feat(prelude): add print function

src/lisp/eval.rs

@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 use super::environment::Environment;
 use super::environment::EnvironmentLayer;
 use super::expression::Expression;
@@ -13,6 +15,19 @@ pub enum EvalError {
     NotASymbol(Expression),
 }
 
+impl Display for EvalError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            EvalError::SymbolNotBound(s) => write!(f, "Symbol {} is not bound", s),
+            EvalError::NotAFunction(e) => write!(f, "Expression {} is not a function", e),
+            EvalError::NotANumber(e) => write!(f, "Expression {} is not a number", e),
+            EvalError::ArgumentError(s) => write!(f, "Argument error: {}", s),
+            EvalError::TypeError(s) => write!(f, "Type error: {}", s),
+            EvalError::NotASymbol(e) => write!(f, "Expression {} is not a symbol", e),
+        }
+    }
+}
+
 /// 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.

src/lisp/expression.rs

@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 use super::environment::Environment;
 use super::eval::CellIterator;
 use super::eval::EvalError;
@@ -36,6 +38,24 @@ impl From<fn(&Environment, Expression) -> Result<Expression, EvalError>> for Exp
     }
 }
 
+impl From<Vec<Expression>> for Expression {
+    fn from(mut value: Vec<Expression>) -> Self {
+        let mut current = Expression::Nil;
+
+        for e in value.iter_mut().rev() {
+            current = Expression::Cell(Box::new(e.to_owned()), Box::new(current));
+        }
+
+        current
+    }
+}
+
+impl From<(Expression, Expression)> for Expression {
+    fn from(value: (Expression, Expression)) -> Self {
+        Expression::Cell(Box::new(value.0), Box::new(value.1))
+    }
+}
+
 impl TryInto<i64> for Expression {
     type Error = EvalError;
     fn try_into(self) -> Result<i64, Self::Error> {
@@ -48,15 +68,27 @@ impl TryInto<i64> for Expression {
     }
 }
 
-impl From<Vec<Expression>> for Expression {
-    fn from(mut value: Vec<Expression>) -> Self {
-        let mut current = Expression::Nil;
-
-        for e in value.iter_mut().rev() {
-            current = Expression::Cell(Box::new(e.to_owned()), Box::new(current));
+impl TryInto<f64> for Expression {
+    type Error = EvalError;
+    fn try_into(self) -> Result<f64, Self::Error> {
+        match self {
+            Expression::Float(f) => Ok(f),
+            _ => Err(EvalError::TypeError(
+                "Expression is not a Float".to_string(),
+            )),
         }
+    }
+}
 
-        current
+impl TryInto<String> for Expression {
+    type Error = EvalError;
+    fn try_into(self) -> Result<String, Self::Error> {
+        match self {
+            Expression::String(s) => Ok(s),
+            _ => Err(EvalError::TypeError(
+                "Expression is not a String".to_string(),
+            )),
+        }
     }
 }
 
@@ -91,3 +123,35 @@ impl TryInto<(Expression, Expression)> for Expression {
         }
     }
 }
+
+impl Display for Expression {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Expression::Cell(a, b) => {
+                match self.clone().try_into() as Result<Vec<Expression>, EvalError> {
+                    Ok(lst) => write!(
+                        f,
+                        "({})",
+                        lst.iter()
+                            .map(|e| e.to_string())
+                            .collect::<Vec<String>>()
+                            .join(" ")
+                    ),
+                    Err(_) => write!(f, "({} . {})", a, b),
+                }
+            }
+            Expression::Function(_) => write!(f, "<function>"),
+            Expression::AnonymousFunction {
+                argument_symbols,
+                body,
+            } => write!(f, "(lambda ({}) {})", argument_symbols.join(" "), body),
+            Expression::Quote(e) => write!(f, "'{}", e),
+            Expression::Symbol(s) => write!(f, "{}", s),
+            Expression::Integer(i) => write!(f, "{}", i),
+            Expression::Float(fl) => write!(f, "{}", fl),
+            Expression::String(s) => write!(f, "\"{}\"", s),
+            Expression::True => write!(f, "true"),
+            Expression::Nil => write!(f, "nil"),
+        }
+    }
+}

src/lisp/prelude.rs

@@ -170,6 +170,13 @@ pub fn prelude_set(env: &Environment, expr: Expression) -> Result<Expression, Ev
     }
 }
 
+pub fn prelude_print(env: &Environment, expr: Expression) -> Result<Expression, EvalError> {
+    let [e] = expr.try_into()?;
+    let e = eval(env, e)?;
+    println!("Prelude: {}", e);
+    Ok(e)
+}
+
 pub fn mk_prelude(layer: &mut EnvironmentLayer) {
     layer.set("+".to_string(), Expression::Function(prelude_add));
     layer.set("-".to_string(), Expression::Function(prelude_sub));
@@ -182,4 +189,5 @@ pub fn mk_prelude(layer: &mut EnvironmentLayer) {
     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));
+    layer.set("print".to_string(), Expression::Function(prelude_print));
 }

src/main.rs

@@ -7,9 +7,9 @@ use crate::lisp::{eval, Environment};
 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 program3 = "(print myvar)";
 
-    let mut environment = Environment::default();
+    let environment = Environment::default();
 
     for r in ExpressionStream::from_char_stream(
         program1
@@ -19,11 +19,13 @@ fn main() {
     ) {
         match r {
             Err(err) => println!("ParserError: {:?}", err),
-            Ok(expr) => println!(
-                "{:?} \n vvvvvvvvvvv \n {:?}\n",
-                expr.clone(),
-                eval(&environment, expr)
-            ),
+            Ok(expr) => {
+                println!("Evaluating: {}", expr.clone());
+                match eval(&environment, expr) {
+                    Ok(e) => println!("=> {}", e),
+                    Err(e) => println!("Error: {}", e),
+                }
+            }
         }
     }