[Bug] Constraints not enforced where `workspace_field ` is absent

[theseyi]

• I'd be willing to implement a fix

Describe the bug
Creating a constraint using the query predicate to enforce the fact for workspace manifests where a field does not exist.
Running yarn constraints does not enforce the constraint.

To Reproduce
Seem minimal reproduction here https://github.com/theseyi/constraints

• init a yarn workspace with one workspace package packages/package-a and the workspace root
• import the constraint plugin yarn plugin import constraints
• touch a constraints.pro file and add in the following constraint
• attempt to enforce the constraint with yarn constraints

% enforce workspace packages without a workspace field to have a scripts.test field
gen_enforced_field(WorkspaceCwd, 'scripts.test', 'someValue') :-
  workspace_field(WorkspaceCwd, 'workspaces', null).

Reproduction

// in workspace root
{
  "name": "constraints",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "workspaces": [
    "packages/*"
  ]
}

// in packages/package-a
{
  "name": "package-a",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT"
}

% enforce workspace packages without a workspace field to have a scripts.test field
gen_enforced_field(WorkspaceCwd, 'scripts.test', 'someValue') :-
  workspace_field(WorkspaceCwd, 'workspaces', null).

Expected:

➤ YN0037: package-a must have a field scripts.test set to someValue, but doesn't
➤ YN0000: Failed with errors in 0s 18ms

IMPORTANT: We strongly prefer reproductions that use Sherlock. Please check our documentation
for more information: https://yarnpkg.com/advanced/sherlock

Unable to use Sherlock here since I'm not familiar with running it over prolog

Environment if relevant (please complete the following information):

• OS: macos
• Node version v12.20.1
• Yarn version 2.4.0

[merceyz]

You can update your constraint to check that it has any value and then negate it to get the result you're after

gen_enforced_field(WorkspaceCwd, 'scripts.test', 'someValue') :-
  \+ workspace_field(WorkspaceCwd, 'workspaces', _).

[theseyi]

Awesome thank you @merceyz 🙏. Although, shouldn't the null value indicate that the field is absent and cause the constraint to be enforced?

[theseyi]

Closing since there is a workaround, but does the original bug still seem valid? @merceyz

[merceyz]

Although, shouldn't the null value indicate that the field is absent and cause the constraint to be enforced?

No, not in this case. null is only special in the gen_* predicates so we should document the negation part, would you like to submit a PR?

File to update: https://github.com/yarnpkg/berry/blob/e8deea6ab56d2da1ece24dbfbd8799aef5e7a20e/packages/gatsby/content/features/constraints.md

[theseyi]

Sure, will get to it today, thanks for following up