(Almost) Lazy Man's https://modmuss50.me/fabric.html

build.gradle

/**
 * Get the newest available loader version.
 *
 * @return loader version.
 */
static String getNewestLoaderVersion() {
	def loaderVersions = new JsonSlurper().parse(new URL("https://meta.fabricmc.net/v2/versions/loader/"))
	assert loaderVersions instanceof ArrayList<LinkedHashMap> // Type check to to remove visual errors
	return loaderVersions.first().version
}

/**
 * Get correct Yarn version for project's MC version.
 *
 * @param newest whether to return the newest available Yarn version, or the oldest. Oldest may be preferred to avoid
 * invalidating genSources' output.
 * @return the correct Yarn version.
 */
String getChosenYarnVersion(boolean newest) {
	def yarnVersions = new JsonSlurper().parse(
			new URL("https://meta.fabricmc.net/v2/versions/yarn/${project.minecraft_version}"))
	assert yarnVersions instanceof ArrayList<LinkedHashMap> // Type check to to remove visual errors
	return newest ? yarnVersions.first().version : yarnVersions.last().version
}

/**
 * Get the latest MC version.
 *
 * @param isStable if snapshots should be ignored or not.
 * @return the MC version.
 */
static String getLatestMc(boolean isStable) {
	def mcVersions = new JsonSlurper().parse(new URL("https://meta.fabricmc.net/v2/versions/game"))
	assert mcVersions instanceof ArrayList<LinkedHashMap> // Type check to to remove visual errors
	if (!isStable) return mcVersions.first().version
	for (LinkedHashMap mcVer : mcVersions) {
		if (mcVer.stable) return mcVer.version
	}
}

/**
 * Get latest Mod Menu version from Fabric's maven. Not keyed to MC version.
 *
 * @return mod menu version.
 */
// I got tired of having to find the right version, ok?
static String getLatestModMenu() {
	def modMenuVersions = new XmlSlurper().parseText(
			new URL("https://maven.fabricmc.net/io/github/prospector/modmenu/maven-metadata.xml").text)
	// For some reason Groovy was not happy with accessing it as a map, so this nonsense exists
	def x = modMenuVersions.getProperty('versioning')
	return x['latest']
}