5 files changed, 234 insertions, 258 deletions

diff --git a/src/lisp/error.rs b/src/lisp/error.rs
index 53681d8..f905e6d 100644
--- a/src/lisp/error.rs
+++ b/src/lisp/error.rs
@@ -1,5 +1,5 @@
 use crate::lisp::{
-    expr::Arity,
+    expr::{Arity, LispExpr},
     lex::{Span, SpanDisplay},
 };
 
@@ -98,6 +98,7 @@ pub enum EvalError {
     TypeMismatch,
     NoFileName,
     AccessEmptyList,
+    AssertionError { expected: LispExpr, got: LispExpr },
     ScriptLoadError(io::Error),
     CustomInternal(&'static str),
     Custom(String),
@@ -124,6 +125,9 @@ impl fmt::Display for EvalError {
             Self::DivByZero => write!(f, "attempt to divide by zero"),
             Self::NoFileName => write!(f, "no file name specified"),
             Self::AccessEmptyList => write!(f, "attempted to access empty list"),
+            Self::AssertionError { expected, got } => {
+                write!(f, "assertion error: expected `{}` got `{}`", expected, got)
+            }
             Self::ScriptLoadError(s) => write!(f, "error while loading script: {}", s),
             Self::CustomInternal(s) => write!(f, "{}", s),
             Self::Custom(s) => write!(f, "error: {}", s),
diff --git a/src/lisp/eval.rs b/src/lisp/eval.rs
index 44540c0..2e46251 100644
--- a/src/lisp/eval.rs
+++ b/src/lisp/eval.rs
@@ -3,7 +3,6 @@ use crate::{
     lisp::{
         error::{EvalError, LispError},
         expr::{Arity, Ident, LispExpr, LispFunction},
-        number::LispNumber,
         Environment,
     },
     type_match,
@@ -13,155 +12,208 @@ use std::convert::TryInto;
 
 use log::{error, info};
 
-pub fn eval(expr: &LispExpr, app: &mut AppState) -> Result<LispExpr, LispError> {
-    match expr {
-        LispExpr::Unit => Ok(expr.clone()),
-        LispExpr::StringLit(_) => Ok(expr.clone()),
-        LispExpr::Char(_) => Ok(expr.clone()),
-        LispExpr::Number(_) => Ok(expr.clone()),
-        LispExpr::BoolLit(_) => Ok(expr.clone()),
-        LispExpr::Ident(ref id) => lookup_extended(&app.lisp_env, id),
-        LispExpr::Quote(item, _) => match item.as_ref() {
-            i @ LispExpr::Unit
-            | i @ LispExpr::StringLit(_)
-            | i @ LispExpr::Char(_)
-            | i @ LispExpr::Number(_)
-            | i @ LispExpr::BoolLit(_) => Ok(i.clone()),
-            _ => Ok(*item.clone()),
-        },
-        LispExpr::List(li) => {
-            let func_expr = &li[0];
-            match func_expr {
-                LispExpr::Ident(s) => match s.as_ref() {
-                    "define" => define_var(&li[1..], app),
-                    "set!" => set_var(&li[1..], app),
-                    "lambda" => create_lambda(&li[1..]),
-                    "if" => eval_if(&li[1..], app),
-                    "quote" => eval_quote(&li[1..]),
-                    _ => {
-                        let mut new_ls = vec![eval(&func_expr, app)?];
-                        new_ls.extend(li[1..].to_vec());
-                        eval(&(LispExpr::List(new_ls)), app)
-                    }
-                },
-                LispExpr::PrimitiveFunc(f) => {
-                    let mut args = Vec::new();
-                    for item in li[1..].iter() {
-                        args.push(eval(item, app)?);
-                    }
-                    f.call(&args, app)
-                }
-                LispExpr::Function(f) => {
-                    let mut args = Vec::new();
-                    for item in li[1..].iter() {
-                        let i = eval(item, app)?;
-                        args.push(i);
+pub type Context = Vec<String>;
+
+pub struct Evaluator<'ctx, 'file, 'global> {
+    pub app: &'global mut AppState<'ctx, 'file>,
+    pub context: Context,
+}
+
+impl<'ctx, 'file, 'global> Evaluator<'ctx, 'file, 'global>
+where
+    'ctx: 'global,
+    'file: 'global,
+{
+    pub fn eval(&mut self, expr: &LispExpr) -> Result<LispExpr, LispError> {
+        match expr {
+            LispExpr::Unit => Ok(expr.clone()),
+            LispExpr::StringLit(_) => Ok(expr.clone()),
+            LispExpr::Char(_) => Ok(expr.clone()),
+            LispExpr::Number(_) => Ok(expr.clone()),
+            LispExpr::BoolLit(_) => Ok(expr.clone()),
+            LispExpr::Ident(ref id) => lookup(&self.app.lisp_env, id),
+            LispExpr::Quote(item, _) => match item.as_ref() {
+                i @ LispExpr::Unit
+                | i @ LispExpr::StringLit(_)
+                | i @ LispExpr::Char(_)
+                | i @ LispExpr::Number(_)
+                | i @ LispExpr::BoolLit(_) => Ok(i.clone()),
+                _ => Ok(*item.clone()),
+            },
+            LispExpr::List(li) => {
+                let func_expr = &li[0];
+                match func_expr {
+                    LispExpr::Ident(s) => match s.as_ref() {
+                        "define" => self.define_var(&li[1..]),
+                        "set!" => self.set_var(&li[1..]),
+                        "lambda" => create_lambda(&li[1..]),
+                        "if" => self.eval_if(&li[1..]),
+                        "quote" => eval_quote(&li[1..]),
+                        _ => {
+                            let mut new_ls = vec![self.eval(&func_expr)?];
+                            new_ls.extend(li[1..].to_vec());
+                            self.eval(&(LispExpr::List(new_ls)))
+                        }
+                    },
+                    LispExpr::PrimitiveFunc(f) => {
+                        let mut args = Vec::new();
+                        // context.push(f.name.to_string());
+                        for item in li[1..].iter() {
+                            args.push(self.eval(item)?);
+                        }
+                        f.call(&args, &mut self.app)
                     }
-                    if f.params.len() != args.len() {
-                        info!("too many or too little number of args");
-                        Err(EvalError::ArgumentCount(Arity::Exact(f.params.len())).into())
-                    } else {
-                        let nested_env: Environment =
-                            f.params.clone().into_iter().zip(args).collect();
-                        app.lisp_env.push(nested_env);
-                        let result = if f.body.is_empty() {
-                            Ok(LispExpr::Unit)
+                    LispExpr::Function(f) => {
+                        let mut args = Vec::new();
+                        for item in li[1..].iter() {
+                            let i = self.eval(item)?;
+                            args.push(i);
+                        }
+                        if f.params.len() != args.len() {
+                            info!("too many or too little number of args");
+                            Err(EvalError::ArgumentCount(Arity::Exact(f.params.len())).into())
                         } else {
-                            eval(&LispExpr::List(f.body.clone()), app)
-                        };
-                        app.lisp_env.pop();
-                        return result;
+                            let nested_env: Environment =
+                                f.params.clone().into_iter().zip(args).collect();
+                            self.app.lisp_env.push(nested_env);
+                            let result = if f.body.is_empty() {
+                                Ok(LispExpr::Unit)
+                            } else {
+                                self.eval(&LispExpr::List(f.body.clone()))
+                            };
+                            self.app.lisp_env.pop();
+                            return result;
+                        }
                     }
+                    LispExpr::List(_) => {
+                        info!("list as funciton");
+                        let func_expr = self.eval(&func_expr)?;
+                        let mut new_ls = vec![func_expr];
+                        new_ls.extend(li[1..].to_vec());
+                        self.eval(&(LispExpr::List(new_ls)))
+                    }
+                    _ => Err(EvalError::BadForm.into()),
                 }
-                LispExpr::List(_) => {
-                    info!("list as funciton");
-                    let func_expr = eval(&func_expr, app)?;
-                    let mut new_ls = vec![func_expr];
-                    new_ls.extend(li[1..].to_vec());
-                    eval(&(LispExpr::List(new_ls)), app)
-                }
-                _ => Err(EvalError::BadForm.into()),
             }
+            _ => Err(EvalError::BadForm.into()),
         }
-        _ => Err(EvalError::BadForm.into()),
     }
-}
 
-pub fn define_var(args: &[LispExpr], app: &mut AppState) -> Result<LispExpr, LispError> {
-    info!("defining");
-    let arity = Arity::Exact(2);
-    if !arity.is_valid(args) {
-        return Err(arity.to_error());
-    }
-    match args {
-        [LispExpr::Ident(id), expr] => {
-            let value = eval(&expr, app)?;
-            let local_env = app.lisp_env.last_mut();
-            if let Some(env) = local_env {
-                env.insert(id.into(), value);
-            } else {
-                error!("Unable to create global definition");
-                return Err(EvalError::BadForm.into());
-            }
-            return Ok(LispExpr::Unit);
+    pub fn define_var(&mut self, args: &[LispExpr]) -> Result<LispExpr, LispError> {
+        let arity = Arity::Exact(2);
+        if !arity.is_valid(args) {
+            return Err(arity.to_error());
         }
-        [LispExpr::List(shorthand), LispExpr::List(body)] => {
-            // (define (func arg) <body>) shorthand
+        match args {
+            [LispExpr::Ident(id), expr] => {
+                let value = self.eval(&expr)?;
+                let local_env = &mut self.app.lisp_env.last_mut();
+                if let Some(env) = local_env {
+                    env.insert(id.into(), value);
+                } else {
+                    error!("Unable to create global definition");
+                    return Err(EvalError::BadForm.into());
+                }
+                return Ok(LispExpr::Unit);
+            }
+            [LispExpr::List(shorthand), LispExpr::List(body)] => {
+                // (define (func arg) <body>) shorthand
 
-            let id = shorthand[0].unwrap_ident();
-            let params = if shorthand.len() > 1 {
-                &shorthand[1..]
-            } else {
-                &[]
+                let id = shorthand[0].unwrap_ident();
+                let params = if shorthand.len() > 1 {
+                    &shorthand[1..]
+                } else {
+                    &[]
+                }
+                .to_vec()
+                .into_iter()
+                .map(|arg| arg.try_into())
+                .collect::<Result<Vec<Ident>, LispError>>()?;
+                let value = LispExpr::Function(LispFunction {
+                    params,
+                    body: body.to_vec(),
+                });
+
+                let local_env = &mut self.app.lisp_env.last_mut();
+                if let Some(env) = local_env {
+                    env.insert(id.into(), value);
+                } else {
+                    error!("Unable to create global definition");
+                    return Err(EvalError::BadForm.into());
+                }
+                return Ok(LispExpr::Unit);
+            }
+            _ => {
+                error!("Invalid usage of `define`");
+                Err(EvalError::BadForm.into())
             }
-            .to_vec()
-            .into_iter()
-            .map(|arg| arg.try_into())
-            .collect::<Result<Vec<Ident>, LispError>>()?;
-            let value = LispExpr::Function(LispFunction {
-                params,
-                body: body.to_vec(),
-            });
+        }
+    }
 
-            let local_env = app.lisp_env.last_mut();
-            if let Some(env) = local_env {
-                info!("creating function {}", id);
-                env.insert(id.into(), value);
-            } else {
-                error!("Unable to create global definition");
+    pub fn set_var(&mut self, args: &[LispExpr]) -> Result<LispExpr, LispError> {
+        let arity = Arity::Exact(2);
+        if !arity.is_valid(args) {
+            return Err(arity.to_error());
+        }
+        match args {
+            [LispExpr::Ident(id), expr] => {
+                let value = self.eval(&expr)?;
+                let local_env = self.app.lisp_env.last_mut();
+                if let Some(env) = local_env {
+                    return env
+                        .insert(id.into(), value)
+                        .ok_or(EvalError::UnboundVariable(id.into()).into());
+                } else {
+                    error!("Unable to set in global env!");
+                    return Err(EvalError::BadForm.into());
+                }
+            }
+            _ => {
+                error!("Invalid usage of `set!`");
                 return Err(EvalError::BadForm.into());
             }
-            return Ok(LispExpr::Unit);
         }
-        _ => {
-            error!("Invalid usage of `define`");
-            Err(EvalError::BadForm.into())
+    }
+
+    pub fn eval_if(&mut self, args: &[LispExpr]) -> Result<LispExpr, LispError> {
+        let arity = Arity::Exact(3);
+        if !arity.is_valid(args) {
+            return Err(arity.to_error());
+        } else {
+            match args {
+                [predicate, then, else_] => {
+                    let predicate = self.eval(&predicate)?;
+                    if matches!(predicate, LispExpr::BoolLit(false)) {
+                        return self.eval(&else_);
+                    } else {
+                        return self.eval(&then);
+                    }
+                }
+                _ => {
+                    panic!("panicked at `if` expression")
+                }
+            }
         }
     }
 }
 
-pub fn set_var(args: &[LispExpr], app: &mut AppState) -> Result<LispExpr, LispError> {
-    let arity = Arity::Exact(2);
+pub fn eval_quote(args: &[LispExpr]) -> Result<LispExpr, LispError> {
+    let arity = Arity::Exact(1);
     if !arity.is_valid(args) {
         return Err(arity.to_error());
-    }
-    match args {
-        [LispExpr::Ident(id), expr] => {
-            let value = eval(&expr, app)?;
-            let local_env = app.lisp_env.last_mut();
-            if let Some(env) = local_env {
-                return env
-                    .insert(id.into(), value)
-                    .ok_or(EvalError::UnboundVariable(id.into()).into());
-            } else {
-                error!("Unable to set in global env!");
-                return Err(EvalError::BadForm.into());
+    } else {
+        match &args[0] {
+            LispExpr::Quote(item, depth) => Ok(LispExpr::Quote(item.clone(), depth + 1)),
+            i @ LispExpr::Unit
+            | i @ LispExpr::StringLit(_)
+            | i @ LispExpr::Char(_)
+            | i @ LispExpr::Number(_)
+            | i @ LispExpr::BoolLit(_) => Ok(i.clone()),
+            _ => {
+                let quoted_expr = Box::new(args[0].clone());
+                Ok(LispExpr::Quote(quoted_expr, 1))
             }
         }
-        _ => {
-            error!("Invalid usage of `set!`");
-            return Err(EvalError::BadForm.into());
-        }
     }
 }
 
@@ -170,7 +222,6 @@ pub fn create_lambda(cdr: &[LispExpr]) -> Result<LispExpr, LispError> {
     if !arity.is_valid(cdr) {
         return Err(arity.to_error());
     }
-    info!("creating lambda");
     match cdr {
         [LispExpr::List(params), LispExpr::List(body)] if type_match!(params, (..) => LispExpr::Ident(_)) =>
         {
@@ -189,48 +240,7 @@ pub fn create_lambda(cdr: &[LispExpr]) -> Result<LispExpr, LispError> {
     }
 }
 
-pub fn eval_if(args: &[LispExpr], app: &mut AppState) -> Result<LispExpr, LispError> {
-    let arity = Arity::Exact(3);
-    if !arity.is_valid(args) {
-        return Err(arity.to_error());
-    } else {
-        match args {
-            [predicate, then, else_] => {
-                let predicate = eval(&predicate, app)?;
-                if matches!(predicate, LispExpr::BoolLit(false)) {
-                    return eval(&else_, app);
-                } else {
-                    return eval(&then, app);
-                }
-            }
-            _ => {
-                panic!("panicked at `if` expression")
-            }
-        }
-    }
-}
-
-pub fn eval_quote(args: &[LispExpr]) -> Result<LispExpr, LispError> {
-    let arity = Arity::Exact(1);
-    if !arity.is_valid(args) {
-        return Err(arity.to_error());
-    } else {
-        match &args[0] {
-            LispExpr::Quote(item, depth) => Ok(LispExpr::Quote(item.clone(), depth + 1)),
-            i @ LispExpr::Unit
-            | i @ LispExpr::StringLit(_)
-            | i @ LispExpr::Char(_)
-            | i @ LispExpr::Number(_)
-            | i @ LispExpr::BoolLit(_) => Ok(i.clone()),
-            _ => {
-                let quoted_expr = Box::new(args[0].clone());
-                Ok(LispExpr::Quote(quoted_expr, 1))
-            }
-        }
-    }
-}
-
-pub fn lookup_extended(env_list: &[Environment], key: &str) -> Result<LispExpr, LispError> {
+pub fn lookup(env_list: &[Environment], key: &str) -> Result<LispExpr, LispError> {
     if env_list.is_empty() {
         return Err(EvalError::UnboundVariable(key.into()).into());
     } else {
@@ -238,55 +248,7 @@ pub fn lookup_extended(env_list: &[Environment], key: &str) -> Result<LispExpr,
         if let Some(val) = local_env.get(key) {
             return Ok(val.clone());
         } else {
-            return lookup_extended(&env_list[..env_list.len() - 1], key);
+            return lookup(&env_list[..env_list.len() - 1], key);
         }
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    use crate::lisp::{expr::LispExpr, lex::Lexer, number::LispNumber, parse::Parser};
-
-    fn run(code: &str, app: &mut AppState) -> LispExpr {
-        let mut parser = Parser::new(Lexer::new(code, 0));
-        eval(&parser.parse_single_expr().unwrap(), app).unwrap()
-    }
-
-    #[test]
-    fn eval_all() {
-        let sdl_context = sdl2::init().unwrap();
-        let ttf_context = sdl2::ttf::init().unwrap();
-        let mut app = AppState::init(100, 100, &sdl_context, &ttf_context, None, None);
-        eval_arithmetic(&mut app);
-        eval_logical(&mut app);
-    }
-
-    fn eval_arithmetic(app: &mut AppState) {
-        assert_eq!(
-            run("(+ 1 2 3)", app),
-            LispExpr::Number(LispNumber::Integer(6))
-        );
-        assert_eq!(
-            run("(+ 1.1 2.2 3.3)", app),
-            LispExpr::Number(LispNumber::Float(6.6))
-        );
-        assert_eq!(
-            run("(* 1 2 3 4 5)", app),
-            LispExpr::Number(LispNumber::Integer(120))
-        );
-        assert_eq!(run("(< 1 2)", app), LispExpr::BoolLit(true));
-        assert_eq!(run("(> 6 5 4 3 2 1)", app), LispExpr::BoolLit(true));
-        assert_eq!(run("(< 1 2 3 4 5 6)", app), LispExpr::BoolLit(true));
-        assert_eq!(run("(>= 5 5 4 3 2 1)", app), LispExpr::BoolLit(true));
-        assert_eq!(run("(<= 2 2 3 4 5 6)", app), LispExpr::BoolLit(true));
-    }
-
-    fn eval_logical(app: &mut AppState) {
-        assert_eq!(run("(and #t #t)", app), LispExpr::BoolLit(true));
-        assert_eq!(run("(or #f #t)", app), LispExpr::BoolLit(true));
-        assert_eq!(run("(not #t)", app), LispExpr::BoolLit(false));
-        assert_eq!(run("(not #f)", app), run("(not (not #t))", app));
-    }
-}
diff --git a/src/lisp/expr.rs b/src/lisp/expr.rs
index 9f2dc8d..1f3e0fb 100644
--- a/src/lisp/expr.rs
+++ b/src/lisp/expr.rs
@@ -3,7 +3,7 @@ use std::{cmp::PartialEq, convert::TryFrom, fmt};
 use crate::app::AppState;
 use crate::lisp::{
     error::{EvalError, LispError},
-    eval::lookup_extended,
+    eval::{lookup, Evaluator},
     number::LispNumber,
     EnvList,
 };
@@ -36,6 +36,7 @@ impl Arity {
 pub struct PrimitiveFunc {
     pub arity: Arity, // minimim arity
     pub closure: fn(&[LispExpr], &mut AppState) -> Result<LispExpr, LispError>,
+    pub name: &'static str,
 }
 
 impl PrimitiveFunc {
@@ -123,8 +124,8 @@ impl LispExpr {
             (LispExpr::Char(s), LispExpr::Char(o)) => Ok(s == o),
             (LispExpr::BoolLit(s), LispExpr::BoolLit(o)) => Ok(s == o),
             (LispExpr::Ident(s), LispExpr::Ident(o)) => {
-                let s = lookup_extended(envs, s)?;
-                let o = lookup_extended(envs, o)?;
+                let s = lookup(envs, s)?;
+                let o = lookup(envs, o)?;
                 s.compare(&o, envs)
             }
             (LispExpr::PrimitiveFunc(s), LispExpr::PrimitiveFunc(o)) => {
diff --git a/src/lisp/prelude.rs b/src/lisp/prelude.rs
index dffd9f4..27e7ad2 100644
--- a/src/lisp/prelude.rs
+++ b/src/lisp/prelude.rs
@@ -2,7 +2,7 @@ use crate::{
     brush::Brush,
     lisp::{
         error::{EvalError, LispError},
-        eval::eval,
+        eval::Evaluator,
         expr::{Arity, LispExpr},
         number::LispNumber,
         Environment,
@@ -21,6 +21,7 @@ macro_rules! primitive {
         let val = crate::lisp::expr::LispExpr::PrimitiveFunc(crate::lisp::expr::PrimitiveFunc {
             arity: $arity,
             closure: $closure,
+            name: $name,
         });
         let _ = $env.insert($name.to_string(), val);
     };
@@ -241,41 +242,15 @@ pub fn new_env() -> Result<Environment, LispError> {
         return Ok(LispExpr::Unit);
     });
 
-    primitive!(env, Arity::Exact(2), "map", |args, app| {
-        let mut apply_map =
-            |func: &LispExpr, ls: &Vec<LispExpr>| -> Result<Vec<LispExpr>, LispError> {
-                ls.into_iter()
-                    .map(|arg| eval(&LispExpr::List(vec![func.clone(), arg.clone()]), app))
-                    .collect()
-            };
-        if matches!(&args[0], LispExpr::Function(_) | LispExpr::PrimitiveFunc(_)) {
-            match &args[1] {
-                LispExpr::List(ls) => return Ok(LispExpr::List(apply_map(&args[0], ls)?)),
-                _ => return Err(EvalError::TypeMismatch.into()),
-            }
-        } else {
-            return Err(EvalError::TypeMismatch.into());
-        }
-    });
-
-    primitive!(env, Arity::Exact(2), "filter", |args, app| {
-        let mut apply_filter =
-            |func: &LispExpr, ls: &Vec<LispExpr>| -> Result<Vec<LispExpr>, LispError> {
-                let mut result = vec![];
-                for arg in ls.into_iter() {
-                    if eval(&LispExpr::List(vec![func.clone(), arg.clone()]), app)?.cast_bool() {
-                        result.push(arg.clone())
-                    }
-                }
-                Ok(result)
-            };
-        if matches!(&args[0], LispExpr::Function(_) | LispExpr::PrimitiveFunc(_)) {
-            match &args[1] {
-                LispExpr::List(ls) => return Ok(LispExpr::List(apply_filter(&args[0], ls)?)),
-                _ => return Err(EvalError::TypeMismatch.into()),
-            }
+    primitive!(env, Arity::Exact(2), "cons", |args, _| {
+        if type_match!(args, 1 => LispExpr::Unit) {
+            return Ok(LispExpr::List(vec![args[0].clone()]));
+        } else if !type_match!(args, 1 => LispExpr::List(_)) {
+            return Ok(LispExpr::List(vec![args[0].clone(), args[1].clone()]));
         } else {
-            return Err(EvalError::TypeMismatch.into());
+            let mut rest = args[1].unwrap_list();
+            rest.insert(0, args[0].clone());
+            return Ok(LispExpr::List(rest));
         }
     });
 
@@ -332,5 +307,17 @@ pub fn new_env() -> Result<Environment, LispError> {
         }
     });
 
+    primitive!(env, Arity::Exact(2), "assert-eq", |args, app| {
+        if args[0].compare(&args[1], &app.lisp_env)? {
+            return Ok(LispExpr::Unit);
+        } else {
+            return Err(EvalError::AssertionError {
+                expected: args[0].clone(),
+                got: args[1].clone(),
+            }
+            .into());
+        }
+    });
+
     Ok(env)
 }
diff --git a/src/lisp/std.lisp b/src/lisp/std.lisp
index c723a13..8350482 100644
--- a/src/lisp/std.lisp
+++ b/src/lisp/std.lisp
@@ -22,5 +22,27 @@
           accumulator
           (cdr ls))))
 
+(define (map func ls)
+  (if (null? ls)
+      '()
+      (cons (func (car ls))
+            (map func (cdr ls)))))
+
+(define (filter pred ls)
+  (if (null? ls)
+      '()
+      (if (pred (car ls))
+          (cons (car ls) (filter pred (cdr ls)))
+          (filter pred (cdr ls)))))
+
+(define (member? item ls)
+  (if (null? ls)
+      #f
+      (or (eq? item (car ls))
+          (member? item (cdr ls)))))
+
+(define (assert expr)
+  (assert-eq #t expr))
+
 (define (sum ls) (fold 0 + ls))
 (define (product ls) (fold 1 * ls))