diff --git a/src/compiler.rs b/src/compiler.rs
new file mode 100644
index 0000000..589931b
--- /dev/null
+++ b/src/compiler.rs
@@ -0,0 +1,83 @@
+use crate::ast::{ExprAst, Prim};
+use crate::dsl::{Composition, Expr, Function, Junction};
+use crate::primitives::{add_pair, cos_f, id, sin_f, triple};
+use crate::support::Arc;
+
+/// Runtime type for a compiled AST node.
+#[derive(Clone)]
+pub enum CompiledExpr {
+    /// f64 -> f64
+    F64(Arc<dyn Expr<f64, Out = f64> + Send + Sync + 'static>),
+    /// f64 -> (f64, f64)
+    Pair(Arc<dyn Expr<f64, Out = (f64, f64)> + Send + Sync + 'static>),
+    /// (f64, f64) -> f64
+    PairToF64(Arc<dyn Expr<(f64, f64), Out = f64> + Send + Sync + 'static>),
+}
+
+impl CompiledExpr {
+}
+
+pub fn compile_ast(ast: &ExprAst) -> Result<CompiledExpr, String> {
+    match ast {
+        ExprAst::Atom(prim) => compile_atom(prim),
+        ExprAst::Composition(first, second) => compile_composition(first, second),
+        ExprAst::Junction(left, right) => compile_junction(left, right),
+    }
+}
+
+pub fn compile_to_scalar(
+    ast: &ExprAst,
+) -> Result<Arc<dyn Expr<f64, Out = f64> + Send + Sync + 'static>, String> {
+    match compile_ast(ast)? {
+        CompiledExpr::F64(expr) => Ok(expr),
+        _ => Err("expected AST that evaluates to f64".to_string()),
+    }
+}
+
+fn compile_atom(prim: &Prim) -> Result<CompiledExpr, String> {
+    let compiled = match prim {
+        Prim::Sin => CompiledExpr::F64(Arc::new(Function { f: sin_f })),
+        Prim::Cos => CompiledExpr::F64(Arc::new(Function { f: cos_f })),
+        Prim::Triple => CompiledExpr::F64(Arc::new(Function { f: triple })),
+        Prim::AddPair => CompiledExpr::PairToF64(Arc::new(Function { f: add_pair })),
+        Prim::Input => CompiledExpr::F64(Arc::new(Function { f: id })),
+    };
+
+    Ok(compiled)
+}
+
+fn compile_composition(
+    first: &ExprAst,
+    second: &ExprAst,
+) -> Result<CompiledExpr, String> {
+    let compiled_first = compile_ast(first)?;
+    let compiled_second = compile_ast(second)?;
+
+    match (compiled_first, compiled_second) {
+        // f64 -> f64 -> f64
+        (CompiledExpr::F64(f1), CompiledExpr::F64(f2)) => Ok(CompiledExpr::F64(Arc::new(
+            Composition { first: f1, second: f2 },
+        ))),
+        // f64 -> f64 -> (f64,f64)
+        (CompiledExpr::F64(f1), CompiledExpr::Pair(f2)) => Ok(CompiledExpr::Pair(Arc::new(
+            Composition { first: f1, second: f2 },
+        ))),
+        // f64 -> (f64,f64) -> f64
+        (CompiledExpr::Pair(f1), CompiledExpr::PairToF64(f2)) => Ok(CompiledExpr::F64(
+            Arc::new(Composition { first: f1, second: f2 }),
+        )),
+        _ => Err("type mismatch in composition".to_string()),
+    }
+}
+
+fn compile_junction(left: &ExprAst, right: &ExprAst) -> Result<CompiledExpr, String> {
+    let compiled_left = compile_ast(left)?;
+    let compiled_right = compile_ast(right)?;
+
+    match (compiled_left, compiled_right) {
+        (CompiledExpr::F64(l), CompiledExpr::F64(r)) => Ok(CompiledExpr::Pair(Arc::new(
+            Junction { left: l, right: r },
+        ))),
+        _ => Err("junction requires two f64-returning branches".to_string()),
+    }
+}
diff --git a/src/primitives.rs b/src/primitives.rs
new file mode 100644
index 0000000..9b7652a
--- /dev/null
+++ b/src/primitives.rs
@@ -0,0 +1,22 @@
+// Reusable primitive functions used by both the DSL and the AST compiler.
+pub fn sin_f(x: f64) -> f64 {
+    x.sin()
+}
+
+pub fn cos_f(x: f64) -> f64 {
+    x.cos()
+}
+
+pub fn triple(x: f64) -> f64 {
+    3.0 * x
+}
+
+/// (a,b) -> a + b
+pub fn add_pair(p: (f64, f64)) -> f64 {
+    p.0 + p.1
+}
+
+/// Identity for the AST `Input` primitive.
+pub fn id(x: f64) -> f64 {
+    x
+}
diff --git a/src/store.rs b/src/store.rs
new file mode 100644
index 0000000..f14e0f5
--- /dev/null
+++ b/src/store.rs
@@ -0,0 +1,69 @@
+use std::collections::HashMap;
+
+use crate::ast::{hash_ast, ExprAst};
+use crate::compiler::{compile_ast, compile_to_scalar, CompiledExpr};
+use crate::support::Arc;
+
+/// In-memory content-addressed store keyed by AST hash.
+#[derive(Default)]
+pub struct ExprStore {
+    compiled: HashMap<u64, Arc<CompiledExpr>>,
+    asts: HashMap<u64, ExprAst>,
+}
+
+impl ExprStore {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Insert an AST, compile it once, and return its hash.
+    /// If the hash already exists, the cached entry is reused.
+    pub fn insert(&mut self, ast: ExprAst) -> Result<u64, String> {
+        let hash = hash_ast(&ast);
+        if !self.compiled.contains_key(&hash) {
+            let compiled = compile_ast(&ast)?;
+            self.compiled.insert(hash, Arc::new(compiled));
+            self.asts.insert(hash, ast);
+        }
+        Ok(hash)
+    }
+
+    /// Fetch a compiled expression by hash.
+    pub fn get(&self, hash: u64) -> Option<Arc<CompiledExpr>> {
+        self.compiled.get(&hash).cloned()
+    }
+
+    /// Fetch the original AST by hash.
+    pub fn get_ast(&self, hash: u64) -> Option<&ExprAst> {
+        self.asts.get(&hash)
+    }
+
+    /// Evaluate a stored scalar expression for the provided input.
+    pub fn eval_scalar(&self, hash: u64, input: f64) -> Result<f64, String> {
+        let compiled = self
+            .get(hash)
+            .ok_or_else(|| format!("hash {:016x} not found", hash))?;
+
+        match compiled.as_ref() {
+            CompiledExpr::F64(expr) => Ok(expr.eval(input)),
+            CompiledExpr::Pair(_) => Err("expression returns a pair, not f64".into()),
+            CompiledExpr::PairToF64(_) => Err("expression expects a pair input, not f64".into()),
+        }
+    }
+
+    /// Insert an AST that must evaluate to f64 -> f64, returning its hash and compiled form.
+    pub fn insert_scalar(
+        &mut self,
+        ast: ExprAst,
+    ) -> Result<
+        (
+            u64,
+            Arc<dyn crate::dsl::Expr<f64, Out = f64> + Send + Sync + 'static>,
+        ),
+        String,
+    > {
+        let hash = self.insert(ast.clone())?;
+        let expr = compile_to_scalar(&ast)?;
+        Ok((hash, expr))
+    }
+}
diff --git a/src/support.rs b/src/support.rs
new file mode 100644
index 0000000..ccec14b
--- /dev/null
+++ b/src/support.rs
@@ -0,0 +1,39 @@
+// Platform helpers to ease future no_std portability.
+// The crate currently depends on `std`, but concentrating platform-specific
+// pieces here makes migration simpler.
+
+#[cfg(feature = "std")]
+pub use std::sync::Arc;
+
+#[cfg(feature = "std")]
+pub fn run_parallel<L, R, OutL, OutR>(left: L, right: R) -> (OutL, OutR)
+where
+    L: FnOnce() -> OutL + Send + 'static,
+    R: FnOnce() -> OutR + Send + 'static,
+    OutL: Send + 'static,
+    OutR: Send + 'static,
+{
+    let h1 = std::thread::spawn(left);
+    let h2 = std::thread::spawn(right);
+    (h1.join().unwrap(), h2.join().unwrap())
+}
+
+#[cfg(not(feature = "std"))]
+mod no_std_support {
+    extern crate alloc;
+    pub use alloc::sync::Arc;
+
+    pub fn run_parallel<L, R, OutL, OutR>(left: L, right: R) -> (OutL, OutR)
+    where
+        L: FnOnce() -> OutL + Send + 'static,
+        R: FnOnce() -> OutR + Send + 'static,
+        OutL: Send + 'static,
+        OutR: Send + 'static,
+    {
+        // No threads available: fall back to sequential execution.
+        (left(), right())
+    }
+}
+
+#[cfg(not(feature = "std"))]
+pub use no_std_support::*;