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//! # ksl::builtin::head
//!
//! Built-in function `Head`.
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)) => {
if elements.is_empty() {
eprintln!(concat!("Error[ksl::builtin::head]: ", "Empty list."));
None
} else {
Some((elements[0].clone(), new_env))
}
}
Some((e, _)) => {
eprintln!(
concat!(
"Error[ksl::builtin::head]: ",
"Expected a list, but got `{:?}`."
),
e
);
None
}
None => {
eprintln!(
concat!(
"Error[ksl::builtin::head]: ",
"Cannot evaluate expression {:?}."
),
lst
);
None
}
}
} else {
eprintln!(
concat!(
"Error[ksl::builtin::head]: ",
"Only accepts 1 argument, but {} were passed."
),
args.len()
);
None
}
}