diff --git a/tree_shared/src/expr.rs b/tree_shared/src/expr.rs
new file mode 100644
index 000000000..9c3f51362
--- /dev/null
+++ b/tree_shared/src/expr.rs
@@ -0,0 +1,118 @@
+#[derive(Clone, Debug, PartialEq)]
+pub enum Order {
+    Parallel,
+    Sequential,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Id {
+    Unique(i64),
+    Shared,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Expr {
+    Num(i64),
+    Boolean(bool),
+    Add(Box<Expr>, Box<Expr>),
+    Sub(Box<Expr>, Box<Expr>),
+    Mul(Box<Expr>, Box<Expr>),
+    LessThan(Box<Expr>, Box<Expr>),
+    And(Box<Expr>, Box<Expr>),
+    Or(Box<Expr>, Box<Expr>),
+    Not(Box<Expr>),
+    Get(Box<Expr>, usize),
+    /// Concat is a convenient built-in way
+    /// to put two tuples together.
+    /// It's not strictly necessary, but
+    /// doing it by constructing a new big tuple is tedius and slow.
+    Concat(Box<Expr>, Box<Expr>),
+    Print(Box<Expr>),
+    Read(Box<Expr>),
+    Write(Box<Expr>, Box<Expr>),
+    All(Id, Order, Vec<Expr>),
+    /// A pred and a list of branches
+    Switch(Box<Expr>, Vec<Expr>),
+    /// Should only be a child of `Switch`
+    /// Represents a single branch of a switch, giving
+    /// it a unique id
+    Branch(Id, Box<Expr>),
+    Loop(Id, Box<Expr>, Box<Expr>),
+    Let(Id, Box<Expr>, Box<Expr>),
+    Arg(Id),
+    Function(Id, Box<Expr>),
+    /// A list of functions, with the first
+    /// being the main function.
+    Program(Vec<Expr>),
+    Call(Id, Box<Expr>),
+}
+
+impl Expr {
+    /// Runs `func` on every child of this expression.
+    pub fn for_each_child(&mut self, mut func: impl FnMut(&mut Expr)) {
+        match self {
+            Expr::Num(_) | Expr::Boolean(_) | Expr::Arg(_) => {}
+            Expr::Add(a, b)
+            | Expr::Sub(a, b)
+            | Expr::Mul(a, b)
+            | Expr::LessThan(a, b)
+            | Expr::And(a, b)
+            | Expr::Or(a, b)
+            | Expr::Concat(a, b)
+            | Expr::Write(a, b) => {
+                func(a);
+                func(b);
+            }
+            Expr::Not(a) | Expr::Print(a) | Expr::Read(a) => {
+                func(a);
+            }
+            Expr::Get(a, _) | Expr::Function(_, a) | Expr::Call(_, a) => {
+                func(a);
+            }
+            Expr::All(_, _, children) => {
+                for child in children {
+                    func(child);
+                }
+            }
+            Expr::Switch(input, children) => {
+                func(input);
+                for child in children {
+                    func(child);
+                }
+            }
+            Expr::Branch(_id, child) => {
+                func(child);
+            }
+            Expr::Loop(_, pred, output) | Expr::Let(_, pred, output) => {
+                func(pred);
+                func(output);
+            }
+            Expr::Program(functions) => {
+                for function in functions {
+                    func(function);
+                }
+            }
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Value {
+    Num(i64),
+    Boolean(bool),
+    Tuple(Vec<Value>),
+}
+
+#[derive(Clone, PartialEq)]
+pub enum Type {
+    Num,
+    Boolean,
+    Tuple(Vec<Type>),
+}
+
+pub enum TypeError {
+    ExpectedType(Expr, Type, Type),
+    ExpectedTupleType(Expr, Type),
+    ExpectedLoopOutputType(Expr, Type),
+    NoArg(Expr),
+}