ksl 0.1.30

KSL core library and interpreter
Documentation 
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//! # ksl::builtin::filter
//!
//! Built-in function `Filter`.

use crate::{Environment, FALSE_SYMBOL, TRUE_SYMBOL, builtin::eval_builtin, eval::apply::eval_apply, value::Value};

pub(crate) fn builtin(args: &[Value], env: Environment) -> Result<Value, std::sync::Arc<str>> {
    if let [list, predict] = args {
        match eval_apply(list, env.clone())? {
            Value::List(elements) => {
                if elements.is_empty() {
                    Ok(Value::List(std::sync::Arc::new([])))
                } else {
                    let mut filtered = Vec::<Value>::with_capacity(elements.len());
                    let fpredict = std::sync::Arc::new(eval_apply(predict, env.clone())?);
                    for element in elements.iter() {
                        let pval = match fpredict.as_ref() {
                            Value::Builtin(name) => eval_builtin(name, std::slice::from_ref(element), env.clone()),
                            Value::Lambda(_, _, _) | Value::Plugin(_, _) => eval_apply(
                                &Value::Apply(vec![element.clone()], fpredict.clone()),
                                env.clone(),
                            ),
                            e => {
                                return Err(std::sync::Arc::from(format!(
                                    concat!("Error[ksl::builtin::Filter]: ", "Unexpected value: `{}`."),
                                    e
                                )));
                            }
                        }?;
                        if match pval {
                            p @ Value::Atom(_) if p == *TRUE_SYMBOL => true,
                            p @ Value::Atom(_) if p == *FALSE_SYMBOL => false,
                            Value::Atom(p) => {
                                return Err(std::sync::Arc::from(format!(
                                    concat!("Error[ksl::builtin::Filter]: ", "`#{}` is not a boolean"),
                                    p
                                )));
                            }
                            e => {
                                return Err(std::sync::Arc::from(format!(
                                    concat!("Error[ksl::builtin::Filter]: ", "Unexpected value: `{}`."),
                                    e
                                )));
                            }
                        } {
                            filtered.push(element.clone());
                        }
                    }
                    Ok(Value::List(std::sync::Arc::from(filtered)))
                }
            }

            e => Err(std::sync::Arc::from(format!(
                concat!("Error[ksl::builtin::Filter]: ", "Unexpected value: `{}`."),
                e
            ))),
        }
    } else {
        Err(std::sync::Arc::from(format!(
            concat!(
                "Error[ksl::builtin::Filter]: ",
                "Expected 2 parameters, but {} were passed."
            ),
            args.len()
        )))
    }
}