CUE v0.16.0-alpha.2 introduced a new optional else clause in comprehensions.

To use this language feature, update your module to target language version v0.16.0 or later with cue mod edit:

$ cue mod edit --language-version v0.16.0

With this change to the language, an if or for comprehension may be followed by an else clause which triggers when the comprehension produced zero values. For example, this occurs when:

• An if clause condition is false
• A for clause iterates over an empty collection
• A for clause has all iterations filtered out by if clauses

An else clause can help avoid repetition or verbosity, for instance:

package p

_foo: true
_bar: true

if _foo && _bar {
	withoutElse: "condition met"
}
if !(_foo && _bar) {
	withoutElse: "condition not met"
}

package p

if _foo && _bar {
	withElse: "condition met"
} else {
	withElse: "condition not met"
}

$ cue export --out yaml
withElse: condition met
withoutElse: condition met

An else clause can also be used as a fallback for loops that produce zero values:

package p

_inputs: ["foo.txt", "bar.xml", "baz.toml"]

for i, name in _inputs
if name =~ #"\.cue$"# {
	cueInputs: (name): i
} else {
	cueInputs: "fallback.cue": -1
}

$ cue export --out yaml
cueInputs:
  fallback.cue: -1