This Commented CUE demonstrates how to use the built-in function list.IsSorted as a field validator to assert that lists are sorted, using either a predefined or custom comparator.

package example

import (

	// The strings package is only imported for some examples in this
	// guide, and is not required when using list.IsSorted.

// For lists of numbers or lists of strings, list.IsSorted can be provided
// with the predefined comparator list.Ascending or list.Descending.
isSorted: [1, 2.2, 3] & list.IsSorted(list.Ascending)

// See related content, below, for a list.IsSortedStrings example.
stringsDescending: list.IsSorted(list.Descending) // validation failure
stringsDescending: ["A", "B", "C"]

// Mixed type lists require a custom comparator containing a "less" field that
// encodes a comparison between its "x" and "y" fields, reflecting x<y. See
// list.Sort for details.
structsCustom: list.IsSorted({x: {}, y: {}, less: x.a < y.a})
structsCustom: [{a: 1}, {a: 2}, {a: 3}]

// This comparator tests if lists of strings are in descending order of how
// many "@" characters they contain.
_sortAtSymbolCountDescending: {
	x:    string
	y:    string
	less: strings.Count(x, "@") > strings.Count(y, "@")
stringsCountDescending: ["@", "@@", "X"]
stringsCountDescending: list.IsSorted(_sortAtSymbolCountDescending) // validation failure
$ cue vet
stringsDescending: invalid value ["A","B","C"] (does not satisfy list.IsSorted({T:number | string,x:number | string,y:number | string,less:_|_(Descending.less: unresolved disjunction number | string (type (string|number)) (and 1 more errors))})):
stringsCountDescending: invalid value ["@","@@","X"] (does not satisfy list.IsSorted({x:string,y:string,less:_|_(_sortAtSymbolCountDescending.less: error in call to strings.Count: non-concrete value string (and 1 more errors))})):