I hadn't come across the "Row" polymorphism term before, but it sounds more like structural typing -- for example TypeScript, and to a lesser degree Go, have structural interfaces that provide "row polymorphic" programming.
You could go further with variant structural typing, basically this is a broader form of type checking based on call compatibility, which answers the question -- is B#foo() callable as an A#foo()?
For instance, your `area` example requires `double` result types, otherwise a row type having `width` and `length` defined as `integer` columns doesn't satisfy `area`. But, result types are naturally covariant -- `integer` is a subset of `double` -- which permits us to accept `integer` in the implementation of `area`.
Similarly, parameter types are naturally contravariant -- `Shape` is contravariant to `Triangle`, thus `B#foo(Shape)` is call-compatible as a `A#foo(Triangle)`, therefore I can pass a Triangle to `B#foo(Shape)`.
The manifold project[1] for Java is one example where the type system is enhanced with this behavior using structural interfaces.
Note, this goes further with parametric types where function result types and parameter types define variance and other constraints.
You could go further with variant structural typing, basically this is a broader form of type checking based on call compatibility, which answers the question -- is B#foo() callable as an A#foo()?
For instance, your `area` example requires `double` result types, otherwise a row type having `width` and `length` defined as `integer` columns doesn't satisfy `area`. But, result types are naturally covariant -- `integer` is a subset of `double` -- which permits us to accept `integer` in the implementation of `area`.
Similarly, parameter types are naturally contravariant -- `Shape` is contravariant to `Triangle`, thus `B#foo(Shape)` is call-compatible as a `A#foo(Triangle)`, therefore I can pass a Triangle to `B#foo(Shape)`.
The manifold project[1] for Java is one example where the type system is enhanced with this behavior using structural interfaces.
Note, this goes further with parametric types where function result types and parameter types define variance and other constraints.
1. https://github.com/manifold-systems/manifold