diff --git a/Cargo.lock b/Cargo.lock
index e0c941d..47a383f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -11,6 +11,12 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "anyhow"
+version = "1.0.71"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c7d0618f0e0b7e8ff11427422b64564d5fb0be1940354bfe2e0529b18a9d9b8"
+
 [[package]]
 name = "ascii-canvas"
 version = "3.0.0"
@@ -272,6 +278,7 @@ checksum = "2dffe52ecf27772e601905b7522cb4ef790d2cc203488bbd0e2fe85fcb74566d"
 name = "mul"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
  "lalrpop",
  "lalrpop-util",
  "regex",
diff --git a/Cargo.toml b/Cargo.toml
index d707db7..a7ec04e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,5 +7,6 @@ edition = "2021"
 lalrpop = "0.19.9"
 
 [dependencies]
+anyhow = "1.0.71"
 lalrpop-util = { version = "0.19.9", features = ["lexer"] }
 regex = "1"
diff --git a/docs/ROADMAP.md b/docs/ROADMAP.md
index 33d2b08..1ea9a13 100644
--- a/docs/ROADMAP.md
+++ b/docs/ROADMAP.md
@@ -1,17 +1,28 @@
 # implementation roadmap
 
-- [ ] parser
-  - [ ] uniform function call syntax
+## stages 
+- [x] parser
   - [ ] lossless syntax trees (rowan + ungrammar)
 - [ ] typechecker
-- [ ] interpreter
+- [x] interpreter
 - [ ] code generator
+- [ ] formatter (pretty-printer)
+
+## features
+- [ ] primitives
+  - [x] booleans
+  - [x] integers
+  - [ ] floating point numbers
+  - [ ] text
 - [ ] functions
-  - [ ] lambdas / closures
+  - [x] lambdas / closures
   - [ ] generic functions
+  - [ ] uniform function call syntax
+  - [ ] operators
 - [ ] algebraic data types
   - [ ] product types
   - [ ] sum types
     - [ ] variant types
 - [ ] pattern matching
-- [ ] trait / interface system
+- [ ] traits
+  - [ ] operator overloading
diff --git a/example.mul b/example.mul
index 35f901b..cb2f735 100644
--- a/example.mul
+++ b/example.mul
@@ -1,9 +1,9 @@
-main = || {
+let main = || {
   let result = sum_equals(1, 2, 3);
   print(result)
-}
+};
 
-sum_equals = |x, y, expected| {
+let sum_equals = |x, y, expected| {
   let sum = add(x, y);
   equals(sum, expected)
-}
+};
diff --git a/src/interpreter.rs b/src/interpreter.rs
new file mode 100644
index 0000000..9691e62
--- /dev/null
+++ b/src/interpreter.rs
@@ -0,0 +1,150 @@
+use std::collections::HashMap;
+
+use crate::syntax::{Expression, Name, Program, Statement};
+
+pub type Environment = HashMap<Name, Value>;
+
+#[derive(Debug, Clone)]
+pub enum Value {
+    Boolean(bool),
+    Integer(i64),
+    Closure(Vec<Name>, Expression, Environment),
+}
+
+pub struct Interpreter {
+    environment: Environment,
+}
+
+#[derive(Debug)]
+pub enum InterpreterError {
+    VariableNotFound(Name),
+    IncorrectArgumentCount(usize, usize),
+    NonFunctionCall(Value),
+    EmptyProgram,
+    ExecutionError(String),
+}
+
+impl From<String> for InterpreterError {
+    fn from(s: String) -> Self {
+        InterpreterError::ExecutionError(s)
+    }
+}
+
+impl std::fmt::Display for InterpreterError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            InterpreterError::VariableNotFound(name) => write!(f, "Variable '{}' not found", name),
+            InterpreterError::IncorrectArgumentCount(expected, actual) => {
+                write!(f, "Expected {} arguments, but got {}", expected, actual)
+            }
+            InterpreterError::NonFunctionCall(value) => {
+                write!(f, "Function call on non-function value: {:?}", value)
+            }
+            InterpreterError::EmptyProgram => write!(f, "Program is empty"),
+            InterpreterError::ExecutionError(s) => write!(f, "Execution error: {}", s),
+        }
+    }
+}
+
+type Result<T> = std::result::Result<T, InterpreterError>;
+
+impl Interpreter {
+    pub fn new() -> Self {
+        Self {
+            environment: Environment::new(),
+        }
+    }
+
+    pub fn interpret_statement(&mut self, statement: &Statement) -> Result<Value> {
+        match statement {
+            Statement::Binding(name, expression) => {
+                let value = self.evaluate(expression)?;
+                self.environment.insert(name.clone(), value.clone());
+                Ok(value)
+            }
+            Statement::Expression(expression) => self.evaluate(expression),
+        }
+    }
+
+    pub fn interpret(&mut self, program: &Program) -> Result<Value> {
+        let mut result = None;
+
+        for statement in program {
+            result = Some(self.interpret_statement(statement)?);
+        }
+
+        match result {
+            Some(value) => Ok(value),
+            None => Err(InterpreterError::EmptyProgram),
+        }
+    }
+
+    fn evaluate(&mut self, expression: &Expression) -> Result<Value> {
+        match expression {
+            Expression::Variable(name) => self
+                .environment
+                .get(name)
+                .cloned()
+                .ok_or_else(|| InterpreterError::VariableNotFound(name.clone())),
+
+            Expression::Boolean(value) => Ok(Value::Boolean(*value)),
+
+            Expression::Integer(value) => Ok(Value::Integer(*value)),
+
+            Expression::Lambda(params, body) => Ok(Value::Closure(
+                params.clone(),
+                *body.clone(),
+                self.environment.clone(),
+            )),
+
+            Expression::Call(function, args) => {
+                let function_value = self.evaluate(function)?;
+
+                match function_value {
+                    Value::Closure(params, body, mut environment) => {
+                        if params.len() != args.len() {
+                            return Err(InterpreterError::IncorrectArgumentCount(
+                                params.len(),
+                                args.len(),
+                            ));
+                        }
+
+                        let mut arg_values = Vec::new();
+                        for arg in args {
+                            arg_values.push(self.evaluate(arg)?);
+                        }
+
+                        // Bind argument values to parameter names in a new environment
+                        for (param, arg_value) in params.iter().zip(arg_values.into_iter()) {
+                            environment.insert(param.clone(), arg_value);
+                        }
+
+                        // Evaluate the body expression in the new environment
+                        let mut interpreter = Interpreter { environment };
+                        interpreter.evaluate(&body)
+                    }
+                    _ => Err(InterpreterError::NonFunctionCall(function_value)),
+                }
+            }
+
+            Expression::Block(statements, expression) => {
+                let new_environment = self.environment.clone();
+
+                for statement in statements {
+                    self.interpret_statement(statement)?;
+                }
+
+                let mut interpreter = Interpreter {
+                    environment: new_environment,
+                };
+                interpreter.evaluate(expression)
+            }
+        }
+    }
+}
+
+impl Default for Interpreter {
+    fn default() -> Self {
+        Self::new()
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 349ddab..bf5927c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,5 @@
 use lalrpop_util::lalrpop_mod;
 
+pub mod interpreter;
 lalrpop_mod!(pub parser);
 pub mod syntax;
diff --git a/src/main.rs b/src/main.rs
index ff787e8..4b6dd04 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,22 +1,17 @@
+use std::env::args;
+use std::fs::read_to_string;
 use std::io::{stdin, stdout, BufRead, Write};
+use std::path::Path;
+use std::process::exit;
 
-use mul::parser::{ExpressionParser, StatementParser};
+use mul::interpreter::Interpreter;
+use mul::parser::{ProgramParser, StatementParser};
 
-macro_rules! parse {
-    ($parser:expr, $line:expr) => {
-        match $parser.parse($line.as_str()) {
-            Ok(parsed) => println!("{parsed:?}"),
-            Err(lalrpop_util::ParseError::UnrecognizedEOF { .. }) => break,
-            Err(e) => println!("Parse error: {e}"),
-        }
-    };
-}
-
-fn main() {
+fn repl() {
     let mut stdin = stdin().lock();
     let mut stdout = stdout().lock();
-    let expression = ExpressionParser::new();
-    let statement = StatementParser::new();
+    let parser = StatementParser::new();
+    let mut intepreter = Interpreter::new();
     loop {
         let mut line = String::new();
         print!("> ");
@@ -24,10 +19,50 @@ fn main() {
         stdin
             .read_line(&mut line)
             .expect("Failed to read from stdin");
-        if line.trim_end().ends_with(';') {
-            parse!(statement, line)
-        } else {
-            parse!(expression, line)
+        if line.trim().is_empty() {
+            println!();
+            continue;
         }
+        if !line.trim_end().ends_with(';') {
+            line.push(';');
+        }
+        let parsed = match parser.parse(&line) {
+            Ok(parsed) => parsed,
+            Err(lalrpop_util::ParseError::UnrecognizedEOF { .. }) => break,
+            Err(e) => {
+                eprintln!("{e}");
+                continue;
+            }
+        };
+        match intepreter.interpret_statement(&parsed) {
+            Ok(value) => println!("{value:?}"),
+            Err(e) => {
+                eprintln!("{e}");
+                continue;
+            }
+        };
+    }
+}
+
+fn build(file: &Path) {
+    let content = read_to_string(file).expect("Failed to read file");
+    let parser = ProgramParser::new();
+    let mut interpreter = Interpreter::new();
+    let program = parser.parse(content.as_str()).unwrap_or_else(|e| {
+        eprintln!("{e}");
+        exit(1);
+    });
+    let v = interpreter.interpret(&program).unwrap_or_else(|e| {
+        eprintln!("{e}");
+        exit(1);
+    });
+    println!("{v:?}")
+}
+
+fn main() {
+    if let Some(file) = args().nth(1) {
+        build(file.as_ref())
+    } else {
+        repl()
     }
 }
diff --git a/src/parser.lalrpop b/src/parser.lalrpop
index e19a959..5fdfd15 100644
--- a/src/parser.lalrpop
+++ b/src/parser.lalrpop
@@ -1,32 +1,40 @@
-use crate::syntax::{Expression, Name, Statement};
+use crate::syntax::{Expression, Name, Program, Statement};
 
 grammar;
 
+pub Program: Program = Statement*;
+
 pub Statement: Statement = {
     "let" <Name> "=" <Expression> ";" => Statement::Binding(<>),
-    <Expression> ";" => Statement::Expression(<>),
+    <Block> ";"? => Statement::Expression(<>),
+    <Simple> ";" => Statement::Expression(<>),
 };
 
-pub Expression: Expression = {
-    "|" <params:Comma<Name>> "|" <body:Expression> => Expression::Lambda(params, Box::new(body)),
+pub Expression = {
+    Simple,
+    Block,
+};
+
+Simple: Expression = {
+    "|" <parameters:Comma<Name>> "|" <body:Expression> => Expression::Lambda(parameters, Box::new(body)),
     Atom,
 };
 
 Atom: Expression = {
     Name => Expression::Variable(<>),
-    "{" <statements:Statement*> <result:Expression> "}" => Expression::Block(statements, Box::new(result)),
-    <callee:Atom> "(" <args:Comma<Expression>> ")" => Expression::Call(Box::new(callee), args),
+    <callee:Atom> "(" <arguments:Comma<Expression>> ")" => Expression::Call(Box::new(callee), arguments),
     Integer => Expression::Integer(<>),
     Boolean => Expression::Boolean(<>),
     "(" <Expression> ")",
 };
 
-// TODO: decide on identifier syntax
-Name: Name =
-    r"[a-zA-Z_]+" => <>.to_string();
+Block: Expression =
+    "{" <statements:Statement*> <result:Expression> "}" => Expression::Block(statements, Box::new(result));
 
-Integer: i64 =
-    r"[0-9]+" => <>.parse::<i64>().unwrap();
+// TODO: decide on identifier syntax
+Name: Name = r"[a-zA-Z_]+" => <>.to_string();
+
+Integer: i64 = r"[0-9]+" => <>.parse::<i64>().unwrap();
 
 Boolean: bool = {
     "true" => true,
diff --git a/src/syntax.rs b/src/syntax.rs
index 53c8ee5..9f91f8b 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -1,23 +1,14 @@
-use std::fmt::Display;
-
 pub type Name = String;
 
-#[derive(Debug)]
+pub type Program = Vec<Statement>;
+
+#[derive(Debug, Clone)]
 pub enum Statement {
     Binding(Name, Expression),
     Expression(Expression),
 }
 
-impl Display for Statement {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            Statement::Binding(name, body) => write!(f, "{name} = {body};"),
-            Statement::Expression(body) => write!(f, "{body};"),
-        }
-    }
-}
-
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum Expression {
     Variable(Name),
     Block(Vec<Statement>, Box<Expression>),
@@ -26,32 +17,3 @@ pub enum Expression {
     Boolean(bool),
     Integer(i64),
 }
-
-impl Display for Expression {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        match self {
-            Expression::Variable(name) => write!(f, "{name}"),
-            Expression::Block(statements, result) => {
-                writeln!(f, "{{")?;
-                for statement in statements {
-                    writeln!(f, "\t{statement}")?;
-                }
-                writeln!(f, "\t{result}")?;
-                writeln!(f, "}}")
-            }
-            Expression::Lambda(params, body) => write!(f, "({}) -> {body}", params.join(", ")),
-            Expression::Call(callee, args) => {
-                write!(f, "{callee}(")?;
-                if let Some(arg) = args.first() {
-                    write!(f, "{arg}")?;
-                }
-                for arg in args.iter().skip(1) {
-                    write!(f, ", {arg}")?;
-                }
-                write!(f, ")")
-            }
-            Expression::Boolean(v) => write!(f, "{v}"),
-            Expression::Integer(v) => write!(f, "{v}"),
-        }
-    }
-}