ksl 0.1.30

KSL core library and interpreter
Documentation 
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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) -> Result<Value, std::sync::Arc<str>> {
    if let [lst] = args {
        match eval_apply(lst, env)? {
            Value::List(elements) => {
                if elements.is_empty() {
                    Err(std::sync::Arc::from(concat!(
                        "Error[ksl::builtin::Head]: ",
                        "Empty list."
                    )))
                } else {
                    Ok(elements[0].clone())
                }
            }
            e => Err(std::sync::Arc::from(format!(
                concat!("Error[ksl::builtin::Head]: ", "Unexpected value: `{}`."),
                e
            ))),
        }
    } else {
        Err(std::sync::Arc::from(format!(
            concat!(
                "Error[ksl::builtin::Head]: ",
                "Expected 1 parameter, but {} were passed."
            ),
            args.len()
        )))
    }
}