diff --git a/purr_core/Cargo.toml b/purr_core/Cargo.toml
new file mode 100644
index 0000000..720a8c3
--- /dev/null
+++ b/purr_core/Cargo.toml
@@ -0,0 +1,4 @@
+[package]
+name = "purr_core"
+version = "0.1.0"
+edition = "2024"
diff --git a/purr_core/src/lib.rs b/purr_core/src/lib.rs
new file mode 100644
index 0000000..6e21d2c
--- /dev/null
+++ b/purr_core/src/lib.rs
@@ -0,0 +1,112 @@
+use std::rc::Rc;
+
+type StructID = usize;
+type EnumID = usize;
+type GenericID = usize;
+type EffectID = usize;
+
+pub type IRString = Rc<str>;
+
+pub struct IRFunctionType {
+    pub params: Vec<IRType>,
+    pub result: Box<IRType>,
+}
+
+pub enum IRType {
+    Integer,
+    Float,
+    String,
+    Bool,
+
+    Array(Box<IRType>),
+    Struct(StructID),
+    Enum(EnumID),
+
+    Func(IRFunctionType),
+
+    Unit,
+    Void,
+
+    Generic(GenericID),
+
+    Effect(EffectID, Box<IRType>),
+}
+
+type StructFieldID = usize;
+type EnumFieldID = usize;
+
+pub struct IRStructField {
+    pub id: StructFieldID,
+    pub ty: IRType,
+}
+
+pub enum IREnumFieldPayload {
+    Unit,
+    Single(IRType),
+    Struct(Vec<IRStructField>),
+}
+
+pub struct IREnumField {
+    pub id: EnumFieldID,
+    pub payload: IREnumFieldPayload,
+}
+
+type FunctionID = usize;
+
+pub enum IRDeclaration {
+    Func {
+        id: FunctionID,
+        ty: IRFunctionType,
+        body: IRBlock,
+    },
+
+    Struct {
+        id: StructID,
+        fields: Vec<IRStructField>,
+    },
+
+    Enum {
+        id: EnumID,
+        fields: Vec<IREnumField>,
+    },
+}
+
+type VariableID = usize;
+
+pub enum IRLiteral {
+    Integer(isize),
+    Float(f32),
+    String(IRString),
+    Bool(bool),
+}
+
+pub enum IRAtom {
+    Literal(IRLiteral),
+    Variable(VariableID),
+}
+
+pub enum IRExpressionKind {
+    Atom(IRAtom),
+    Call(FunctionID, Vec<IRExpression>),
+    If {
+        condition: Box<IRAtom>,
+        then_branch: IRBlock,
+        else_branch: IRBlock,
+    },
+}
+
+pub struct IRExpression {
+    pub expr: Box<IRExpressionKind>,
+    pub ty: IRType,
+}
+
+pub enum IRStatement {
+    Define(VariableID, IRExpression),
+    Expression(IRExpression),
+    Return(IRExpression),
+}
+
+pub struct IRBlock {
+    pub statements: Vec<IRStatement>,
+    pub result: Option<IRExpression>,
+}