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This feature is still incubating, and its exact API might change from a release to the next. It means it's the perfect time for you to get involved and let us hear your feedback!


To access this feature, first install the constraints plugin: yarn plugin import constraints

Constraints are a solution to a very basic need: I have a lot of workspaces, and I need to make sure they use the same version of their dependencies. Or that they don't depend on a specific package. Or that they use a specific type of dependency. Anyway, you see the point: whatever is the exact logic, my goal is the same; I want to automatically enforce some kind of rule across all my workspaces. That's exactly what constraints allow you to do.

Creating a constraint

Constraints are created by adding a file at the root of your project (repository). The .pro extension might leave you perplex: this is because constraints aren't written in JavaScript (!) but rather in Prolog, a fact-based rule engine. The goal of this section isn't to teach you Prolog (good tutorials already exist, such as Learn Prolog in Y Minutes), but rather to show you why we chose it and the value it brings.

As we mentioned, Prolog is a fact-based engine. It starts with a list of facts that are always true, and a list of predicates that basically read as "predicate f(X) is true if u(X) and v(X) are both true". By computing for which values of X are u(X) and v(X) true, Prolog is able to automatically compute the list of values for which f(X) would be true. This is particularly useful for constraints, because it allows you to write very simple but powerful rules that have the ability to affect all your workspaces in very few lines.

Going back to the constraint engine, the facts are the definitions created by the package manager (such as "fact: the root workspace depends on Lodash version 4.4.2 in devDependencies"), and the predicates are the set of rules that you want to enforce across your project (check below for some recipes).

Query predicate

The following predicates provide information about the current state of your project and are meant to be used in the dependencies of your own rules (check the recipes for examples how to use them in practice). Note that the /<number> syntax listed at the end simply is the predicate arity (number of arguments it takes).

The notation on this page uses -, + and ? as prefix for the predicate parameters. These values are used commonly in prolog documentation and mean

  • +: this value is considered input and must be instantiated
  • -: this value is considered output and will be instantiated by the predicate, though you can provide a value to verify that the value matches the predicate
  • ?: this value can be instantiated or not, both will work



True for only three values: dependencies, devDependencies and peerDependencies.



True if the workspace described by the specified WorkspaceCwd exists.



True if the workspace described by the specified WorkspaceCwd exists and if it has the specified WorkspaceIdent.



True if the workspace described by the specified WorkspacedCwd exists and if it has the specified WorkspaceVersion.



True if the workspace described by the specified WorkspaceCwd depends on the dependency described by the specified DependencyIdent and DependencyRange combination in the dependencies block of the given DependencyType.



True if the workspace described by the WorkspaceCwd has the given FieldValue in the manifest at FieldPath.

The FieldPath can target properties of properties via . notation, e.g. a FieldPath of 'publishConfig.registry' will set FieldValue to the value of the registry inside publishConfig.



True if the workspace described by the WorkspaceCwd has a value in the manifest at FieldPath, and if this value passes the check of CheckCode.

The CheckCode script is meant to be written in JavaScript, with the special variable $$ representing the value obtained from the manifest. This makes workspace_field_test an escape hatch for some operations that would be too inconvenient to implement in Prolog (for example checking that a value is present within a JS array, etc).

The Arguments parameter is expected to be an optional Prolog list of atoms that will be passed to CheckCode through $0, $1, etc.

Constraint predicates

The following predicates will affect the behavior of the yarn constraints and yarn constraints --fix commands.

The parameters to the predicates are prefixed with + and -. These have the same meaning as in the query predicates. In this context they mean

  • - These are the output, they will not have a value when the predicate is invoked and the predicate must ensure a value is set
  • + These are the input, they will already have a value when the predicate is invoked



The gen_enforced_dependency rule offers a neat way to inform the package manager that a specific workspace MUST either depend on a specific range of a specific dependency (if DependencyRange is non-null) or not depend at all on the dependency (if DependencyRange is null) in the DependencyType dependencies block.

Running yarn constraints --fix will instruct Yarn to fix the detected errors the best it can, but in some cases ambiguities will arise. Those will have to be solved manually, although Yarn will help you in the process.



The gen_enforced_field predicate tells the package manager that a specific workspace must have the given FieldValue in the manifest via the FieldPath. A FieldValue of null means the field has to be absent:

? gen_enforced_field(WorkspaceCwd, FieldPath, null).

Note that the value will be interpreted in JSON if possible, or as a regular string otherwise. So if you need to put a null value into a field, use the JSON syntax:

? gen_enforced_field(WorkspaceCwd, FieldPath, 'null').

Finally, if you need to put a string containing null into a field, use the JSON string syntax:

? gen_enforced_field(WorkspaceCwd, FieldPath, '"null"').

Running yarn constraints --fix will instruct Yarn to fix the detected errors the best it can, but in some cases ambiguities will arise. Those will have to be solved manually, although Yarn will help you in the process.

Constraint recipes

The following constraints are a good starting point to figure out how to write your own rules. If you build one that you think would be a good fit for this section, open a PR and we'll add them here!

Quick note about the Prolog syntax

Be aware that in prolog X :- Y basically means "X is true for each Y that's true". Similarly, know that UpperCamelCase names are variables that get "replaced" by every compatible value possible. Finally, the special variable name _ simply discards the parameter value.

Prevent all workspaces from depending on a specific package

gen_enforced_dependency(WorkspaceCwd, 'tslib', null, DependencyType) :-
  workspace_has_dependency(WorkspaceCwd, 'tslib', _, DependencyType).

We define a rule that says that for each dependency of each workspace in our project, if this dependency name is tslib, then it exists a similar rule of the gen_enforced_dependency type that forbids the workspace from depending on tslib. This will cause the package manager to see that the rule isn't met, and autofix it when requested by removing the dependency from the workspace.

Prevent two workspaces from depending on conflicting versions of a same dependency

gen_invalid_dependency(WorkspaceCwd, DependencyIdent, DependencyType, 'This dependency conflicts with another one from another workspace') :-
  workspace_has_dependency(WorkspaceCwd, DependencyIdent, DependencyRange, DependencyType),
  workspace_has_dependency(_, DependencyIdent, DependencyRange2, _),
  DependencyRange \= DependencyRange2.

We define a gen_invalid_dependency rule that is true for each dependency of each package (first workspace_has_dependency) if it also exists another dependency of another package (second workspace_has_dependency) that has the same name but a different range (\= operator).

Force all workspace dependencies to be made explicit

gen_enforced_dependency(WorkspaceCwd, DependencyIdent, 'workspace:*', DependencyType) :-
  workspace_ident(_, DependencyIdent),
  workspace_has_dependency(WorkspaceCwd, DependencyIdent, _, DependencyType).

We define a gen_enforced_dependency that requires the dependency range workspace:* to be used if the dependency name is also the name of a valid workspace. The final workspace_has_dependency check is there to ensure that this rule is only applied on workspace that currently depend on the specified workspace in the first place (if it wasn't there, the rule would instead force all workspaces to depend on one another).