PureScript 0.15 Row Polymorphism: Ambiguity in Record Field Selection

Answer

In PureScript 0.15, row‑polymorphic record selection is resolved via the `Has` type class. When two modules expose records that share the same label, each module supplies its own `Has` instance for that label. If both modules are imported (unqualified or qualified) into the same file, the type checker sees two overlapping `Has` instances and reports an ambiguous type error. Orphan instances (instances where neither the class nor the type is defined in the declaring module) are only accepted when the module that defines the instance is explicitly imported; this rule, introduced in 0.13, means the ambiguity only appears when both orphan‑instance modules are in scope. There is no compiler flag to suppress the ambiguity; the error is removed by qualifying the record construction or field access, or by adding an explicit type annotation that selects the desired `Has` instance. If an orphan `Has` instance is required, define it in a dedicated module and import that module wherever the record is used.

Likely explanation

The compiler searches for a `Has` instance for the label in every imported module. Overlapping instances from different modules create a type‑class ambiguity that the type checker cannot disambiguate automatically.

Confirmed facts

1. Ambiguous field selection errors arise because the `Has` class is resolved by name‑lookup across all modules in scope.
2. PureScript does not provide a flag to suppress these ambiguities; the only remedies are qualification or explicit type annotations.
3. Orphan instances are rejected unless the module that defines the instance is explicitly introduced, a rule added in version 0.13.
4. Kind‑polymorphic type classes added in 0.15 make the `Has` class more generic but do not change the ambiguity resolution mechanism.

Steps to resolve

1. Qualify the record construction or field access with the module name (e.g., `A.{ x = 1 }` or `A.x`).
2. Add an explicit type annotation to the field selection (`r.x :: Int`) to pin down which `Has` instance to use.
3. If an orphan `Has` instance is needed, define it in a dedicated module (e.g., `Instances.purs`) and import that module wherever the record is used.
4. Re‑compile; the ambiguous error should disappear and the program type‑checks.

Missing diagnostic detail

Which modules are currently imported unqualified in the file where the error occurs?