This Commented CUE
demonstrates how to validate the parity of an integer value using two
equivalent definitions for #Odd and #Even.
Both pairs of definitions display custom error messages using
the built-in function error,
which requires CUE v0.14.0 or later.
Using error is optional and can be omitted.
Using div
file.cue
package example
#Even: X=(2 * div(X, 2)) | error("\(X) is not even")
#Odd: X=(1 + 2*div(X, 2)) | error("\(X) is not odd")
even: [...#Even]
even: [42, 13, 0]
odd: [...#Odd]
odd: [42, 13, 0]TERMINAL
$ cue vet -c
even.1: 13 is not even:
./file.cue:3:28
./file.cue:3:11
./file.cue:6:11
./file.cue:7:12
odd.0: 42 is not odd:
./file.cue:4:30
./file.cue:4:11
./file.cue:9:10
./file.cue:10:7
odd.2: 0 is not odd:
./file.cue:4:30
./file.cue:4:11
./file.cue:9:10
./file.cue:10:15
Using math.MultipleOf
file.cue
package example
import "math"
#Even: X=math.MultipleOf(2) | error("\(X) is not even")
#Odd: X=matchN(0, [#Even]) | error("\(X) is not odd")
// #Odd can also be defined without #Even:
#Odd: matchN(0, [math.MultipleOf(2)])
even: [...#Even]
even: [42, 13, 0]
odd: [...#Odd]
odd: [42, 13, 0]TERMINAL
$ cue vet -c
even.1: 13 is not even:
./file.cue:5:31
./file.cue:5:26
./file.cue:11:11
./file.cue:12:12
odd.0: 42 is not odd:
./file.cue:6:31
./file.cue:9:7
./file.cue:9:14
./file.cue:14:10
./file.cue:15:7
odd.2: 0 is not odd:
./file.cue:6:31
./file.cue:9:7
./file.cue:9:14
./file.cue:14:10
./file.cue:15:15Issue #943 tracks the proposed built-in function
not, which would permit #Odd to be defined directly in terms of
math.MultipleOf and without the use of matchN.