diff --git a/kalk/src/ast.rs b/kalk/src/ast.rs
index 47f572c..c4ebd74 100644
--- a/kalk/src/ast.rs
+++ b/kalk/src/ast.rs
@@ -1,4 +1,5 @@
 use crate::lexer::TokenKind;
+use crate::parser::CalcError;
 use crate::parser::Unit;
 
 #[derive(Debug, Clone, PartialEq)]
@@ -27,11 +28,11 @@ impl TokenKind {
         }
     }
 
-    pub fn to_unit(&self) -> Result<Unit, String> {
+    pub fn to_unit(&self) -> Result<Unit, CalcError> {
         match self {
             TokenKind::Deg => Ok(Unit::Degrees),
             TokenKind::Rad => Ok(Unit::Radians),
-            _ => Err(String::from("Invalid unit.")),
+            _ => Err(CalcError::InvalidUnit),
         }
     }
 }
diff --git a/kalk/src/interpreter.rs b/kalk/src/interpreter.rs
index 73e0a79..8a2c6ea 100644
--- a/kalk/src/interpreter.rs
+++ b/kalk/src/interpreter.rs
@@ -1,5 +1,6 @@
 use crate::ast::{Expr, Stmt};
 use crate::lexer::TokenKind;
+use crate::parser::CalcError;
 use crate::parser::Unit;
 use crate::prelude;
 use crate::symbol_table::SymbolTable;
@@ -21,7 +22,7 @@ impl<'a> Context<'a> {
         }
     }
 
-    pub fn interpret(&mut self, statements: Vec<Stmt>) -> Result<Option<Float>, String> {
+    pub fn interpret(&mut self, statements: Vec<Stmt>) -> Result<Option<Float>, CalcError> {
         for (i, stmt) in statements.iter().enumerate() {
             let value = eval_stmt(self, stmt);
 
@@ -36,7 +37,7 @@ impl<'a> Context<'a> {
     }
 }
 
-fn eval_stmt(context: &mut Context, stmt: &Stmt) -> Result<Float, String> {
+fn eval_stmt(context: &mut Context, stmt: &Stmt) -> Result<Float, CalcError> {
     match stmt {
         Stmt::VarDecl(identifier, _) => eval_var_decl_stmt(context, stmt, identifier),
         Stmt::FnDecl(_, _, _) => eval_fn_decl_stmt(context),
@@ -48,20 +49,20 @@ fn eval_var_decl_stmt(
     context: &mut Context,
     stmt: &Stmt,
     identifier: &str,
-) -> Result<Float, String> {
+) -> Result<Float, CalcError> {
     context.symbol_table.insert(&identifier, stmt.clone());
     Ok(Float::with_val(context.precision, 1))
 }
 
-fn eval_fn_decl_stmt(context: &mut Context) -> Result<Float, String> {
+fn eval_fn_decl_stmt(context: &mut Context) -> Result<Float, CalcError> {
     Ok(Float::with_val(context.precision, 1)) // 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<Float, String> {
+fn eval_expr_stmt(context: &mut Context, expr: &Expr) -> Result<Float, CalcError> {
     eval_expr(context, &expr)
 }
 
-fn eval_expr(context: &mut Context, expr: &Expr) -> Result<Float, String> {
+fn eval_expr(context: &mut Context, expr: &Expr) -> Result<Float, CalcError> {
     match expr {
         Expr::Binary(left, op, right) => eval_binary_expr(context, &left, op, &right),
         Expr::Unary(op, expr) => eval_unary_expr(context, op, expr),
@@ -80,7 +81,7 @@ fn eval_binary_expr(
     left: &Expr,
     op: &TokenKind,
     right: &Expr,
-) -> Result<Float, String> {
+) -> Result<Float, CalcError> {
     let left = eval_expr(context, &left)?;
     let right = eval_expr(context, &right)?;
 
@@ -94,7 +95,7 @@ fn eval_binary_expr(
     })
 }
 
-fn eval_unary_expr(context: &mut Context, op: &TokenKind, expr: &Expr) -> Result<Float, String> {
+fn eval_unary_expr(context: &mut Context, op: &TokenKind, expr: &Expr) -> Result<Float, CalcError> {
     let expr_value = eval_expr(context, &expr)?;
 
     match op {
@@ -103,11 +104,15 @@ fn eval_unary_expr(context: &mut Context, op: &TokenKind, expr: &Expr) -> Result
             context.precision,
             prelude::special_funcs::factorial(expr_value),
         )),
-        _ => Err(String::from("Invalid operator for unary expression.")),
+        _ => Err(CalcError::InvalidOperator),
     }
 }
 
-fn eval_unit_expr(context: &mut Context, expr: &Expr, kind: &TokenKind) -> Result<Float, String> {
+fn eval_unit_expr(
+    context: &mut Context,
+    expr: &Expr,
+    kind: &TokenKind,
+) -> Result<Float, CalcError> {
     let x = eval_expr(context, &expr);
     let unit = kind.to_unit()?;
 
@@ -126,7 +131,7 @@ fn eval_unit_expr(context: &mut Context, expr: &Expr, kind: &TokenKind) -> Resul
     }
 }
 
-fn eval_var_expr(context: &mut Context, identifier: &str) -> Result<Float, String> {
+fn eval_var_expr(context: &mut Context, identifier: &str) -> Result<Float, CalcError> {
     // If there is a constant with this name, return a literal expression with its value
     if let Some(value) = prelude::CONSTANTS.get(identifier) {
         return eval_expr(context, &Expr::Literal((*value).to_string()));
@@ -136,18 +141,18 @@ fn eval_var_expr(context: &mut Context, identifier: &str) -> Result<Float, Strin
     let var_decl = context.symbol_table.get(identifier).cloned();
     match var_decl {
         Some(Stmt::VarDecl(_, expr)) => eval_expr(context, &expr),
-        _ => Err(format!("Undefined variable: '{}'.", identifier)),
+        _ => Err(CalcError::UndefinedVar(identifier.into())),
     }
 }
 
-fn eval_literal_expr(context: &mut Context, value: &str) -> Result<Float, String> {
+fn eval_literal_expr(context: &mut Context, value: &str) -> Result<Float, CalcError> {
     match Float::parse(value) {
         Ok(parsed_value) => Ok(Float::with_val(context.precision, parsed_value)),
-        Err(_) => Err(format!("Invalid number literal: '{}'.", value)),
+        Err(_) => Err(CalcError::InvalidNumberLiteral(value.into())),
     }
 }
 
-fn eval_group_expr(context: &mut Context, expr: &Expr) -> Result<Float, String> {
+fn eval_group_expr(context: &mut Context, expr: &Expr) -> Result<Float, CalcError> {
     eval_expr(context, expr)
 }
 
@@ -155,7 +160,7 @@ fn eval_fn_call_expr(
     context: &mut Context,
     identifier: &str,
     expressions: &[Expr],
-) -> Result<Float, String> {
+) -> Result<Float, CalcError> {
     // Prelude
     let prelude_func = match expressions.len() {
         1 => {
@@ -179,9 +184,10 @@ fn eval_fn_call_expr(
         "sum" | "Σ" => {
             // Make sure exactly 3 arguments were supplied.
             if expressions.len() != 3 {
-                return Err(format!(
-                    "Expected 3 arguments but got {}.",
-                    expressions.len()
+                return Err(CalcError::IncorrectAmountOfArguments(
+                    3,
+                    "sum".into(),
+                    expressions.len(),
                 ));
             }
 
@@ -214,11 +220,10 @@ fn eval_fn_call_expr(
     match stmt_definition {
         Some(Stmt::FnDecl(_, arguments, fn_body)) => {
             if arguments.len() != expressions.len() {
-                return Err(format!(
-                    "Expected {} arguments in function '{}' but found {}.",
+                return Err(CalcError::IncorrectAmountOfArguments(
                     arguments.len(),
-                    identifier,
-                    expressions.len()
+                    identifier.into(),
+                    expressions.len(),
                 ));
             }
 
@@ -232,7 +237,7 @@ fn eval_fn_call_expr(
 
             eval_expr(context, &*fn_body)
         }
-        _ => Err(format!("Undefined function: '{}'.", identifier)),
+        _ => Err(CalcError::UndefinedFn(identifier.into())),
     }
 }
 
@@ -244,7 +249,7 @@ mod tests {
 
     const PRECISION: u32 = 53;
 
-    fn interpret(stmt: Stmt) -> Result<Option<Float>, String> {
+    fn interpret(stmt: Stmt) -> Result<Option<Float>, CalcError> {
         let mut symbol_table = SymbolTable::new();
         let mut context = Context::new(&mut symbol_table, &Unit::Radians, PRECISION);
         context.interpret(vec![stmt])
@@ -318,7 +323,7 @@ mod tests {
 
         assert_eq!(
             interpret(stmt),
-            Err(String::from("Undefined variable: 'x'."))
+            Err(CalcError::UndefinedVar(String::from("x")))
         );
     }
 
diff --git a/kalk/src/parser.rs b/kalk/src/parser.rs
index d4fb686..d970f75 100644
--- a/kalk/src/parser.rs
+++ b/kalk/src/parser.rs
@@ -12,11 +12,6 @@ pub struct Context {
     symbol_table: SymbolTable,
     angle_unit: Unit,
 }
-#[derive(Debug, Clone, PartialEq)]
-pub enum Unit {
-    Radians,
-    Degrees,
-}
 
 impl Context {
     pub fn new() -> Self {
@@ -41,7 +36,29 @@ impl Default for Context {
     }
 }
 
-pub fn parse(context: &mut Context, input: &str, precision: u32) -> Result<Option<Float>, String> {
+#[derive(Debug, Clone, PartialEq)]
+pub enum Unit {
+    Radians,
+    Degrees,
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum CalcError {
+    IncorrectAmountOfArguments(usize, String, usize),
+    InvalidNumberLiteral(String),
+    InvalidOperator,
+    InvalidUnit,
+    UnexpectedToken(TokenKind),
+    UndefinedFn(String),
+    UndefinedVar(String),
+    Unknown,
+}
+
+pub fn parse(
+    context: &mut Context,
+    input: &str,
+    precision: u32,
+) -> Result<Option<Float>, CalcError> {
     context.tokens = Lexer::lex(input);
     context.pos = 0;
 
@@ -54,7 +71,7 @@ pub fn parse(context: &mut Context, input: &str, precision: u32) -> Result<Optio
     interpreter.interpret(statements)
 }
 
-fn parse_stmt(context: &mut Context) -> Result<Stmt, String> {
+fn parse_stmt(context: &mut Context) -> Result<Stmt, CalcError> {
     if match_token(context, TokenKind::Identifier) {
         return Ok(match peek_next(context).kind {
             TokenKind::Equals => parse_var_decl_stmt(context)?,
@@ -66,7 +83,7 @@ fn parse_stmt(context: &mut Context) -> Result<Stmt, String> {
     Ok(Stmt::Expr(Box::new(parse_expr(context)?)))
 }
 
-fn parse_identifier_stmt(context: &mut Context) -> Result<Stmt, String> {
+fn parse_identifier_stmt(context: &mut Context) -> Result<Stmt, CalcError> {
     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.
 
@@ -99,7 +116,7 @@ fn parse_identifier_stmt(context: &mut Context) -> Result<Stmt, String> {
             return Ok(fn_decl);
         }
 
-        Err("Parsing error.".into())
+        Err(CalcError::Unknown)
     } else {
         // It is a function call, not a function declaration.
         // Redo the parsing for this specific part.
@@ -108,7 +125,7 @@ fn parse_identifier_stmt(context: &mut Context) -> Result<Stmt, String> {
     }
 }
 
-fn parse_var_decl_stmt(context: &mut Context) -> Result<Stmt, String> {
+fn parse_var_decl_stmt(context: &mut Context) -> Result<Stmt, CalcError> {
     let identifier = advance(context).clone();
     advance(context); // Equal sign
     let expr = parse_expr(context)?;
@@ -116,11 +133,11 @@ fn parse_var_decl_stmt(context: &mut Context) -> Result<Stmt, String> {
     Ok(Stmt::VarDecl(identifier.value, Box::new(expr)))
 }
 
-fn parse_expr(context: &mut Context) -> Result<Expr, String> {
+fn parse_expr(context: &mut Context) -> Result<Expr, CalcError> {
     Ok(parse_sum(context)?)
 }
 
-fn parse_sum(context: &mut Context) -> Result<Expr, String> {
+fn parse_sum(context: &mut Context) -> Result<Expr, CalcError> {
     let mut left = parse_factor(context)?;
 
     while match_token(context, TokenKind::Plus) || match_token(context, TokenKind::Minus) {
@@ -134,7 +151,7 @@ fn parse_sum(context: &mut Context) -> Result<Expr, String> {
     Ok(left)
 }
 
-fn parse_factor(context: &mut Context) -> Result<Expr, String> {
+fn parse_factor(context: &mut Context) -> Result<Expr, CalcError> {
     let mut left = parse_unary(context)?;
 
     while match_token(context, TokenKind::Star)
@@ -155,7 +172,7 @@ fn parse_factor(context: &mut Context) -> Result<Expr, String> {
     Ok(left)
 }
 
-fn parse_unary(context: &mut Context) -> Result<Expr, String> {
+fn parse_unary(context: &mut Context) -> Result<Expr, CalcError> {
     if match_token(context, TokenKind::Minus) {
         let op = advance(context).kind.clone();
         let expr = Box::new(parse_unary(context)?);
@@ -165,7 +182,7 @@ fn parse_unary(context: &mut Context) -> Result<Expr, String> {
     Ok(parse_exponent(context)?)
 }
 
-fn parse_exponent(context: &mut Context) -> Result<Expr, String> {
+fn parse_exponent(context: &mut Context) -> Result<Expr, CalcError> {
     let left = parse_factorial(context)?;
 
     if match_token(context, TokenKind::Power) {
@@ -177,7 +194,7 @@ fn parse_exponent(context: &mut Context) -> Result<Expr, String> {
     Ok(left)
 }
 
-fn parse_factorial(context: &mut Context) -> Result<Expr, String> {
+fn parse_factorial(context: &mut Context) -> Result<Expr, CalcError> {
     let expr = parse_primary(context)?;
 
     Ok(if match_token(context, TokenKind::Exclamation) {
@@ -188,7 +205,7 @@ fn parse_factorial(context: &mut Context) -> Result<Expr, String> {
     })
 }
 
-fn parse_primary(context: &mut Context) -> Result<Expr, String> {
+fn parse_primary(context: &mut Context) -> Result<Expr, CalcError> {
     let expr = match peek(context).kind {
         TokenKind::OpenParenthesis => parse_group(context)?,
         TokenKind::Pipe => parse_abs(context)?,
@@ -203,7 +220,7 @@ fn parse_primary(context: &mut Context) -> Result<Expr, String> {
     }
 }
 
-fn parse_group(context: &mut Context) -> Result<Expr, String> {
+fn parse_group(context: &mut Context) -> Result<Expr, CalcError> {
     advance(context);
     let group_expr = Expr::Group(Box::new(parse_expr(context)?));
     consume(context, TokenKind::ClosedParenthesis)?;
@@ -211,7 +228,7 @@ fn parse_group(context: &mut Context) -> Result<Expr, String> {
     Ok(group_expr)
 }
 
-fn parse_abs(context: &mut Context) -> Result<Expr, String> {
+fn parse_abs(context: &mut Context) -> Result<Expr, CalcError> {
     advance(context);
     let group_expr = Expr::Group(Box::new(parse_expr(context)?));
     consume(context, TokenKind::Pipe)?;
@@ -219,7 +236,7 @@ fn parse_abs(context: &mut Context) -> Result<Expr, String> {
     Ok(Expr::FnCall(String::from("abs"), vec![group_expr]))
 }
 
-fn parse_identifier(context: &mut Context) -> Result<Expr, String> {
+fn parse_identifier(context: &mut Context) -> Result<Expr, CalcError> {
     let identifier = advance(context).clone();
 
     // Eg. sqrt64
@@ -294,12 +311,12 @@ fn advance(context: &mut Context) -> &Token {
     previous(context)
 }
 
-fn consume(context: &mut Context, kind: TokenKind) -> Result<&Token, String> {
-    if match_token(context, kind) {
+fn consume(context: &mut Context, kind: TokenKind) -> Result<&Token, CalcError> {
+    if match_token(context, kind.clone()) {
         return Ok(advance(context));
     }
 
-    Err("Unexpected token".into())
+    Err(CalcError::UnexpectedToken(kind))
 }
 
 fn is_at_end(context: &mut Context) -> bool {
@@ -313,13 +330,13 @@ mod tests {
     use crate::test_helpers::*;
     use test_case::test_case;
 
-    fn parse_with_context(context: &mut Context, tokens: Vec<Token>) -> Result<Stmt, String> {
+    fn parse_with_context(context: &mut Context, tokens: Vec<Token>) -> Result<Stmt, CalcError> {
         context.tokens = tokens;
 
         parse_stmt(context)
     }
 
-    fn parse(tokens: Vec<Token>) -> Result<Stmt, String> {
+    fn parse(tokens: Vec<Token>) -> Result<Stmt, CalcError> {
         let mut context = Context::new();
         context.tokens = tokens;
 
diff --git a/kalk_cli/src/output.rs b/kalk_cli/src/output.rs
index 3787e73..87db08e 100644
--- a/kalk_cli/src/output.rs
+++ b/kalk_cli/src/output.rs
@@ -1,5 +1,5 @@
 use ansi_term::Colour::Red;
-use kalk::parser::{self};
+use kalk::parser::{self, CalcError, CalcError::*};
 
 pub fn eval(parser: &mut parser::Context, input: &str) {
     match parser::parse(parser, input, 53) {
@@ -8,7 +8,7 @@ pub fn eval(parser: &mut parser::Context, input: &str) {
             let exp = if let Some(exp) = exp_option { exp } else { 0 };
 
             if result.is_infinite() {
-                err("Too big to process.");
+                print_err("Too big to process.");
             /*} else if result.clone().fract() == 0 {
             println!("{}", result.to_integer().unwrap());*/
             } else {
@@ -37,10 +37,26 @@ pub fn eval(parser: &mut parser::Context, input: &str) {
             }
         }
         Ok(None) => print!(""),
-        Err(msg) => err(&msg),
+        Err(err) => print_calc_err(err),
     }
 }
 
-fn err(msg: &str) {
+fn print_calc_err(err: CalcError) {
+    print_err(&match err {
+        IncorrectAmountOfArguments(expected, func, got) => format!(
+            "Expected {} arguments for function {}, but got {}.",
+            expected, func, got
+        ),
+        InvalidNumberLiteral(x) => format!("Invalid number literal: '{}'.", x),
+        InvalidOperator => format!("Invalid operator."),
+        InvalidUnit => format!("Invalid unit."),
+        UnexpectedToken(kind) => format!("Unexpected token: '{:?}'.", kind),
+        UndefinedFn(name) => format!("Undefined function: '{}'.", name),
+        UndefinedVar(name) => format!("Undefined variable: '{}'.", name),
+        Unknown => format!("Unknown error."),
+    });
+}
+
+fn print_err(msg: &str) {
     println!("{}", Red.paint(msg));
 }