Types and are Values
Specifying duplicate fields with conflicting values results in an error or bottom. Bottom is a special value in CUE, denoted _|_, that indicates an error such as conflicting values. Any error in CUE results in _|_. Logically all errors are equal, although errors may be associated with metadata such as an error message.
Note that an error is different from null: null is a valid value, whereas _|_ is not.
bottom.cue:
a: 4 a: 5 l: [ 1, 2 ] l: [ 1, 3 ] list: [0, 1, 2] val: list[3]
$ cue eval -i bottom.cue
list: [0, 1, 2] a: _|_ // conflicting values 4 and 5 l: [1, _|_, // conflicting values 2 and 3 ] val: _|_ // index 3 out of bounds