Row types are an alternative to subtyping for record/object types. With subtyping you say that if you have a record Foo and you leave out some fields to get a r

Row types are an alternative to subtyping for record/object types. With subtyping you say that if you have a record Foo and you leave out some fields to get a record Bar then Foo is a subtype of Bar. For example the type {a:int, b:string} is a subtype of {a:int}. This means that if you have a function f : {a:int} -> ResultType, then you can pass a value of type {a:int, b:string} to it, since it has all the required fields. A problem with subtyping for records/objects is that it doesn't work very well with type inference. With row types you use parametric polymorphism to make the extra fields explicit. You have types of the form {a:int, ...B} where B indicates that the value may have additional fields. In this system you give f the type forall B. {a:int, ...B} -> ResultType. Row types are more powerful than subtyping in some ways but less powerful in other ways. For example with subtyping you can put two records {a:3, b:"hello"} and {a:5} in the same list. With row types you can't do that (you'd need existential types to do that). It turns out that these restrictions make type inference for row types decidable.