ksl 0.1.5

KSL core library and interpreter
Documentation 
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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
    }
}