From d1b33826bcc94e1feec4eb99f5781506223e3676 Mon Sep 17 00:00:00 2001
From: Akshay <nerdy@peppe.rs>
Date: Wed, 31 Mar 2021 20:21:53 +0530
Subject: add let forms

---
 src/lisp/eval.rs | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

(limited to 'src/lisp')

diff --git a/src/lisp/eval.rs b/src/lisp/eval.rs
index d11ad30..9276ef5 100644
--- a/src/lisp/eval.rs
+++ b/src/lisp/eval.rs
@@ -42,6 +42,7 @@ where
                         "if" => self.eval_if(&li[1..]),
                         "cond" => self.eval_cond(&li[1..]),
                         "quote" => Ok(apply_quote(&li[1])),
+                        "let" => self.eval_let(&li[1..]),
                         _ => {
                             let mut new_ls = vec![self.eval(&func_expr)?];
                             new_ls.extend(li[1..].to_vec());
@@ -213,6 +214,48 @@ where
             return Ok(LispExpr::Unit);
         }
     }
+
+    pub fn eval_let(&mut self, args: &[LispExpr]) -> Result<LispExpr, LispError> {
+        let arity = Arity::Exact(2);
+        let valid_binding_stmt =
+            |expr: &LispExpr| matches!(expr, LispExpr::List(v) if v.len() == 2);
+        if !arity.check(args) {
+            return Err(arity.to_error());
+        } else {
+            let nested_env = Environment::new();
+            self.app.lisp_env.push(nested_env);
+            match args {
+                [LispExpr::List(bindings), body] => {
+                    for binding_stmt in bindings {
+                        if valid_binding_stmt(binding_stmt) {
+                            match &binding_stmt.unwrap_list()[..] {
+                                [LispExpr::Ident(id), bind_val] => {
+                                    let value = self.eval(&bind_val)?;
+                                    if let Some(env) = self.app.lisp_env.last_mut() {
+                                        env.insert(id.into(), value);
+                                    }
+                                }
+                                _ => {
+                                    error!("bad let binding form");
+                                    return Err(EvalError::BadForm.into());
+                                }
+                            }
+                        } else {
+                            error!("bad `let` form");
+                            return Err(EvalError::BadForm.into());
+                        }
+                    }
+                    let result = self.eval(&body);
+                    self.app.lisp_env.pop();
+                    return result;
+                }
+                _ => {
+                    error!("bad `let` form");
+                    return Err(EvalError::BadForm.into());
+                }
+            }
+        }
+    }
 }
 
 pub fn apply_quote(arg: &LispExpr) -> LispExpr {
@@ -222,6 +265,7 @@ pub fn apply_quote(arg: &LispExpr) -> LispExpr {
         | i @ LispExpr::Char(_)
         | i @ LispExpr::Number(_)
         | i @ LispExpr::BoolLit(_) => i.clone(),
+        LispExpr::Quote(expr, depth) => LispExpr::Quote(Box::clone(expr), depth + 1),
         LispExpr::List(ls) => LispExpr::List(ls.iter().map(|a| apply_quote(a)).collect::<Vec<_>>()),
         _ => arg.clone(),
     }
@@ -332,6 +376,7 @@ mod tests {
             run("(caar (cdr '(1 (4 5))))", app),
             LispExpr::Number(LispNumber::Integer(4))
         );
+        assert_eq!(run("''1", app), LispExpr::Number(LispNumber::Integer(1)));
     }
 
     fn eval_logical(app: &mut AppState) {
-- 
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