feat: add shared environment/set

src/lisp/environment.rs

@@ -1,5 +1,10 @@
-use super::expression::Expression;
-use std::collections::HashMap;
+use super::{expression::Expression, prelude::mk_prelude};
+use std::{
+    borrow::BorrowMut,
+    cell::{RefCell, RefMut},
+    collections::HashMap,
+    rc::Rc,
+};
 
 #[derive(PartialEq, Clone, Debug)]
 /// A Environment is a stack of `EnvironmentLayer`s. Each `EnvironmentLayer` is a mapping from
@@ -9,6 +14,8 @@ pub struct Environment<'a> {
     layer: EnvironmentLayer,
     /// The outer _fallback_ mapping.
     outer: Option<&'a Environment<'a>>,
+    /// A shared layer taking precendence over the outer layer, but not the current layer.
+    shared: Rc<RefCell<EnvironmentLayer>>,
 }
 
 #[derive(PartialEq, Clone, Debug)]
@@ -31,8 +38,8 @@ impl EnvironmentLayer {
     }
 
     /// Get a value in the `EnvironmentLayer`.
-    pub fn get(&self, key: &str) -> Option<&Expression> {
-        self.symbols.get(key)
+    pub fn get(&self, key: &str) -> Option<Expression> {
+        self.symbols.get(key).cloned()
     }
 }
 
@@ -48,12 +55,17 @@ impl<'a> Environment<'a> {
         Environment {
             layer: EnvironmentLayer::new(),
             outer: None,
+            shared: Rc::new(RefCell::new(EnvironmentLayer::new())),
         }
     }
 
     /// Construct an `Environment` from a `EnvironmentLayer` with no outer `Environment`.
     pub fn from_layer(layer: EnvironmentLayer) -> Self {
-        Environment { layer, outer: None }
+        Environment {
+            layer,
+            outer: None,
+            shared: Rc::new(RefCell::new(EnvironmentLayer::new())),
+        }
     }
 
     /// Construct a new `Environment` with `self` as the outer `Environment`.
@@ -61,6 +73,7 @@ impl<'a> Environment<'a> {
         Environment {
             layer: EnvironmentLayer::new(),
             outer: Some(self),
+            shared: self.shared.clone(),
         }
     }
 
@@ -69,15 +82,43 @@ impl<'a> Environment<'a> {
         Environment {
             layer,
             outer: Some(&self),
+            shared: Rc::new(RefCell::new(EnvironmentLayer::new())),
+        }
+    }
+
+    /// Set a value in the shared layer.
+    ///
+    /// Panics:
+    /// - if the shared layer cannot be borrowed mutably.
+    pub fn shared_set(&self, key: String, value: Expression) {
+        match self.shared.try_borrow_mut() {
+            Ok(mut shared) => shared.set(key, value),
+            Err(e) => panic!("Cannot borrow shared layer mutably. ({})", e),
+        }
+    }
+
+    /// Get a value from the shared layer.
+    pub fn shared_get(&self, key: &str) -> Option<Expression> {
+        self.shared.borrow().get(key)
+    }
+
+    /// Get a value from the `Environment`, without looking at the shared layer.
+    pub fn layer_get(&self, key: &str) -> Option<Expression> {
+        if let Some(e) = self.layer.get(key) {
+            Some(e)
+        } else {
+            self.outer?.layer_get(key).clone()
         }
     }
 
     /// Get a value from the `Environment`.
-    pub fn get(&self, key: &str) -> Option<&Expression> {
+    pub fn get(&self, key: &str) -> Option<Expression> {
         if let Some(e) = self.layer.get(key) {
             Some(e)
+        } else if let Some(e) = self.shared_get(key) {
+            Some(e)
         } else {
-            self.outer?.get(key)
+            self.outer?.layer_get(key).clone()
         }
     }
 
@@ -87,6 +128,19 @@ impl<'a> Environment<'a> {
     }
 }
 
+impl Default for Environment<'_> {
+    /// Get the default prelude layer
+    fn default() -> Self {
+        let mut prelude = EnvironmentLayer::new();
+        mk_prelude(&mut prelude);
+        Environment {
+            layer: prelude,
+            outer: None,
+            shared: Rc::new(RefCell::new(EnvironmentLayer::new())),
+        }
+    }
+}
+
 #[test]
 fn test_environment() {
     let mut env = Environment::new();
@@ -94,9 +148,9 @@ fn test_environment() {
     env.set("b".to_string(), Expression::Integer(2));
     let mut inner = env.mk_inner();
     inner.set("a".to_string(), Expression::Integer(3));
-    assert_eq!(inner.get("a"), Some(&Expression::Integer(3)));
-    assert_eq!(inner.get("b"), Some(&Expression::Integer(2)));
-    assert_eq!(env.get("a"), Some(&Expression::Integer(1)));
-    assert_eq!(env.get("b"), Some(&Expression::Integer(2)));
+    assert_eq!(inner.get("a"), Some(Expression::Integer(3)));
+    assert_eq!(inner.get("b"), Some(Expression::Integer(2)));
+    assert_eq!(env.get("a"), Some(Expression::Integer(1)));
+    assert_eq!(env.get("b"), Some(Expression::Integer(2)));
     assert_eq!(env.get("c"), None);
 }

src/lisp/eval.rs

@@ -0,0 +1,94 @@
+use super::environment::Environment;
+use super::environment::EnvironmentLayer;
+use super::expression::Expression;
+
+#[derive(Debug)]
+/// All possible evaluation errors
+pub enum EvalError {
+    SymbolNotBound(String),
+    NotAFunction(Expression),
+    NotANumber(Expression),
+    ArgumentError(String),
+    TypeError(String),
+    NotASymbol(Expression),
+}
+
+/// A CellIterator is a convenience struct to iterate a linked cons list.
+/// The Iterator returns Ok(Expression) as long, as there are elements in the list.
+/// Err(EvalError) is returned when the right side of a cons cell is not another cons cell or nil.
+pub struct CellIterator {
+    expr: Option<Expression>,
+}
+
+impl CellIterator {
+    pub fn new(expr: Expression) -> CellIterator {
+        CellIterator { expr: Some(expr) }
+    }
+}
+
+impl Iterator for CellIterator {
+    type Item = Result<Expression, EvalError>;
+    fn next(&mut self) -> Option<Self::Item> {
+        if let Some(expr) = self.expr.take() {
+            match expr {
+                Expression::Cell(head, tail) => {
+                    self.expr = Some(*tail);
+                    return Some(Ok(*head));
+                }
+                Expression::Nil => {
+                    return None;
+                }
+                _ => {
+                    return Some(Err(EvalError::TypeError(
+                        "Expected a cell or nil".to_string(),
+                    )));
+                }
+            }
+        } else {
+            None
+        }
+    }
+}
+
+/// Dispatch an anonymous function call. Evaluates `body` in `env`, binding `args` to `argument_symbols`
+fn dispatch_anonymous_function(
+    env: &Environment,
+    argument_symbols: Vec<String>,
+    body: Expression,
+    args: Expression,
+) -> Result<Expression, EvalError> {
+    let mut args: Vec<Expression> = args.try_into()?;
+
+    let mut overlay = EnvironmentLayer::new();
+
+    if args.len() != argument_symbols.len() {
+        return Err(EvalError::ArgumentError(format!(
+            "Exprected {} arguments, got {}",
+            argument_symbols.len(),
+            args.len()
+        )));
+    }
+
+    for (arg, symbol) in args.iter_mut().zip(argument_symbols.iter()) {
+        overlay.set(symbol.to_owned(), arg.to_owned());
+    }
+
+    eval(&env.overlay(overlay), body)
+}
+
+/// Evaluate an expression inside an environment
+pub fn eval(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    match expr {
+        Expression::Cell(lhs, rhs) => match eval(env, *lhs)? {
+            Expression::Function(f) => f(env, *rhs),
+            Expression::AnonymousFunction {
+                argument_symbols,
+                body,
+            } => dispatch_anonymous_function(env, argument_symbols, *body, *rhs),
+            a => Err(EvalError::NotAFunction(a)),
+        },
+        Expression::Quote(e) => Ok(*e),
+        Expression::Symbol(s) => eval(env, env.get(&s).ok_or(EvalError::SymbolNotBound(s))?),
+        x => Ok(x),
+    }
+}

src/lisp/expression.rs

@@ -1,5 +1,6 @@
-use super::environment::{Environment, EnvironmentLayer};
-use std::collections::HashMap;
+use super::environment::Environment;
+use super::eval::CellIterator;
+use super::eval::EvalError;
 
 #[derive(Clone, Debug, PartialEq, PartialOrd)]
 /// A sum type of all possible lisp expressions.
@@ -29,54 +30,6 @@ pub enum Expression {
     Nil,
 }
 
-#[derive(Debug)]
-/// All possible evaluation errors
-pub enum EvalError {
-    SymbolNotBound(String),
-    NotAFunction(Expression),
-    NotANumber(Expression),
-    ArgumentError(String),
-    TypeError(String),
-    NotASymbol(Expression),
-}
-
-/// A CellIterator is a convenience struct to iterate a linked cons list.
-/// The Iterator returns Ok(Expression) as long, as there are elements in the list.
-/// Err(EvalError) is returned when the right side of a cons cell is not another cons cell or nil.
-pub struct CellIterator {
-    expr: Option<Expression>,
-}
-
-impl CellIterator {
-    pub fn new(expr: Expression) -> CellIterator {
-        CellIterator { expr: Some(expr) }
-    }
-}
-
-impl Iterator for CellIterator {
-    type Item = Result<Expression, EvalError>;
-    fn next(&mut self) -> Option<Self::Item> {
-        if let Some(expr) = self.expr.take() {
-            match expr {
-                Expression::Cell(head, tail) => {
-                    self.expr = Some(*tail);
-                    return Some(Ok(*head));
-                }
-                Expression::Nil => {
-                    return None;
-                }
-                _ => {
-                    return Some(Err(EvalError::TypeError(
-                        "Expected a cell or nil".to_string(),
-                    )));
-                }
-            }
-        } else {
-            None
-        }
-    }
-}
-
 impl From<fn(&Environment, Expression) -> Result<Expression, EvalError>> for Expression {
     fn from(f: fn(&Environment, Expression) -> Result<Expression, EvalError>) -> Self {
         Expression::Function(f)
@@ -138,219 +91,3 @@ impl TryInto<(Expression, Expression)> for Expression {
         }
     }
 }
-
-/// Dispatch an anonymous function call. Evaluates `body` in `env`, binding `args` to `argument_symbols`
-fn dispatch_anonymous_function(
-    env: &Environment,
-    argument_symbols: Vec<String>,
-    body: Expression,
-    args: Expression,
-) -> Result<Expression, EvalError> {
-    let mut args: Vec<Expression> = args.try_into()?;
-
-    let mut overlay = EnvironmentLayer::new();
-
-    if args.len() != argument_symbols.len() {
-        return Err(EvalError::ArgumentError(format!(
-            "Exprected {} arguments, got {}",
-            argument_symbols.len(),
-            args.len()
-        )));
-    }
-
-    for (arg, symbol) in args.iter_mut().zip(argument_symbols.iter()) {
-        overlay.set(symbol.to_owned(), arg.to_owned());
-    }
-
-    eval(&env.overlay(overlay), body)
-}
-
-/// Evaluate an expression inside an environment
-fn eval(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    match expr {
-        Expression::Cell(lhs, rhs) => match eval(env, *lhs)? {
-            Expression::Function(f) => f(env, *rhs),
-            Expression::AnonymousFunction {
-                argument_symbols,
-                body,
-            } => dispatch_anonymous_function(env, argument_symbols, *body, *rhs),
-            a => Err(EvalError::NotAFunction(a)),
-        },
-        Expression::Quote(e) => Ok(*e),
-        Expression::Symbol(s) => eval(
-            env,
-            env.get(&s).ok_or(EvalError::SymbolNotBound(s)).cloned()?,
-        ),
-        x => Ok(x),
-    }
-}
-
-//==================Prelude evaluation environment==============================
-
-fn prelude_add(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [a, b] = expr.try_into()?;
-
-    match eval(env, a)? {
-        Expression::Integer(a) => match eval(env, b)? {
-            Expression::Integer(b) => Ok(Expression::Integer(a + b)),
-            Expression::Float(b) => Ok(Expression::Float(a as f64 + b)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        Expression::Float(a) => match eval(env, b)? {
-            Expression::Float(b) => Ok(Expression::Float(a + b)),
-            Expression::Integer(b) => Ok(Expression::Float(a + b as f64)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        x => Err(EvalError::NotANumber(x)),
-    }
-}
-
-fn prelude_sub(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [a, b] = expr.try_into()?;
-
-    match eval(env, a)? {
-        Expression::Integer(a) => match eval(env, b)? {
-            Expression::Integer(b) => Ok(Expression::Integer(a - b)),
-            Expression::Float(b) => Ok(Expression::Float(a as f64 - b)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        Expression::Float(a) => match eval(env, b)? {
-            Expression::Float(b) => Ok(Expression::Float(a - b)),
-            Expression::Integer(b) => Ok(Expression::Float(a - b as f64)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        x => Err(EvalError::NotANumber(x)),
-    }
-}
-
-fn prelude_mul(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [a, b] = expr.try_into()?;
-
-    match eval(env, a)? {
-        Expression::Integer(a) => match eval(env, b)? {
-            Expression::Integer(b) => Ok(Expression::Integer(a * b)),
-            Expression::Float(b) => Ok(Expression::Float(a as f64 * b)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        Expression::Float(a) => match eval(env, b)? {
-            Expression::Float(b) => Ok(Expression::Float(a * b)),
-            Expression::Integer(b) => Ok(Expression::Float(a * b as f64)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        x => Err(EvalError::NotANumber(x)),
-    }
-}
-
-fn prelude_div(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [a, b] = expr.try_into()?;
-
-    match eval(env, a)? {
-        Expression::Integer(a) => match eval(env, b)? {
-            Expression::Integer(b) => Ok(Expression::Integer(a / b)),
-            Expression::Float(b) => Ok(Expression::Float(a as f64 / b)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        Expression::Float(a) => match eval(env, b)? {
-            Expression::Float(b) => Ok(Expression::Float(a / b)),
-            Expression::Integer(b) => Ok(Expression::Float(a / b as f64)),
-            x => Err(EvalError::NotANumber(x)),
-        },
-        x => Err(EvalError::NotANumber(x)),
-    }
-}
-
-fn prelude_lambda(_env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [args, body] = expr.try_into()?;
-    let mut arg_exprs: Vec<Expression> = args.try_into()?;
-    let argument_symbols: Vec<String> = arg_exprs
-        .iter_mut()
-        .map(|a| match a {
-            Expression::Symbol(s) => Ok(s.to_owned()),
-            x => Err(EvalError::NotASymbol(x.to_owned())),
-        })
-        .collect::<Result<Vec<String>, EvalError>>()?;
-    Ok(Expression::AnonymousFunction {
-        argument_symbols,
-        body: Box::new(body),
-    })
-}
-
-fn prelude_let(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [bindings, body] = expr.try_into()?;
-
-    let bindings = CellIterator::new(bindings)
-        .map(|e| {
-            let (s, e) = e?.try_into()?;
-            if let Expression::Symbol(s) = s {
-                Ok((s, eval(env, e)?))
-            } else {
-                Err(EvalError::ArgumentError(
-                    "Let bindings must be an alist with elements (symbol . expr)".to_string(),
-                ))
-            }
-        })
-        .collect::<Result<HashMap<String, Expression>, EvalError>>()?;
-
-    eval(&env.overlay(bindings.into()), body)
-}
-
-fn prelude_if(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [predicate, e_then, e_else] = expr.try_into()?;
-
-    match eval(env, predicate)? {
-        Expression::Nil => eval(env, e_else),
-        _ => eval(env, e_then),
-    }
-}
-
-fn prelude_eq(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [a, b] = expr.try_into()?;
-    let a = eval(env, a)?;
-    let b = eval(env, b)?;
-
-    if a == b {
-        Ok(Expression::True)
-    } else {
-        Ok(Expression::Nil)
-    }
-}
-
-fn prelude_lt(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [a, b] = expr.try_into()?;
-    let a = eval(env, a)?;
-    let b = eval(env, b)?;
-
-    if a < b {
-        Ok(Expression::True)
-    } else {
-        Ok(Expression::Nil)
-    }
-}
-
-fn prelude_gt(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
-    let [a, b] = expr.try_into()?;
-    let a = eval(env, a)?;
-    let b = eval(env, b)?;
-
-    if a > b {
-        Ok(Expression::True)
-    } else {
-        Ok(Expression::Nil)
-    }
-}
-
-pub fn eval_prelude(expr: Expression) -> Result<Expression, EvalError> {
-    let mut prelude = Environment::new();
-    prelude.set("+".to_string(), Expression::Function(prelude_add));
-    prelude.set("-".to_string(), Expression::Function(prelude_sub));
-    prelude.set("*".to_string(), Expression::Function(prelude_mul));
-    prelude.set("/".to_string(), Expression::Function(prelude_div));
-    prelude.set("lambda".to_string(), Expression::Function(prelude_lambda));
-    prelude.set("if".to_string(), Expression::Function(prelude_if));
-    prelude.set("==".to_string(), Expression::Function(prelude_eq));
-    prelude.set("<".to_string(), Expression::Function(prelude_lt));
-    prelude.set(">".to_string(), Expression::Function(prelude_gt));
-    prelude.set("let".to_string(), Expression::Function(prelude_let));
-
-    eval(&prelude, expr)
-}

src/lisp/mod.rs

@@ -1,8 +1,10 @@
 pub mod environment;
+pub mod eval;
 pub mod expression;
+pub mod prelude;
 
 pub use environment::Environment;
 pub use environment::EnvironmentLayer;
-pub use expression::eval_prelude;
-pub use expression::EvalError;
+pub use eval::eval;
+pub use eval::EvalError;
 pub use expression::Expression;

src/lisp/prelude.rs

@@ -0,0 +1,185 @@
+use super::environment::Environment;
+use super::environment::EnvironmentLayer;
+use super::eval::eval;
+use super::eval::CellIterator;
+use super::eval::EvalError;
+use super::expression::Expression;
+use std::collections::HashMap;
+
+pub fn prelude_add(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [a, b] = expr.try_into()?;
+
+    match eval(env, a)? {
+        Expression::Integer(a) => match eval(env, b)? {
+            Expression::Integer(b) => Ok(Expression::Integer(a + b)),
+            Expression::Float(b) => Ok(Expression::Float(a as f64 + b)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        Expression::Float(a) => match eval(env, b)? {
+            Expression::Float(b) => Ok(Expression::Float(a + b)),
+            Expression::Integer(b) => Ok(Expression::Float(a + b as f64)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        x => Err(EvalError::NotANumber(x)),
+    }
+}
+
+pub fn prelude_sub(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [a, b] = expr.try_into()?;
+
+    match eval(env, a)? {
+        Expression::Integer(a) => match eval(env, b)? {
+            Expression::Integer(b) => Ok(Expression::Integer(a - b)),
+            Expression::Float(b) => Ok(Expression::Float(a as f64 - b)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        Expression::Float(a) => match eval(env, b)? {
+            Expression::Float(b) => Ok(Expression::Float(a - b)),
+            Expression::Integer(b) => Ok(Expression::Float(a - b as f64)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        x => Err(EvalError::NotANumber(x)),
+    }
+}
+
+pub fn prelude_mul(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [a, b] = expr.try_into()?;
+
+    match eval(env, a)? {
+        Expression::Integer(a) => match eval(env, b)? {
+            Expression::Integer(b) => Ok(Expression::Integer(a * b)),
+            Expression::Float(b) => Ok(Expression::Float(a as f64 * b)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        Expression::Float(a) => match eval(env, b)? {
+            Expression::Float(b) => Ok(Expression::Float(a * b)),
+            Expression::Integer(b) => Ok(Expression::Float(a * b as f64)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        x => Err(EvalError::NotANumber(x)),
+    }
+}
+
+pub fn prelude_div(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [a, b] = expr.try_into()?;
+
+    match eval(env, a)? {
+        Expression::Integer(a) => match eval(env, b)? {
+            Expression::Integer(b) => Ok(Expression::Integer(a / b)),
+            Expression::Float(b) => Ok(Expression::Float(a as f64 / b)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        Expression::Float(a) => match eval(env, b)? {
+            Expression::Float(b) => Ok(Expression::Float(a / b)),
+            Expression::Integer(b) => Ok(Expression::Float(a / b as f64)),
+            x => Err(EvalError::NotANumber(x)),
+        },
+        x => Err(EvalError::NotANumber(x)),
+    }
+}
+
+pub fn prelude_lambda(_env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [args, body] = expr.try_into()?;
+    let mut arg_exprs: Vec<Expression> = args.try_into()?;
+    let argument_symbols: Vec<String> = arg_exprs
+        .iter_mut()
+        .map(|a| match a {
+            Expression::Symbol(s) => Ok(s.to_owned()),
+            x => Err(EvalError::NotASymbol(x.to_owned())),
+        })
+        .collect::<Result<Vec<String>, EvalError>>()?;
+    Ok(Expression::AnonymousFunction {
+        argument_symbols,
+        body: Box::new(body),
+    })
+}
+
+pub fn prelude_let(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [bindings, body] = expr.try_into()?;
+
+    let bindings = CellIterator::new(bindings)
+        .map(|e| {
+            let (s, e) = e?.try_into()?;
+            if let Expression::Symbol(s) = s {
+                Ok((s, eval(env, e)?))
+            } else {
+                Err(EvalError::ArgumentError(
+                    "Let bindings must be an alist with elements (symbol . expr)".to_string(),
+                ))
+            }
+        })
+        .collect::<Result<HashMap<String, Expression>, EvalError>>()?;
+
+    eval(&env.overlay(bindings.into()), body)
+}
+
+pub fn prelude_if(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [predicate, e_then, e_else] = expr.try_into()?;
+
+    match eval(env, predicate)? {
+        Expression::Nil => eval(env, e_else),
+        _ => eval(env, e_then),
+    }
+}
+
+pub fn prelude_eq(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [a, b] = expr.try_into()?;
+    let a = eval(env, a)?;
+    let b = eval(env, b)?;
+
+    if a == b {
+        Ok(Expression::True)
+    } else {
+        Ok(Expression::Nil)
+    }
+}
+
+pub fn prelude_lt(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [a, b] = expr.try_into()?;
+    let a = eval(env, a)?;
+    let b = eval(env, b)?;
+
+    if a < b {
+        Ok(Expression::True)
+    } else {
+        Ok(Expression::Nil)
+    }
+}
+
+pub fn prelude_gt(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [a, b] = expr.try_into()?;
+    let a = eval(env, a)?;
+    let b = eval(env, b)?;
+
+    if a > b {
+        Ok(Expression::True)
+    } else {
+        Ok(Expression::Nil)
+    }
+}
+
+pub fn prelude_set(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [s, e] = expr.try_into()?;
+
+    match s {
+        Expression::Symbol(s) => {
+            env.shared_set(s, e);
+            Ok(Expression::Nil)
+        }
+        x => Err(EvalError::NotASymbol(x)),
+    }
+}
+
+pub fn mk_prelude(layer: &mut EnvironmentLayer) {
+    layer.set("+".to_string(), Expression::Function(prelude_add));
+    layer.set("-".to_string(), Expression::Function(prelude_sub));
+    layer.set("*".to_string(), Expression::Function(prelude_mul));
+    layer.set("/".to_string(), Expression::Function(prelude_div));
+    layer.set("lambda".to_string(), Expression::Function(prelude_lambda));
+    layer.set("if".to_string(), Expression::Function(prelude_if));
+    layer.set("==".to_string(), Expression::Function(prelude_eq));
+    layer.set("<".to_string(), Expression::Function(prelude_lt));
+    layer.set(">".to_string(), Expression::Function(prelude_gt));
+    layer.set("let".to_string(), Expression::Function(prelude_let));
+    layer.set("set".to_string(), Expression::Function(prelude_set));
+}

src/main.rs

@@ -1,21 +1,32 @@
 mod lisp;
 mod parser;
-use lisp::eval_prelude;
 use parser::ExpressionStream;
 
-fn main() {
-    let program = "((lambda (x y) (+ (if (< x 10) (* x 11) x) y)) 2 20)";
+use crate::lisp::{eval, Environment};
 
-    for r in ExpressionStream::from_char_stream(program.chars()) {
+fn main() {
+    let program1 = "((lambda (x y) (+ (if (< x 10) (* x 11) x) y)) 2 20)";
+    let program2 = "(set myvar \"hello world!\")";
+    let program3 = "myvar";
+
+    let mut environment = Environment::default();
+
+    for r in ExpressionStream::from_char_stream(
+        program1
+            .chars()
+            .chain(program2.chars())
+            .chain(program3.chars()),
+    ) {
         match r {
             Err(err) => println!("ParserError: {:?}", err),
             Ok(expr) => println!(
                 "{:?} \n vvvvvvvvvvv \n {:?}\n",
                 expr.clone(),
-                eval_prelude(expr)
+                eval(&environment, expr)
             ),
         }
     }
 
     println!("Interpreter Done!");
+    println!("Environment: {:?}", environment);
 }