Null¶

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. — Tony Hoare

One of the intended use cases of RCL is to be an abstraction layer for formats with no or poor abstraction abilities, such as json and yaml. Many tools take input in such format, and it is not up to RCL to make demands about the schema of that data. RCL has to provide a way to emit null.

Fortunately, the problems commonly attributed to null are not problems with having a null value per se, they are problems with implicit nullability, which RCL does not have. null is the sole value of type Null, which is distinct from and does not unify with other types. In this sense, null behaves like the unit type in languages such as Haskell and Rust (written () there).

Although it is possible to use null as a sentinel value, this is not idiomatic in RCL. For example, indexing into a dict with a key that is not present does not return null or some other representation of “undefined”, it aborts evaluation with an error. To handle possibly absent keys, there is Dict.get which requires specifying a default value.