Improve parser errors around `is`

[john-h-kastner-aws]

Category

Other

Describe the feature you'd like to request

There is some room to provide more specific parse errors for malformed is expressions.

To make this issue concrete: is currently parsers expecting a Name for the entity type. If the following text doesn't parse as a Name the user sees a cryptic "expected token" error message (see #176). We could improve these errors message by instead expecting an Expr, and then raising an specific error in the CST to AST conversion if the Expr isn't a Name.

Other parse error improvements improvements are covered by #176, #391, #407

Describe alternatives you've considered

.

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

[cdisselkoen]

To be even more concrete, here are some is parse errors (as of #396) that could be improved:

permit(principal in Group::"friends" is User, action, resource);
current error: "expected a entity uid or template slot"

permit(principal is User::"alice", action, resource);
current error: "unexpected token `"alice"`"

permit(principal is User in 1, action, resource);
current error: "expected a entity uid or template slot, found a `literal` statement"

permit(principal, action, resource is 1);
current error: "unexpected token `1`"

permit(principal is User in ?resource, action, resource);
current error: "expected a entity uid or template slot"

permit(principal, action, resource) when { principal is 1 };
current error: "unexpected token `1`"