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Expressions

Null Coalescing

With null coalescing we really mean error, or bottom, coalescing. The defaults mechanism for disjunctions can also be used to provide fallback values in case an expression evaluates to bottom.

In the example the fallback values are specified for a and b in case the list index is out of bounds.

To do actual null coalescing one can unify a result with the desired type to force an error. In that case the default will be used if either the lookup fails or the result is not of the desired type.

coalesce.cue:

list: [ "Cat", "Mouse", "Dog" ]

a: *list[0] | "None"
b: *list[5] | "None"

n: [null]
v: *n[0]&string | "default"

$ cue eval coalesce.cue

list: ["Cat", "Mouse", "Dog"]
a: "Cat"
b: "None"
n: [null]
v: "default"