ksl 0.1.30

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

use crate::{Environment, eval::apply::eval_apply, is_invalid_symbol, value::Value};

pub(crate) fn builtin(args: &[Value], env: Environment) -> Result<Value, std::sync::Arc<str>> {
    if let [func_expr, sym_expr, list_expr] = args {
        // iterator
        let list_val = eval_apply(list_expr, env.clone())?;
        match (list_val, sym_expr) {
            (Value::List(vindeces), Value::Symbol(index_sym)) => {
                if is_invalid_symbol(index_sym) {
                    return Err(std::sync::Arc::from(format!(
                        concat!(
                            "Error[ksl::builtin::Do]: ",
                            "Rebinding of symbol `{}` is not permitted."
                        ),
                        index_sym
                    )));
                }

                // try cache iter index symbol for naming crashing
                let old_value = env.store.write().get(index_sym).cloned();

                // do loop
                for vindex in vindeces.iter() {
                    // update index
                    env.store
                        .write()
                        .insert(index_sym.clone(), std::sync::Arc::new(vindex.clone()));
                    // iter body
                    eval_apply(func_expr, env.clone())?;
                }

                // recover index symbol
                let _ = if let Some(original) = old_value {
                    env.store.write().insert(index_sym.clone(), original)
                } else {
                    env.store.write().remove(index_sym)
                };

                Ok(Value::Unit)
            }

            (Value::List(_), e) => Err(std::sync::Arc::from(format!(
                "Error[ksl::builtin::Do]: Expected a Symbol for the index variable, but got `{}`.",
                e
            ))),
            (e, _) => Err(std::sync::Arc::from(format!(
                "Error[ksl::builtin::Do]: Expected a List to iterate over, but got `{}`.",
                e
            ))),
        }
    } else {
        Err(std::sync::Arc::from(format!(
            concat!(
                "Error[ksl::builtin::Do]: ",
                "Expected 3 parameters, but {} were passed."
            ),
            args.len()
        )))
    }
}