diff --git a/src/interpreter.rs b/src/interpreter.rs
index 7a3e2a2..d734468 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -2,15 +2,15 @@ use std::mem;
 
 use crate::lexer::TokenKind;
 use crate::parser::{Expr, Stmt, Unit};
-use crate::prelude::{self};
-use crate::{symbol_table::SymbolTable, visitor::Visitor};
+use crate::prelude;
+use crate::symbol_table::SymbolTable;
 
-pub struct Interpreter<'a> {
+pub struct Context<'a> {
     symbol_table: &'a mut SymbolTable,
     angle_unit: Unit,
 }
 
-impl<'a> Interpreter<'a> {
+impl<'a> Context<'a> {
     pub fn new(angle_unit: Unit, symbol_table: &'a mut SymbolTable) -> Self {
         for constant in prelude::CONSTANTS {
             symbol_table.insert(
@@ -22,33 +22,33 @@ impl<'a> Interpreter<'a> {
             );
         }
 
-        Interpreter {
+        Context {
             angle_unit: angle_unit.clone(),
             symbol_table,
         }
     }
 
-    pub fn interpret(&mut self, statements: Vec<Stmt>) -> Option<f64> {
+    pub fn interpret(&mut self, statements: Vec<Stmt>) -> Result<Option<f64>, String> {
         for (i, stmt) in statements.iter().enumerate() {
-            let value = self.visit_stmt(stmt);
+            let value = eval_stmt(self, stmt);
 
             if i == statements.len() - 1 {
                 if let Stmt::Expr(_) = stmt {
-                    return Some(value);
+                    return Ok(Some(value?));
                 }
             }
         }
 
-        return None;
+        Ok(None)
     }
 }
 
 impl TokenKind {
-    fn to_unit(&self) -> Unit {
+    fn to_unit(&self) -> Result<Unit, String> {
         match self {
-            TokenKind::Deg => Unit::Degrees,
-            TokenKind::Rad => Unit::Radians,
-            _ => panic!("Invalid unit."),
+            TokenKind::Deg => Ok(Unit::Degrees),
+            TokenKind::Rad => Ok(Unit::Radians),
+            _ => Err(String::from("Invalid unit.")),
         }
     }
 }
@@ -60,142 +60,145 @@ impl Unit {
     }
 }
 
-impl<'a> Visitor<f64, f64> for Interpreter<'a> {
-    fn visit_stmt(&mut self, stmt: &Stmt) -> f64 {
-        match stmt {
-            Stmt::VarDecl(identifier, _) => self.eval_var_decl_stmt(stmt, identifier),
-            Stmt::FnDecl(_, _, _) => self.eval_fn_decl_stmt(),
-            Stmt::Expr(expr) => self.eval_expr_stmt(&expr),
-        }
+fn eval_stmt(context: &mut Context, stmt: &Stmt) -> Result<f64, String> {
+    match stmt {
+        Stmt::VarDecl(identifier, _) => eval_var_decl_stmt(context, stmt, identifier),
+        Stmt::FnDecl(_, _, _) => eval_fn_decl_stmt(context),
+        Stmt::Expr(expr) => eval_expr_stmt(context, &expr),
     }
+}
 
-    fn visit_expr(&mut self, expr: &Expr) -> f64 {
-        match expr {
-            Expr::Binary(left, op, right) => self.eval_binary_expr(&left, op, &right),
-            Expr::Unary(_, expr) => self.eval_unary_expr(expr),
-            Expr::Unit(expr, kind) => self.eval_unit_expr(expr, kind),
-            Expr::Var(identifier) => self.eval_var_expr(identifier),
-            Expr::Literal(value) => self.eval_literal_expr(value),
-            Expr::Group(expr) => self.eval_group_expr(&expr),
-            Expr::FnCall(identifier, expressions) => {
-                self.eval_fn_call_expr(identifier, expressions)
-            }
+fn eval_var_decl_stmt(context: &mut Context, stmt: &Stmt, identifier: &str) -> Result<f64, String> {
+    context.symbol_table.insert(&identifier, stmt.clone());
+    Ok(0f64)
+}
+
+fn eval_fn_decl_stmt(_: &mut Context) -> Result<f64, String> {
+    Ok(0f64) // Nothing needs to happen here, since the parser will already have added the FnDecl's to the symbol table.
+}
+
+fn eval_expr_stmt(context: &mut Context, expr: &Expr) -> Result<f64, String> {
+    eval_expr(context, &expr)
+}
+
+fn eval_expr(context: &mut Context, expr: &Expr) -> Result<f64, String> {
+    match expr {
+        Expr::Binary(left, op, right) => eval_binary_expr(context, &left, op, &right),
+        Expr::Unary(_, expr) => eval_unary_expr(context, expr),
+        Expr::Unit(expr, kind) => eval_unit_expr(context, expr, kind),
+        Expr::Var(identifier) => eval_var_expr(context, identifier),
+        Expr::Literal(value) => eval_literal_expr(context, value),
+        Expr::Group(expr) => eval_group_expr(context, &expr),
+        Expr::FnCall(identifier, expressions) => {
+            eval_fn_call_expr(context, identifier, expressions)
         }
     }
 }
 
-impl<'a> Interpreter<'a> {
-    fn eval_var_decl_stmt(&mut self, stmt: &Stmt, identifier: &str) -> f64 {
-        self.symbol_table.insert(&identifier, stmt.clone());
-        0f64
+fn eval_binary_expr(
+    context: &mut Context,
+    left: &Expr,
+    op: &TokenKind,
+    right: &Expr,
+) -> Result<f64, String> {
+    let left = eval_expr(context, &left)?;
+    let right = eval_expr(context, &right)?;
+
+    Ok(match op {
+        TokenKind::Plus => left + right,
+        TokenKind::Minus => left - right,
+        TokenKind::Star => left * right,
+        TokenKind::Slash => left / right,
+        TokenKind::Power => left.powf(right),
+        _ => 0f64,
+    })
+}
+
+fn eval_unary_expr(context: &mut Context, expr: &Expr) -> Result<f64, String> {
+    eval_expr(context, &expr).clone()
+}
+
+fn eval_unit_expr(context: &mut Context, expr: &Expr, kind: &TokenKind) -> Result<f64, String> {
+    let x = eval_expr(context, &expr);
+    let unit = kind.to_unit()?;
+
+    // Don't do any angle conversions if the defauly angle unit is the same as the unit kind
+    match unit {
+        Unit::Degrees | Unit::Radians => {
+            if context.angle_unit.compare(&unit) {
+                return x;
+            }
+        }
     }
 
-    fn eval_fn_decl_stmt(&mut self) -> f64 {
-        0f64 // Nothing needs to happen here, since the parser will already have added the FnDecl's to the symbol table.
-    }
-
-    fn eval_expr_stmt(&mut self, expr: &Expr) -> f64 {
-        self.visit_expr(&expr)
+    match unit {
+        Unit::Degrees => Ok(x?.to_radians()),
+        Unit::Radians => Ok(x?.to_degrees()),
     }
 }
 
-impl<'a> Interpreter<'a> {
-    fn eval_binary_expr(&mut self, left: &Expr, op: &TokenKind, right: &Expr) -> f64 {
-        let left = self.visit_expr(&left);
-        let right = self.visit_expr(&right);
+fn eval_var_expr(context: &mut Context, identifier: &str) -> Result<f64, String> {
+    let var_decl = context.symbol_table.get(identifier).cloned();
+    match var_decl {
+        Some(Stmt::VarDecl(_, expr)) => eval_expr(context, &expr),
+        _ => Err(String::from("Undefined variable.")),
+    }
+}
 
-        match op {
-            TokenKind::Plus => left + right,
-            TokenKind::Minus => left - right,
-            TokenKind::Star => left * right,
-            TokenKind::Slash => left / right,
-            TokenKind::Power => left.powf(right),
-            _ => 0f64,
+fn eval_literal_expr(_: &mut Context, value: &str) -> Result<f64, String> {
+    match value.parse() {
+        Ok(parsed_value) => Ok(parsed_value),
+        Err(_) => Err(String::from("Invalid number literal.")),
+    }
+}
+
+fn eval_group_expr(context: &mut Context, expr: &Expr) -> Result<f64, String> {
+    eval_expr(context, expr)
+}
+
+fn eval_fn_call_expr(
+    context: &mut Context,
+    identifier: &str,
+    expressions: &Vec<Expr>,
+) -> Result<f64, String> {
+    let prelude_func = match expressions.len() {
+        1 => {
+            let x = eval_expr(context, &expressions[0])?;
+            prelude::call_unary_func(identifier, x, &context.angle_unit)
         }
-    }
-
-    fn eval_unary_expr(&mut self, expr: &Expr) -> f64 {
-        self.visit_expr(&expr).clone()
-    }
-
-    fn eval_unit_expr(&mut self, expr: &Expr, kind: &TokenKind) -> f64 {
-        let x = self.visit_expr(&expr);
-
-        // Don't do any angle conversions if the defauly angle unit is the same as the unit kind
-        if (kind.compare(&TokenKind::Deg) || kind.compare(&TokenKind::Rad))
-            && self.angle_unit.compare(&kind.to_unit())
-        {
-            return x;
+        2 => {
+            let x = eval_expr(context, &expressions[0])?;
+            let y = eval_expr(context, &expressions[1])?;
+            prelude::call_binary_func(identifier, x, y, &context.angle_unit)
         }
+        _ => None,
+    };
 
-        match kind {
-            TokenKind::Deg => x.to_radians(),
-            TokenKind::Rad => x.to_degrees(),
-            _ => panic!("Invalid unit."),
-        }
+    if let Some(result) = prelude_func {
+        return Ok(result);
     }
 
-    fn eval_var_expr(&mut self, identifier: &str) -> f64 {
-        let value = self
-            .symbol_table
-            .get(identifier)
-            .expect("Undefined variable.")
-            .clone();
-        if let Stmt::VarDecl(_, expr) = value {
-            return self.visit_expr(&expr);
-        }
+    let stmt_definition = context
+        .symbol_table
+        .get(&format!("{}()", identifier))
+        .cloned();
 
-        panic!("Unknown error.");
-    }
-
-    fn eval_literal_expr(&mut self, value: &str) -> f64 {
-        value.parse().unwrap()
-    }
-
-    fn eval_group_expr(&mut self, expr: &Expr) -> f64 {
-        self.visit_expr(expr)
-    }
-
-    fn eval_fn_call_expr(&mut self, identifier: &str, expressions: &Vec<Expr>) -> f64 {
-        let prelude_func = match expressions.len() {
-            1 => {
-                let x = self.visit_expr(&expressions[0]);
-                prelude::call_unary_func(identifier, x, &self.angle_unit)
-            }
-            2 => {
-                let x = self.visit_expr(&expressions[0]);
-                let y = self.visit_expr(&expressions[1]);
-                prelude::call_binary_func(identifier, x, y, &self.angle_unit)
-            }
-            _ => None,
-        };
-
-        if let Some(result) = prelude_func {
-            return result;
-        }
-
-        let stmt = self
-            .symbol_table
-            .get(&format!("{}()", identifier))
-            .expect("Undefined function")
-            .clone();
-
-        if let Stmt::FnDecl(_, arguments, fn_body) = stmt {
+    match stmt_definition {
+        Some(Stmt::FnDecl(_, arguments, fn_body)) => {
             if arguments.len() != expressions.len() {
-                panic!("Incorrect amount of arguments.");
+                return Err(String::from("Incorrect amount of arguments."));
             }
 
             // Initialise the arguments as their own variables.
             for (i, argument) in arguments.iter().enumerate() {
-                self.visit_stmt(&Stmt::VarDecl(
-                    argument.clone(),
-                    Box::new(expressions[i].clone()),
-                ));
+                eval_stmt(
+                    context,
+                    &Stmt::VarDecl(argument.clone(), Box::new(expressions[i].clone())),
+                )?;
             }
 
-            return self.visit_expr(&*fn_body);
+            return eval_expr(context, &*fn_body);
         }
-
-        panic!("Unexpected error.");
+        _ => Err(String::from("Undefined function.")),
     }
 }
diff --git a/src/main.rs b/src/main.rs
index 70885c9..bf0c60d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,14 +5,13 @@ mod lexer;
 mod parser;
 mod prelude;
 mod symbol_table;
-mod visitor;
-use parser::{ParserContext, Unit};
+use parser::Unit;
 
 use rustyline::error::ReadlineError;
 use rustyline::Editor;
 
 fn main() {
-    let mut parser = ParserContext::new();
+    let mut parser = parser::Context::new();
 
     // Command line argument input, execute it and exit.
     if let Some(expr) = env::args().skip(1).next() {
@@ -37,7 +36,7 @@ fn main() {
     }
 }
 
-fn eval_repl(parser: &mut ParserContext, input: &str) {
+fn eval_repl(parser: &mut parser::Context, input: &str) {
     match input {
         "" => eprint!(""),
         "clear" => print!("\x1B[2J"),
@@ -46,10 +45,11 @@ fn eval_repl(parser: &mut ParserContext, input: &str) {
     }
 }
 
-fn eval(parser: &mut ParserContext, input: &str) {
+fn eval(parser: &mut parser::Context, input: &str) {
     match parser::parse(parser, input, get_angle_unit()) {
-        Ok(result) => println!("{}", result),
-        Err(_) => println!("Invalid expression"),
+        Ok(Some(result)) => println!("{}", result),
+        Ok(None) => print!(""),
+        Err(err) => println!("{}", err),
     }
 }
 
diff --git a/src/parser.rs b/src/parser.rs
index 4f4513d..fa723a4 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1,7 +1,7 @@
 use std::mem;
 
 use crate::{
-    interpreter::Interpreter,
+    interpreter,
     lexer::{Lexer, Token, TokenKind},
     symbol_table::SymbolTable,
 };
@@ -30,7 +30,7 @@ pub enum Unit {
     Degrees,
 }
 
-pub struct ParserContext {
+pub struct Context {
     //angle_unit: Unit,
     tokens: Vec<Token>,
     pos: usize,
@@ -50,9 +50,9 @@ impl TokenKind {
     }
 }
 
-impl ParserContext {
+impl Context {
     pub fn new() -> Self {
-        ParserContext {
+        Context {
             tokens: Vec::new(),
             pos: 0,
             symbol_table: SymbolTable::new(),
@@ -60,19 +60,20 @@ impl ParserContext {
     }
 }
 
-pub fn parse(context: &mut ParserContext, input: &str, angle_unit: Unit) -> Result<f64, String> {
+pub fn parse(context: &mut Context, input: &str, angle_unit: Unit) -> Result<Option<f64>, String> {
     context.tokens = Lexer::lex(input);
+    context.pos = 0;
 
     let mut statements: Vec<Stmt> = Vec::new();
     while !is_at_end(context) {
         statements.push(parse_stmt(context)?);
     }
 
-    let mut interpreter = Interpreter::new(angle_unit, &mut context.symbol_table);
-    Ok(interpreter.interpret(statements).unwrap())
+    let mut interpreter = interpreter::Context::new(angle_unit, &mut context.symbol_table);
+    interpreter.interpret(statements)
 }
 
-fn parse_stmt(context: &mut ParserContext) -> Result<Stmt, String> {
+fn parse_stmt(context: &mut Context) -> Result<Stmt, String> {
     if match_token(context, TokenKind::Identifier) {
         return Ok(match peek_next(context).kind {
             TokenKind::Equals => parse_var_decl_stmt(context)?,
@@ -84,7 +85,7 @@ fn parse_stmt(context: &mut ParserContext) -> Result<Stmt, String> {
     Ok(Stmt::Expr(Box::new(parse_expr(context)?)))
 }
 
-fn parse_identifier_stmt(context: &mut ParserContext) -> Result<Stmt, String> {
+fn parse_identifier_stmt(context: &mut Context) -> Result<Stmt, String> {
     let began_at = context.pos;
     let primary = parse_primary(context)?; // Since function declarations and function calls look the same at first, simply parse a "function call", and re-use the data.
 
@@ -126,7 +127,7 @@ fn parse_identifier_stmt(context: &mut ParserContext) -> Result<Stmt, String> {
     }
 }
 
-fn parse_var_decl_stmt(context: &mut ParserContext) -> Result<Stmt, String> {
+fn parse_var_decl_stmt(context: &mut Context) -> Result<Stmt, String> {
     let identifier = advance(context).clone();
     advance(context); // Equal sign
     let expr = parse_expr(context)?;
@@ -134,11 +135,11 @@ fn parse_var_decl_stmt(context: &mut ParserContext) -> Result<Stmt, String> {
     Ok(Stmt::VarDecl(identifier.value, Box::new(expr)))
 }
 
-fn parse_expr(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_expr(context: &mut Context) -> Result<Expr, String> {
     Ok(parse_sum(context)?)
 }
 
-fn parse_sum(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_sum(context: &mut Context) -> Result<Expr, String> {
     let mut left = parse_factor(context)?;
 
     while match_token(context, TokenKind::Plus) || match_token(context, TokenKind::Minus) {
@@ -152,7 +153,7 @@ fn parse_sum(context: &mut ParserContext) -> Result<Expr, String> {
     Ok(left)
 }
 
-fn parse_factor(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_factor(context: &mut Context) -> Result<Expr, String> {
     let mut left = parse_unary(context)?;
 
     while match_token(context, TokenKind::Star)
@@ -175,7 +176,7 @@ fn parse_factor(context: &mut ParserContext) -> Result<Expr, String> {
     Ok(left)
 }
 
-fn parse_unary(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_unary(context: &mut Context) -> Result<Expr, String> {
     if match_token(context, TokenKind::Minus) {
         let op = advance(context).kind.clone();
         let expr = Box::new(parse_unary(context)?);
@@ -185,7 +186,7 @@ fn parse_unary(context: &mut ParserContext) -> Result<Expr, String> {
     Ok(parse_exponent(context)?)
 }
 
-fn parse_exponent(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_exponent(context: &mut Context) -> Result<Expr, String> {
     let left = parse_primary(context)?;
 
     if match_token(context, TokenKind::Power) {
@@ -197,7 +198,7 @@ fn parse_exponent(context: &mut ParserContext) -> Result<Expr, String> {
     Ok(left)
 }
 
-fn parse_primary(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_primary(context: &mut Context) -> Result<Expr, String> {
     let expr = match peek(context).kind {
         TokenKind::OpenParenthesis => parse_group(context)?,
         TokenKind::Pipe => parse_abs(context)?,
@@ -212,7 +213,7 @@ fn parse_primary(context: &mut ParserContext) -> Result<Expr, String> {
     }
 }
 
-fn parse_group(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_group(context: &mut Context) -> Result<Expr, String> {
     advance(context);
     let group_expr = Expr::Group(Box::new(parse_expr(context)?));
     consume(context, TokenKind::ClosedParenthesis)?;
@@ -220,7 +221,7 @@ fn parse_group(context: &mut ParserContext) -> Result<Expr, String> {
     Ok(group_expr)
 }
 
-fn parse_abs(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_abs(context: &mut Context) -> Result<Expr, String> {
     advance(context);
     let group_expr = Expr::Group(Box::new(parse_expr(context)?));
     consume(context, TokenKind::Pipe)?;
@@ -228,7 +229,7 @@ fn parse_abs(context: &mut ParserContext) -> Result<Expr, String> {
     Ok(Expr::FnCall(String::from("abs"), vec![group_expr]))
 }
 
-fn parse_identifier(context: &mut ParserContext) -> Result<Expr, String> {
+fn parse_identifier(context: &mut Context) -> Result<Expr, String> {
     let identifier = advance(context).clone();
 
     // Eg. sqrt64
@@ -261,19 +262,19 @@ fn parse_identifier(context: &mut ParserContext) -> Result<Expr, String> {
     Ok(Expr::Var(identifier.value))
 }
 
-fn peek<'a>(context: &'a mut ParserContext) -> &'a Token {
+fn peek<'a>(context: &'a mut Context) -> &'a Token {
     &context.tokens[context.pos]
 }
 
-fn peek_next<'a>(context: &'a mut ParserContext) -> &'a Token {
+fn peek_next<'a>(context: &'a mut Context) -> &'a Token {
     &context.tokens[context.pos + 1]
 }
 
-fn previous<'a>(context: &'a mut ParserContext) -> &'a Token {
+fn previous<'a>(context: &'a mut Context) -> &'a Token {
     &context.tokens[context.pos - 1]
 }
 
-fn match_token(context: &mut ParserContext, kind: TokenKind) -> bool {
+fn match_token(context: &mut Context, kind: TokenKind) -> bool {
     if is_at_end(context) {
         return false;
     }
@@ -281,12 +282,12 @@ fn match_token(context: &mut ParserContext, kind: TokenKind) -> bool {
     peek(context).kind.compare(&kind)
 }
 
-fn advance<'a>(context: &'a mut ParserContext) -> &'a Token {
+fn advance<'a>(context: &'a mut Context) -> &'a Token {
     context.pos += 1;
     previous(context)
 }
 
-fn consume<'a>(context: &'a mut ParserContext, kind: TokenKind) -> Result<&'a Token, String> {
+fn consume<'a>(context: &'a mut Context, kind: TokenKind) -> Result<&'a Token, String> {
     if match_token(context, kind) {
         return Ok(advance(context));
     }
@@ -294,6 +295,6 @@ fn consume<'a>(context: &'a mut ParserContext, kind: TokenKind) -> Result<&'a To
     Err("Unexpected token".into())
 }
 
-fn is_at_end(context: &mut ParserContext) -> bool {
+fn is_at_end(context: &mut Context) -> bool {
     context.pos >= context.tokens.len() || peek(context).kind.compare(&TokenKind::EOF)
 }
diff --git a/src/visitor.rs b/src/visitor.rs
deleted file mode 100644
index 25fe8c5..0000000
--- a/src/visitor.rs
+++ /dev/null
@@ -1,6 +0,0 @@
-use crate::parser::{Expr, Stmt};
-
-pub trait Visitor<T, U> {
-    fn visit_stmt(&mut self, stmt: &Stmt) -> T;
-    fn visit_expr(&mut self, expr: &Expr) -> U;
-}