--- a/crates/jrsonnet-cli/src/stdlib.rs
+++ b/crates/jrsonnet-cli/src/stdlib.rs
@@ -10,8 +10,33 @@
 	pub value: String,
 }
 
+/// Parses a string like `name=<value>`, or `name` and reads value from env variable.
+/// With no value it will be read from env variable.
+/// If env variable is not found then it will be an error.
+/// Value can contain `=` symbol.
+///
+/// ```
+/// use std::str::FromStr;
+/// use jrsonnet_cli::ExtStr;
+///
+/// let ext = ExtStr::from_str("name=value").unwrap();
+/// assert_eq!(ext.name, "name");
+/// assert_eq!(ext.value, "value");
+///
+/// std::env::set_var("name", "value");
+///
+/// let ext = ExtStr::from_str("name").unwrap();
+/// assert_eq!(ext.name, "name");
+/// assert_eq!(ext.value, "value");
+///
+/// let ext = ExtStr::from_str("name=value=with=equals").unwrap();
+/// assert_eq!(ext.name, "name");
+/// assert_eq!(ext.value, "value=with=equals");
+/// ```
+///
 impl FromStr for ExtStr {
 	type Err = &'static str;
+
 	fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
 		match s.find('=') {
 			Some(idx) => Ok(ExtStr {