diff --git a/src/main.rs b/src/main.rs
index 0a0b5d9..52952d8 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -10,9 +10,11 @@ fn main() -> io::Result<()> {
 
     match cli.action {
         Some(cli::Action::Evaluate(args)) => {
-            let expr = args.expression.join(" ");
-            let expr = math::parse(&expr);
-            println!("{}", math::eval(expr));
+            let mut ctx = math::Context::default();
+
+            let result = ctx.run_string(&args.expression.join(" "));
+            println!("{}", result);
+
             Ok(())
         }
         None => repl(),
@@ -20,6 +22,8 @@ fn main() -> io::Result<()> {
 }
 
 fn repl() -> io::Result<()> {
+    let mut ctx = math::Context::default();
+
     if io::stdin().is_terminal() {
         println!("mcl: math command line | ctrl + d to exit");
     }
@@ -31,9 +35,8 @@ fn repl() -> io::Result<()> {
             break;
         }
 
-        let line = line.unwrap();
-        let parsed = math::parse(line.trim());
-        println!("{}", math::eval(parsed));
+        let result = ctx.run_string(&line.unwrap());
+        println!("{}", result);
 
         prompt()?;
     }
diff --git a/src/math/constants.rs b/src/math/constants.rs
index bce5a39..0924ac3 100644
--- a/src/math/constants.rs
+++ b/src/math/constants.rs
@@ -1,9 +1,10 @@
 use rust_decimal::Decimal;
 
-pub fn get(name: &str) -> Option<Decimal> {
-    match &name.to_lowercase()[..] {
-        "pi" => Some(Decimal::PI),
-        "e" => Some(Decimal::E),
-        _ => None,
+use crate::math::{context::Context, types::Expression};
+
+impl Context {
+    pub fn register_math_constants(&mut self) {
+        self.set_variable("pi", Expression::Number(Decimal::PI));
+        self.set_variable("e", Expression::Number(Decimal::E));
     }
 }
diff --git a/src/math/context.rs b/src/math/context.rs
new file mode 100644
index 0000000..caabdcc
--- /dev/null
+++ b/src/math/context.rs
@@ -0,0 +1,57 @@
+use std::collections::HashMap;
+
+use crate::math::{lexer, parser, types::Expression};
+
+type FunctionCallback = Box<dyn Fn(Vec<Expression>) -> Expression>;
+
+pub struct Context {
+    variables: HashMap<String, Expression>,
+    functions: HashMap<String, FunctionCallback>,
+}
+
+impl Context {
+    pub fn new() -> Self {
+        Self {
+            variables: HashMap::new(),
+            functions: HashMap::new(),
+        }
+    }
+
+    pub fn default() -> Self {
+        let mut instance = Self::new();
+        instance.register_math_constants();
+        instance.register_builtin_functions();
+        instance
+    }
+
+    pub fn register_function<F>(&mut self, name: &str, callback: F)
+    where
+        F: Fn(Vec<Expression>) -> Expression + 'static,
+    {
+        self.functions.insert(name.to_owned(), Box::new(callback));
+    }
+
+    pub fn apply_function(&self, name: &str, params: Vec<Expression>) -> Expression {
+        let func = self.functions.get(name);
+        if func.is_none() {
+            panic!("unable to find function: {}", name);
+        }
+
+        let func = func.unwrap();
+        func(params)
+    }
+
+    pub fn get_variable(&self, name: &str) -> Option<&Expression> {
+        self.variables.get(name)
+    }
+
+    pub fn set_variable(&mut self, name: &str, expr: Expression) {
+        self.variables.insert(name.to_owned(), expr);
+    }
+
+    pub fn run_string(&mut self, code: &str) -> Expression {
+        let lexer = lexer::from(code);
+        let mut parser = parser::from(lexer.collect());
+        self.eval_expr(parser.parse())
+    }
+}
diff --git a/src/math/eval.rs b/src/math/eval.rs
index f21904a..334e17f 100644
--- a/src/math/eval.rs
+++ b/src/math/eval.rs
@@ -1,48 +1,45 @@
 use rust_decimal::MathematicalOps;
 
 use crate::math::{
-    constants, functions,
+    context::Context,
     types::{Expression, Operator},
 };
 
-pub fn eval_expr(expr: Expression) -> Expression {
-    match expr {
-        Expression::Number(_) => expr,
-        Expression::Variable(name) => {
-            if let Some(value) = constants::get(&name) {
-                Expression::Number(value)
-            } else {
-                Expression::Variable(name)
+impl Context {
+    pub fn eval_expr(&self, expr: Expression) -> Expression {
+        match expr {
+            Expression::Number(_) => expr,
+            Expression::Variable(name) => {
+                if let Some(value) = self.get_variable(&name) {
+                    value.clone()
+                } else {
+                    Expression::Variable(name)
+                }
             }
-        }
-        Expression::Fraction(numberator, denominator) => {
-            // TODO: implement proper fraction handling
-            eval_expr(Expression::Operation(
-                Operator::Divide,
-                numberator,
-                denominator,
-            ))
-        }
-        Expression::Operation(operator, left, right) => eval_operation(operator, *left, *right),
-        Expression::Function(name, params) => {
-            let params = params.into_iter().map(eval_expr).collect();
-            functions::eval_function(&name, params)
+            Expression::Fraction(numberator, denominator) => {
+                // TODO: implement proper fraction handling
+                self.eval_expr(Expression::Operation(
+                    Operator::Divide,
+                    numberator,
+                    denominator,
+                ))
+            }
+            Expression::Operation(operator, left, right) => {
+                let left = self.eval_expr(*left);
+                let right = self.eval_expr(*right);
+                match (&left, &right) {
+                    (Expression::Number(left), Expression::Number(right)) => match operator {
+                        Operator::Add => Expression::Number(left + right),
+                        Operator::Subtract => Expression::Number(left - right),
+                        Operator::Multiply => Expression::Number(left * right),
+                        Operator::Divide => Expression::Number(left / right),
+                        Operator::Modulo => Expression::Number(left % right),
+                        Operator::Power => Expression::Number(left.powd(*right)),
+                    },
+                    _ => Expression::Operation(operator, Box::new(left), Box::new(right)),
+                }
+            }
+            Expression::Function(name, params) => self.apply_function(&name, params),
         }
     }
 }
-
-fn eval_operation(operator: Operator, left: Expression, right: Expression) -> Expression {
-    let left = eval_expr(left);
-    let right = eval_expr(right);
-    match (&left, &right) {
-        (Expression::Number(left), Expression::Number(right)) => match operator {
-            Operator::Add => Expression::Number(left + right),
-            Operator::Subtract => Expression::Number(left - right),
-            Operator::Multiply => Expression::Number(left * right),
-            Operator::Divide => Expression::Number(left / right),
-            Operator::Modulo => Expression::Number(left % right),
-            Operator::Power => Expression::Number(left.powd(*right)),
-        },
-        _ => Expression::Operation(operator, Box::new(left), Box::new(right)),
-    }
-}
diff --git a/src/math/functions.rs b/src/math/functions.rs
index 16d53d4..ee0f59c 100644
--- a/src/math/functions.rs
+++ b/src/math/functions.rs
@@ -1,19 +1,35 @@
+use crate::math::context::Context;
 use crate::math::types::Expression;
 use rust_decimal::Decimal;
 use rust_decimal::MathematicalOps;
 use rust_decimal::prelude::ToPrimitive;
 
-pub fn eval_function(func_name: &str, params: Vec<Expression>) -> Expression {
-    match func_name {
-        "sin" => map_decimal(&params[0], |d| d.sin()),
-        "cos" => map_decimal(&params[0], |d| d.cos()),
-        "tan" => map_decimal(&params[0], |d| d.tan()),
-        "asin" => map_decimal(&params[0], |d| map_f64(&d, |f| f.asin())),
-        "acos" => map_decimal(&params[0], |d| map_f64(&d, |f| f.acos())),
-        "atan" => map_decimal(&params[0], |d| map_f64(&d, |f| f.atan())),
-        "abs" => map_decimal(&params[0], |d| d.abs()),
-        "sqrt" => map_decimal(&params[0], |d| d.abs().sqrt().expect("impossible")),
-        _ => panic!("unknown function"),
+impl Context {
+    pub fn register_builtin_functions(&mut self) {
+        self.register_function("abs", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| d.abs())
+        });
+        self.register_function("sqrt", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| d.abs().sqrt().unwrap())
+        });
+        self.register_function("sin", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| d.sin())
+        });
+        self.register_function("cos", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| d.cos())
+        });
+        self.register_function("tan", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| d.tan())
+        });
+        self.register_function("asin", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| map_f64(&d, |f| f.asin()))
+        });
+        self.register_function("acos", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| map_f64(&d, |f| f.acos()))
+        });
+        self.register_function("atan", |params: Vec<Expression>| {
+            map_decimal(&params[0], |d| map_f64(&d, |f| f.atan()))
+        });
     }
 }
 
diff --git a/src/math/mod.rs b/src/math/mod.rs
index 105326f..2ed2081 100644
--- a/src/math/mod.rs
+++ b/src/math/mod.rs
@@ -4,6 +4,7 @@ use crate::math::types::Expression;
 mod tests;
 
 mod constants;
+mod context;
 mod display;
 mod eval;
 mod functions;
@@ -12,12 +13,4 @@ mod parser;
 
 pub mod types;
 
-pub fn parse(source: &str) -> Expression {
-    let lexer = lexer::from(source);
-    let mut parser = parser::from(lexer.collect());
-    parser.parse()
-}
-
-pub fn eval(expr: Expression) -> Expression {
-    eval::eval_expr(expr)
-}
+pub use context::Context;
diff --git a/src/math/tests/eval.rs b/src/math/tests/eval.rs
index 958f721..65a8ddd 100644
--- a/src/math/tests/eval.rs
+++ b/src/math/tests/eval.rs
@@ -1,26 +1,29 @@
 use rust_decimal::{Decimal, dec};
 
 use crate::math::{
-    eval::eval_expr,
+    self,
     types::{Expression, Operator},
 };
 
 #[test]
 fn test_eval_expr_number() {
     let expr = Expression::Number(dec!(1));
-    assert_eq!(eval_expr(expr), Expression::Number(dec!(1)));
+    let ctx = math::Context::default();
+    assert_eq!(ctx.eval_expr(expr), Expression::Number(dec!(1)));
 }
 
 #[test]
 fn test_eval_expr_variable() {
     let expr = Expression::Variable("x".to_owned());
-    assert_eq!(eval_expr(expr), Expression::Variable("x".to_owned()));
+    let ctx = math::Context::default();
+    assert_eq!(ctx.eval_expr(expr), Expression::Variable("x".to_owned()));
 }
 
 #[test]
 fn test_eval_expr_variable_constant() {
     let expr = Expression::Variable("pi".to_owned());
-    assert_eq!(eval_expr(expr), Expression::Number(Decimal::PI));
+    let ctx = math::Context::default();
+    assert_eq!(ctx.eval_expr(expr), Expression::Number(Decimal::PI));
 }
 
 #[test]
@@ -29,13 +32,15 @@ fn test_eval_expr_fraction() {
         Box::new(Expression::Number(dec!(1))),
         Box::new(Expression::Number(dec!(2))),
     );
-    assert_eq!(eval_expr(expr), Expression::Number(dec!(0.5)));
+    let ctx = math::Context::default();
+    assert_eq!(ctx.eval_expr(expr), Expression::Number(dec!(0.5)));
 }
 
 #[test]
 fn test_eval_expr_function() {
     let expr = Expression::Function("sqrt".to_owned(), vec![Expression::Number(dec!(4))]);
-    assert_eq!(eval_expr(expr), Expression::Number(dec!(2)));
+    let ctx = math::Context::default();
+    assert_eq!(ctx.eval_expr(expr), Expression::Number(dec!(2)));
 }
 
 #[test]
@@ -45,5 +50,6 @@ fn test_eval_operation() {
         Box::new(Expression::Number(dec!(2))),
         Box::new(Expression::Number(dec!(3))),
     );
-    assert_eq!(eval_expr(expr), Expression::Number(dec!(8)));
+    let ctx = math::Context::default();
+    assert_eq!(ctx.eval_expr(expr), Expression::Number(dec!(8)));
 }
diff --git a/src/math/types.rs b/src/math/types.rs
index 680a140..b5e01eb 100644
--- a/src/math/types.rs
+++ b/src/math/types.rs
@@ -1,6 +1,6 @@
 use rust_decimal::Decimal;
 
-#[derive(Debug, PartialEq)]
+#[derive(Debug, PartialEq, Clone)]
 pub enum Expression {
     Number(Decimal),
     Fraction(Box<Expression>, Box<Expression>),