This Commented CUE demonstrates using an optional field marker to constrain a field’s value - but only if it is present.
schema.cue
package example
// CUE indicates optional fields with a question
// mark: "?"
// f1 is optional. If it is present it can be any
// value
f1?: _
// f-2 is optional. If it is present it must be a
// string
"f-2"?: string
// s1 is a regular field containing a struct
// which will be present no matter what the input
// data contains
s1: {
// f3 is optional. If it is present it
// must be an integer less than 10
f3?: int & <10
}data.yml
# f1 is a nested struct
f1:
f1.1:
f1.2: some string
# f-2 is floating point number, not a string
f-2: 42.137
# s1 only contains the field x, not f3.
s1:
x: some other stringTERMINAL
$ cue vet .:example data.yml
"f-2": conflicting values 42.137 and string (mismatched types float and string):
./data.yml:7:6
./schema.cue:12:9