# 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 <abbr>RCL</abbr> 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 <abbr>RCL</abbr> 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
<abbr>RCL</abbr> 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 <abbr>RCL</abbr>. 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`](type_dict.md#get) which requires specifying a default value.