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//! # ksl::builtin::tail
//!
//! Built-in function `Tail`.
use crate::{Environment, eval::apply::eval_apply, value::Value};
pub(crate) fn builtin(args: &[Value], env: &Environment) -> Option<(Value, Environment)> {
if let [lst] = &args[..] {
match eval_apply(lst, env) {
Some((Value::List(elements), new_env)) => Some((
Value::List(if elements.is_empty() {
Vec::new()
} else {
elements[1..].to_vec()
}),
new_env,
)),
Some((e, _)) => {
eprintln!(
concat!(
"Error[ksl::builtin::tail]: ",
"Expected a list, but got `{:?}`."
),
e
);
None
}
None => {
eprintln!(
concat!(
"Error[ksl::builtin::tail]: ",
"Cannot evaluate expression {:?}."
),
lst
);
None
}
}
} else {
eprintln!(
concat!(
"Error[ksl::builtin::tail]: ",
"Only accepts 1 argument, but {} were passed."
),
args.len()
);
None
}
}