`nix repl` to interactively evaluate Nix expressions. `:p` if output is not full.
 `nix-instantiate --eval <file>.nix` to evaluate a Nix expression from a file. `--strict`.

Nix is like JSON, but with functions. 
Recursive attribute sets can reference values declared earlier in the same set. 
```nix
rec {
  one = 1;
  two = one + 1;
}
```
 
A `let` binding is used to assign names to values just as attribute sets, they can then be used in expressions. Let bindings have a local scope.

A `with` allows referencing attributes of attribute sets without referencing the set.