diff --git a/src/math/display.rs b/src/math/display.rs
index 23ae892..76a24d9 100644
--- a/src/math/display.rs
+++ b/src/math/display.rs
@@ -19,7 +19,7 @@ impl Display for Operator {
 impl Display for Expression {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Expression::Number(num) => write!(f, "{}", num),
+            Expression::Number(num) => write!(f, "{}", num.normalize()),
             Expression::Variable(var) => write!(f, "{}", var),
             Expression::Fraction(numerator, denominator) => {
                 write!(f, "({} / {})", numerator, denominator)
diff --git a/src/math/eval.rs b/src/math/eval.rs
index 6c0145f..f5771ab 100644
--- a/src/math/eval.rs
+++ b/src/math/eval.rs
@@ -1,11 +1,20 @@
 use rust_decimal::MathematicalOps;
 
-use crate::math::types::{Expression, Operator};
+use crate::math::{
+    functions, math_constants,
+    types::{Expression, Operator},
+};
 
 pub fn eval_expr(expr: Expression) -> Expression {
     match expr {
         Expression::Number(_) => expr,
-        Expression::Variable(_) => expr,
+        Expression::Variable(name) => {
+            if let Some(value) = math_constants::get(&name) {
+                Expression::Number(value)
+            } else {
+                Expression::Variable(name)
+            }
+        }
         Expression::Fraction(numberator, denominator) => {
             // TODO: implement proper fraction handling
             eval_expr(Expression::Operation(
@@ -15,7 +24,10 @@ pub fn eval_expr(expr: Expression) -> Expression {
             ))
         }
         Expression::Operation(operator, left, right) => eval_operation(operator, *left, *right),
-        Expression::Function(_name, _params) => todo!(),
+        Expression::Function(name, params) => {
+            let params = params.into_iter().map(eval_expr).collect();
+            functions::eval_function(&name, params)
+        }
     }
 }
 
@@ -31,7 +43,6 @@ fn eval_operation(operator: Operator, left: Expression, right: Expression) -> Ex
             Operator::Modulo => Expression::Number(left % right),
             Operator::Power => Expression::Number(left.powd(*right)),
         },
-        // TODO: implement handling for variables
         _ => Expression::Operation(operator, Box::new(left), Box::new(right)),
     }
 }
diff --git a/src/math/functions.rs b/src/math/functions.rs
new file mode 100644
index 0000000..16d53d4
--- /dev/null
+++ b/src/math/functions.rs
@@ -0,0 +1,31 @@
+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"),
+    }
+}
+
+fn map_decimal(expr: &Expression, f: fn(Decimal) -> Decimal) -> Expression {
+    match expr {
+        Expression::Number(d) => Expression::Number(f(*d)),
+        _ => panic!("expected decimal"),
+    }
+}
+
+fn map_f64(decimal: &Decimal, f: fn(f64) -> f64) -> Decimal {
+    let num = decimal.to_f64().expect("unable to unwrap decimal");
+    let num = f(num);
+    Decimal::from_f64_retain(num).expect("unable to wrap decimal")
+}
diff --git a/src/math/math_constants.rs b/src/math/math_constants.rs
new file mode 100644
index 0000000..bce5a39
--- /dev/null
+++ b/src/math/math_constants.rs
@@ -0,0 +1,9 @@
+use rust_decimal::Decimal;
+
+pub fn get(name: &str) -> Option<Decimal> {
+    match &name.to_lowercase()[..] {
+        "pi" => Some(Decimal::PI),
+        "e" => Some(Decimal::E),
+        _ => None,
+    }
+}
diff --git a/src/math/mod.rs b/src/math/mod.rs
index c272e33..e8e4b3a 100644
--- a/src/math/mod.rs
+++ b/src/math/mod.rs
@@ -2,7 +2,9 @@ use crate::math::types::Expression;
 
 mod display;
 mod eval;
+mod functions;
 mod lexer;
+mod math_constants;
 mod parser;
 
 pub mod types;