need to warn about those since they cause hard failures. However, we need some limited support for type-level strings and the `Symbol` kind, for type-level numbers and the `Nat` kind, as well as for some stuff from `DA.Generics`. Type-level numbers and the `Nat` kind work reasonable well since we have them in DAML-LF too. The `Symbol` kind, and the types and kinds from `DA.Generics` are "erased" during conversion to DAML-LF by replacing them with either a special `Erased` type or the star kind. Thus, these entities are not restored properly with data-dependencies. In combination with type classes this leads easily to instances that for different types in the original code but are at conflict during interface reconstruction. Long story short, erased types/kinds and type classes don't work with data-dependencies. We have a hack to somehow preserve type-level string since we need that for our record system and the `HasField` type class. I have no desire to support this hack for arbitrary type classes coming from the user. Thus, I consider type-level strings to be erased as well here. This PR adds a warning whenever a user is attempting to define a type class that involves a kind that will be erased or an instance that involves a type that will be erased. The warning tells the user that this does not work with data-dependencies and is hence not upgradable. There is one caveat: the `HasField` instances produced by the propressor fall in the category we should warn about as well. Since not supporting them would make our record system unusable, we need to give them special treatment in data-dependencies. Thus, warning about them would be unjustified noise and we explicitly exclude them in the code producing the warnings. CHANGELOG_BEGIN [DAML Compiler] Issue warning when advanced types are with type classes in a way that is supported by data-dependencies. CHANGELOG_END |
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|---|---|---|
| .. | ||
| daml-extension | ||
| daml-lf-ast | ||
| daml-lf-proto | ||
| daml-lf-reader | ||
| daml-lf-tools | ||
| daml-lf-verify | ||
| daml-licenses | ||
| damlc | ||
| ghcide | ||
| hie-core | ||
| lsp-tests | ||
| repl-service | ||
| scenario-service | ||
| README.md | ||
DAML Compiler
This directory contains several libraries used by and the executable for the DAML compiler.
Libraries
daml-lf-ast: The DAML-LF AST and type checker.