Other kinds of abstract definitions

[ppolesiuk]

The abstr visibility mode for types means that the type is public, but constructors (values) are not. The abstr with the same semantics would be useful for other kinds of definitions. A motivating example is to create a global handler with public effect, but private capability that is used for defining other useful function. For instance, consider the following implementation of logging, assuming that hState is a library defined handler for state (btw, we have not such handler yet).

abstr handle logger / Log with hState

pub let log msg =
  logger := msg :: logger.get ()

...

Here, the Log effect should be public, but the capability logger should be private.