ksl 0.1.7

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

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

pub(crate) fn builtin(args: &[Value], env: &Environment) -> Option<(Value, Environment)> {
    if let [Value::Identity(id), val] = &args[..] {
        if BULTIN_FUNCTIONS.contains(&id.as_str()) {
            eprintln!(
                concat!(
                    "Error[ksl::builtin::let]: ",
                    "Built-in function `{}` cannot be rebound."
                ),
                id
            );
            None
        } else {
            match eval_apply(val, env) {
                Some((result, _)) => Some((Value::Unit, Environment::from([(id.clone(), result)]))),
                None => {
                    eprintln!(
                        concat!(
                            "Error[ksl::builtin::let]: ",
                            "Cannot evaluate expression {:?}."
                        ),
                        val
                    );
                    None
                }
            }
        }
    } else {
        eprintln!(
            concat!(
                "Error[ksl::builtin::let]: ",
                "Only accepts a single symbol and a value as arguments, ",
                "but {:?} were passed."
            ),
            args
        );
        None
    }
}