diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index 5a0cf2c..7df6b41 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -1,13 +1,10 @@
 mod tree_node;
 
 use crate::tokenizer::{
-    token::{Symbol, Token, TokenType},
+    token::{Keyword, Symbol, Token, TokenType},
     Tokenizer, TokenizerBuffer, TokenizerError,
 };
-use std::{
-    collections::VecDeque,
-    io::{Read, Seek, SeekFrom},
-};
+use std::io::{Read, Seek};
 use thiserror::Error;
 use tree_node::*;
 
@@ -15,7 +12,7 @@ use tree_node::*;
 pub enum ParseError {
     #[error(transparent)]
     TokenizerError(#[from] TokenizerError),
-    #[error("Unexpected token\n\nLine: {0}, Column: {1}\nToken: {2}", token.line, token.column, token.token_type)]
+    #[error("Unexpected token\n\nLine: {0}, Column: {1}\nToken: {2}\n", token.line, token.column, token.token_type)]
     UnexpectedToken { token: Token },
     #[error("Unexpected EOF")]
     UnexpectedEOF,
@@ -23,6 +20,71 @@ pub enum ParseError {
     UnknownError,
 }
 
+macro_rules! self_matches_peek {
+    ($self:ident, $pattern:pat) => {
+        matches!($self.tokenizer.peek()?, Some(Token { token_type: $pattern, .. }))
+    };
+    ($self:ident, $pattern:pat if $cond:expr) => {
+        matches!($self.tokenizer.peek()?, Some(Token { token_type: $pattern, .. }) if $cond)
+    };
+}
+
+macro_rules! token_from_option {
+    ($token:expr) => {
+        match $token {
+            Some(ref token) => token.clone(),
+            None => return Err(ParseError::UnexpectedEOF),
+        }
+    };
+}
+
+macro_rules! extract_token_data {
+    ($token:ident, $pattern:pat, $extraction:expr) => {
+        match $token.token_type {
+            $pattern => $extraction,
+            _ => {
+                return Err(ParseError::UnexpectedToken {
+                    token: $token.clone(),
+                })
+            }
+        }
+    };
+    ($token:expr, $pattern:pat, $extraction:expr) => {
+        match $token.token_type {
+            $pattern => $extraction,
+            _ => {
+                return Err(ParseError::UnexpectedToken {
+                    token: $token.clone(),
+                })
+            }
+        }
+    };
+}
+
+macro_rules! self_matches_current {
+    ($self:ident, $pattern:pat) => {
+        matches!($self.current_token, Some(Token { token_type: $pattern, .. }))
+    };
+    ($self:ident, $pattern:pat if $cond:expr) => {
+        matches!($self.current_token, Some(Token { token_type: $pattern, .. }) if $cond)
+    };
+}
+
+macro_rules! token_matches {
+    ($token:ident, $pattern:pat) => {
+        matches!($token.token_type, $pattern)
+    };
+    ($token:expr, $pattern:pat) => {
+        matches!($token.token_type, $pattern)
+    };
+    ($token:ident, $pattern:pat if $cond:expr) => {
+        matches!($token.token_type, $pattern if $cond)
+    };
+    ($token:expr, $pattern:pat if $cond:expr) => {
+        matches!($token.token_type, $pattern if $cond)
+    };
+}
+
 pub struct Parser<R: Read + Seek> {
     tokenizer: TokenizerBuffer<R>,
     current_token: Option<Token>,
@@ -39,57 +101,40 @@ where
         }
     }
 
+    /// Parses the input from the tokenizer buffer and returns the resulting expression
     pub fn parse(&mut self) -> Result<Option<tree_node::Expression>, ParseError> {
-        self.current_token = self.tokenizer.next()?;
+        self.assign_next()?;
         self.expression()
     }
 
-    fn expression(&mut self) -> Result<Option<tree_node::Expression>, ParseError> {
-        /// Helper macro to match the next token in the tokenizer buffer to a pattern
-        /// with an optional if condition. The token is peeked and not consumed.
-        macro_rules! matches_peek {
-            ($pattern:pat) => {
-                matches!(self.tokenizer.peek()?, Some(Token { token_type: $pattern, .. }))
-            };
-            ($pattern:pat if $cond:expr) => {
-                matches!(self.tokenizer.peek()?, Some(Token { token_type: $pattern, .. }) if $cond)
-            };
-        }
+    /// Assigns the next token in the tokenizer buffer to the current token
+    fn assign_next(&mut self) -> Result<(), ParseError> {
+        self.current_token = self.tokenizer.next()?;
+        Ok(())
+    }
 
+    fn get_next(&mut self) -> Result<Option<&Token>, ParseError> {
+        self.assign_next()?;
+        Ok(self.current_token.as_ref())
+    }
+
+    fn expression(&mut self) -> Result<Option<tree_node::Expression>, ParseError> {
         let Some(current_token) = self.current_token.as_ref() else {
             return Ok(None);
         };
 
-        Ok(match current_token.token_type {
-            // Assignment expression
-            TokenType::Identifier(_) if matches_peek!(TokenType::Symbol(Symbol::Assign)) => {
-                Some(Expression::AssignmentExpression(self.assignment()?))
-            }
+        let to_return = Some(match current_token.token_type {
+            // match declarations with a `let` keyword
+            TokenType::Keyword(Keyword::Let) => self.declaration()?,
 
-            // Negation expression
-            TokenType::Symbol(Symbol::Minus) if matches_peek!(TokenType::Number(_)) => {
-                self.tokenizer.next()?;
-                Some(Expression::Negation(Box::new(
-                    self.parse()?.ok_or(ParseError::UnexpectedEOF)?,
-                )))
-            }
+            // match functions with a `fn` keyword
+            TokenType::Keyword(Keyword::Fn) => Expression::FunctionExpression(self.function()?),
 
-            // Literal expression
+            // match literal expressions with a semi-colon afterwards
             TokenType::Number(_) | TokenType::String(_)
-                if !matches_peek!(
-                    TokenType::Symbol(s) if s.is_operator() || s.is_comparison() || s.is_logical()
-                ) =>
+                if self_matches_peek!(self, TokenType::Symbol(Symbol::Semicolon)) =>
             {
-                Some(Expression::Literal(self.literal()?))
-            }
-
-            // Logical expression
-            TokenType::Number(_) | TokenType::String(_)
-                if matches_peek!(
-                    TokenType::Symbol(s) if s.is_comparison() || s.is_logical()
-                ) =>
-            {
-                Some(Expression::LogicalExpression(self.logical()?))
+                Expression::Literal(self.literal()?)
             }
 
             _ => {
@@ -97,82 +142,66 @@ where
                     token: current_token.clone(),
                 })
             }
-        })
-    }
-
-    fn assignment(&mut self) -> Result<tree_node::AssignmentExpression, ParseError> {
-        let Some(Token {
-            token_type: TokenType::Identifier(identifier),
-            ..
-        }) = self.current_token.as_ref()
-        else {
-            return Err(ParseError::UnexpectedToken {
-                // Safety: We have already checked that `self.current_token` is `Some` in the `parse()` function
-                token: self.current_token.clone().unwrap(),
-            });
-        };
-
-        // make sure the next token is `=` for sanity
-        if let Some(Token {
-            token_type: TokenType::Symbol(Symbol::Assign),
-            ..
-        }) = self.tokenizer.next()?
-        {
-        } else {
-            self.tokenizer.seek(SeekFrom::Current(-1))?;
-            return Err(ParseError::UnexpectedToken {
-                token: self.tokenizer.next()?.unwrap(),
-            });
-        };
-
-        Ok(AssignmentExpression {
-            identifier: identifier.clone(),
-            expression: Box::new(self.parse()?.ok_or(ParseError::UnexpectedEOF)?),
-        })
-    }
-
-    fn logical(&mut self) -> Result<tree_node::LogicalExpression, ParseError> {
-        let Some(current_token) = self.current_token.as_ref() else {
-            return Err(ParseError::UnexpectedEOF);
-        };
-
-        todo!()
-    }
-
-    fn binary(&mut self) -> Result<tree_node::BinaryExpression, ParseError> {
-        let Some(current_token) = self.current_token.as_ref() else {
-            return Err(ParseError::UnexpectedEOF);
-        };
-
-        todo!()
-    }
-
-    fn literal(&mut self) -> Result<tree_node::Literal, ParseError> {
-        let Some(current_token) = self.current_token.as_ref() else {
-            return Err(ParseError::UnexpectedEOF);
-        };
-
-        let to_return = match current_token.token_type {
-            TokenType::Number(n) => Literal::Number(n),
-            TokenType::String(ref s) => Literal::String(s.clone()),
-            _ => {
-                return Err(ParseError::UnexpectedToken {
-                    token: current_token.clone(),
-                })
-            }
-        };
-
-        // Advance the tokenizer if the next token is a semicolon
-        if let Some(Token {
-            token_type: TokenType::Symbol(Symbol::Semicolon),
-            ..
-        }) = self.tokenizer.peek()?
-        {
-            self.tokenizer.next()?;
-        }
+        });
 
         Ok(to_return)
     }
+
+    fn declaration(&mut self) -> Result<Expression, ParseError> {
+        let current_token = token_from_option!(self.current_token);
+        if !self_matches_current!(self, TokenType::Keyword(Keyword::Let)) {
+            return Err(ParseError::UnexpectedToken {
+                token: current_token.clone(),
+            });
+        }
+        let identifier = extract_token_data!(
+            token_from_option!(self.get_next()?),
+            TokenType::Identifier(ref id),
+            id.clone()
+        );
+
+        let current_token = token_from_option!(self.get_next()?).clone();
+
+        if !token_matches!(current_token, TokenType::Symbol(Symbol::Assign)) {
+            return Err(ParseError::UnexpectedToken {
+                token: current_token,
+            });
+        }
+
+        let assignment_expression = self.parse()?.ok_or(ParseError::UnexpectedEOF)?;
+
+        // make sure the next token is a semi-colon
+        let current_token = token_from_option!(self.get_next()?);
+        if !token_matches!(current_token, TokenType::Symbol(Symbol::Semicolon)) {
+            return Err(ParseError::UnexpectedToken {
+                token: current_token.clone(),
+            });
+        }
+
+        Ok(Expression::DeclarationExpression(
+            identifier,
+            Box::new(assignment_expression),
+        ))
+    }
+
+    fn literal(&mut self) -> Result<Literal, ParseError> {
+        let current_token = token_from_option!(self.current_token);
+        let literal = match current_token.token_type {
+            TokenType::Number(ref num) => Literal::Number(num.clone()),
+            TokenType::String(ref string) => Literal::String(string.clone()),
+            _ => {
+                return Err(ParseError::UnexpectedToken {
+                    token: current_token.clone(),
+                })
+            }
+        };
+
+        Ok(literal)
+    }
+
+    fn function(&mut self) -> Result<FunctionExpression, ParseError> {
+        todo!("Implement function parsing")
+    }
 }
 
 #[cfg(test)]
@@ -181,40 +210,20 @@ mod tests {
     use anyhow::Result;
 
     #[test]
-    fn test_assignment() -> Result<()> {
+    fn test_declarations() -> Result<()> {
         let input = r#"
-        x = 10;
-        y = "testing";
+        let x = 5;
+        // The below line should fail
+        let y = 234
         "#;
         let tokenizer = Tokenizer::from(input.to_owned());
         let mut parser = Parser::new(tokenizer);
 
-        let expr = parser.parse()?.unwrap();
+        let expression = parser.parse()?.unwrap();
 
-        assert_eq!("x = 10", format!("{}", expr));
+        assert_eq!("(let x = 5)", expression.to_string());
 
-        let expr = parser.parse()?.unwrap();
-
-        assert_eq!("y = \"testing\"", format!("{}", expr));
-
-        Ok(())
-    }
-
-    #[test]
-    fn test_literal() -> Result<()> {
-        let input = r#"
-            10;
-            "testing";
-        "#;
-
-        let tokenizer = Tokenizer::from(input.to_owned());
-        let mut parser = Parser::new(tokenizer);
-
-        let expr = parser.parse()?.unwrap();
-        assert_eq!("10", format!("{}", expr));
-
-        let expr = parser.parse()?.unwrap();
-        assert_eq!("\"testing\"", format!("{}", expr));
+        assert!(parser.parse().is_err());
 
         Ok(())
     }
diff --git a/src/parser/tree_node.rs b/src/parser/tree_node.rs
index 0b960d8..19235eb 100644
--- a/src/parser/tree_node.rs
+++ b/src/parser/tree_node.rs
@@ -1,4 +1,6 @@
-use crate::tokenizer::token::{Number, TokenType};
+use std::collections::HashSet;
+
+use crate::tokenizer::token::Number;
 
 #[derive(Debug, Eq, PartialEq)]
 pub enum Literal {
@@ -71,7 +73,30 @@ pub struct AssignmentExpression {
 
 impl std::fmt::Display for AssignmentExpression {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "{} = {}", self.identifier, self.expression)
+        write!(f, "({} = {})", self.identifier, self.expression)
+    }
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct FunctionExpression {
+    pub name: String,
+    pub arguments: HashSet<String>,
+    pub body: Box<Expression>,
+}
+
+impl std::fmt::Display for FunctionExpression {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "(fn {}({}) {{ {} }})",
+            self.name,
+            self.arguments
+                .iter()
+                .cloned()
+                .collect::<Vec<String>>()
+                .join(", "),
+            self.body
+        )
     }
 }
 
@@ -82,6 +107,8 @@ pub enum Expression {
     BinaryExpression(BinaryExpression),
     LogicalExpression(LogicalExpression),
     AssignmentExpression(AssignmentExpression),
+    DeclarationExpression(String, Box<Expression>),
+    FunctionExpression(FunctionExpression),
 }
 
 impl std::fmt::Display for Expression {
@@ -92,6 +119,8 @@ impl std::fmt::Display for Expression {
             Expression::BinaryExpression(e) => write!(f, "{}", e),
             Expression::LogicalExpression(e) => write!(f, "{}", e),
             Expression::AssignmentExpression(e) => write!(f, "{}", e),
+            Expression::DeclarationExpression(id, e) => write!(f, "(let {} = {})", id, e),
+            Expression::FunctionExpression(e) => write!(f, "{}", e),
         }
     }
 }
diff --git a/src/tokenizer/mod.rs b/src/tokenizer/mod.rs
index 6aa3b73..ccebf61 100644
--- a/src/tokenizer/mod.rs
+++ b/src/tokenizer/mod.rs
@@ -520,27 +520,6 @@ mod tests {
         }
     "#;
 
-    #[test]
-    fn test_tokenizer_buffer_seek_from_current() -> Result<()> {
-        let tokenizer = Tokenizer::from(TEST_STRING.to_owned());
-        let mut buffer = TokenizerBuffer::new(tokenizer);
-
-        let token = buffer.next()?;
-        assert_eq!(token.unwrap().token_type, TokenType::Keyword(Keyword::Fn));
-
-        buffer.seek(SeekFrom::Current(1))?;
-
-        let token = buffer.next()?;
-        assert_eq!(token.unwrap().token_type, TokenType::Symbol(Symbol::LParen));
-
-        buffer.seek(SeekFrom::Current(-1))?;
-
-        let token = buffer.next()?;
-        assert_eq!(token.unwrap().token_type, TokenType::Symbol(Symbol::LParen));
-
-        Ok(())
-    }
-
     #[test]
     fn test_tokenizer_from_path_ok() {
         let tokenizer = Tokenizer::from_path(TEST_FILE);
diff --git a/src/tokenizer/token.rs b/src/tokenizer/token.rs
index 46465dd..5a04c91 100644
--- a/src/tokenizer/token.rs
+++ b/src/tokenizer/token.rs
@@ -1,4 +1,4 @@
-#[derive(Debug, PartialEq, Clone)]
+#[derive(Debug, PartialEq, Eq, Clone)]
 pub struct Token {
     /// The type of the token
     pub token_type: TokenType,