`{a : "str"}` evaluates into a function about nickel HOT 2 CLOSED

That's a bad error message indeed. Thanks for reporting. For the record, here is what happens: {a : "str"} is interpreted as a type expression as per the syntax rules of the unified types/terms syntax (a record with only field without definition and : annotations). Indeed, {a: Number} is a record type.

When used at run-time, a type evaluates to its contract. Its contract is a function of the form fun label value => ... (at least once evaluated), and is built from the internals built-in contracts. In practice, this is elaborated to $record ..some_params.. where $record is a builtin internal function. This has type Label -> Dyn -> Dyn, and indeed is a contract waiting to be applied, which isn't serializable.

That types are automatically translated to their contract function is dubious and has actually been discussed in #1775. A better behavior would probably to keep it as a Type node in the AST, which would greatly improve error messages like this one (we could keep transforming them when they're used as a contract somewhere).

Separately, we should probably emit a warning when a value is used as a type/contract but has no chance to ever make sense (currently constants indeed don't have any reasonable meaning, although this could change, where {foo | "bar"} could mean {foo | MustEquals "bar"}, although I'm not sure this is a great idea - just mentioning that this could change one day).

from nickel.

Separately, we should probably emit a warning when a value is used as a type/contract but has no chance to ever make sense (currently constants indeed don't have any reasonable meaning, although this could change, where {foo | "bar"} could mean {foo | MustEquals "bar"}, although I'm not sure this is a great idea - just mentioning that this could change one day).

I think if {foo : "bar"} has no valid use case today, it makes sense to just throw an error. It can always be re-enabled in the future without breaking compatibility. No decision about future use of {foo : "bar"} has to be made right now.

from nickel.