Null coalescing is a technique that allows your CUE to evaluate successfully, despite data having invalid, unexpected, or missing values. By “null coalescing”, we really mean error (or bottom) coalescing.

It uses a disjunction’s default marker (*) to prefer the value of an expression that might evaluate to bottom (_|_), alongside an alternative, fallback value that the disjunction will select if the expression does produce bottom. This isn’t a separate language feature, but is the expected outcome from CUE’s design that _|_ | value evaluates to value.

This technique can guard against situations such as list indexes being out of bounds, and type checks, with a fallback in the case of a type mismatch.

#pets: ["Cat", "Mouse", "Dog"]

// Guard against out of bounds list indexes.
pet0: *#pets[0] | "Pet not found"
pet5: *#pets[5] | "Pet not found"

#nums: [7, "8", "9"]

// Perform a type check.
num0: *(#nums[0] & int) | "Not an integer"
num1: *(#nums[1] & int) | "Not an integer"

$ cue eval -c file.cue
pet0: "Cat"
pet5: "Pet not found"
num0: 7
num1: "Not an integer"