diff --git a/src/ast.rs b/src/ast.rs
index 20dd2a7..70e773e 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1,5 +1,5 @@
 use std::collections::hash_map::Entry;
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::num::NonZeroUsize;
 use std::str::FromStr;
 use std::sync::Arc;
@@ -721,21 +721,60 @@ impl Program {
     pub fn analyze(from: &parse::Program) -> Result<Self, RichError> {
         let unit = ResolvedType::unit();
         let mut scope = Scope::default();
-        let items = from
+
+        // Pass 1: Process type aliases in file order.
+        for item in from.items() {
+            if let parse::Item::TypeAlias(alias) = item {
+                scope
+                    .insert_alias(alias.name().clone(), alias.ty().clone())
+                    .with_span(alias)?;
+            }
+        }
+
+        // Collect all non-main custom function definitions.
+        let func_items: Vec<&parse::Function> = from
             .items()
             .iter()
-            .map(|s| Item::analyze(s, &unit, &mut scope))
-            .collect::<Result<Vec<Item>, RichError>>()?;
+            .filter_map(|item| match item {
+                parse::Item::Function(f) if f.name().as_inner() != "main" => Some(f),
+                _ => None,
+            })
+            .collect();
+
+        // Reject duplicate non-main function names before building the call graph.
+        check_no_duplicate_functions(&func_items)?;
+
+        // Build a call graph, reject calls to main, and reject recursive cycles.
+        let call_graph = build_call_graph(&func_items);
+        check_no_calls_to_main(&func_items, &call_graph)?;
+        check_for_cycles(&func_items, &call_graph)?;
+
+        // Pass 2: Analyze custom functions in dependency order so that a
+        // callee is always in scope before its callers are analyzed.
+        for func in topological_sort(&func_items, &call_graph) {
+            Function::analyze(func, &unit, &mut scope)?;
+        }
+
+        // Pass 3: Find and analyze the main function.
+        let mut main_expr: Option<Expression> = None;
+        let mut seen_main = false;
+        for item in from.items() {
+            if let parse::Item::Function(f) = item {
+                if f.name().as_inner() == "main" {
+                    if seen_main {
+                        return Err(Error::FunctionRedefined(FunctionName::main())).with_span(f);
+                    }
+                    seen_main = true;
+                    if let Function::Main(expr) = Function::analyze(f, &unit, &mut scope)? {
+                        main_expr = Some(expr);
+                    }
+                }
+            }
+        }
+
+        let main = main_expr.ok_or(Error::MainRequired).with_span(from)?;
         debug_assert!(scope.is_topmost());
         let (parameters, witness_types, call_tracker) = scope.destruct();
-        let mut iter = items.into_iter().filter_map(|item| match item {
-            Item::Function(Function::Main(expr)) => Some(expr),
-            _ => None,
-        });
-        let main = iter.next().ok_or(Error::MainRequired).with_span(from)?;
-        if iter.next().is_some() {
-            return Err(Error::FunctionRedefined(FunctionName::main())).with_span(from);
-        }
         Ok(Self {
             main,
             parameters,
@@ -745,6 +784,152 @@ impl Program {
     }
 }
 
+/// Scan a parse expression for every custom function it calls directly.
+/// Covers plain calls, `array_fold`, `fold`, and `for_while` references.
+fn collect_custom_calls(expr: &parse::Expression) -> HashSet<FunctionName> {
+    let mut calls = HashSet::new();
+    for node in parse::ExprTree::Expression(expr).pre_order_iter() {
+        if let parse::ExprTree::Call(call) = node {
+            match call.name() {
+                parse::CallName::Custom(name)
+                | parse::CallName::ArrayFold(name, _)
+                | parse::CallName::Fold(name, _)
+                | parse::CallName::ForWhile(name) => {
+                    calls.insert(name.clone());
+                }
+                _ => {}
+            }
+        }
+    }
+    calls
+}
+
+/// Return an error if any two non-main functions share the same name.
+fn check_no_duplicate_functions(func_items: &[&parse::Function]) -> Result<(), RichError> {
+    let mut seen: HashSet<&FunctionName> = HashSet::new();
+    for func in func_items {
+        if !seen.insert(func.name()) {
+            return Err(Error::FunctionRedefined(func.name().clone())).with_span(*func);
+        }
+    }
+    Ok(())
+}
+
+/// Build a map from each function name to the set of custom functions it calls.
+fn build_call_graph(
+    func_items: &[&parse::Function],
+) -> HashMap<FunctionName, HashSet<FunctionName>> {
+    func_items
+        .iter()
+        .map(|func| (func.name().clone(), collect_custom_calls(func.body())))
+        .collect()
+}
+
+/// Return an error if any function directly calls `main`.
+fn check_no_calls_to_main(
+    func_items: &[&parse::Function],
+    graph: &HashMap<FunctionName, HashSet<FunctionName>>,
+) -> Result<(), RichError> {
+    let main = FunctionName::main();
+    for func in func_items {
+        if let Some(deps) = graph.get(func.name()) {
+            if deps.contains(&main) {
+                return Err(Error::MainNotCallable).with_span(*func);
+            }
+        }
+    }
+    Ok(())
+}
+
+/// DFS helper that returns the name of a node involved in a cycle, if any.
+fn dfs_find_cycle(
+    name: &FunctionName,
+    graph: &HashMap<FunctionName, HashSet<FunctionName>>,
+    visiting: &mut HashSet<FunctionName>,
+    visited: &mut HashSet<FunctionName>,
+) -> Option<FunctionName> {
+    if visiting.contains(name) {
+        return Some(name.clone());
+    }
+    if visited.contains(name) {
+        return None;
+    }
+    visiting.insert(name.clone());
+    if let Some(deps) = graph.get(name) {
+        for dep in deps {
+            if let Some(cycle) = dfs_find_cycle(dep, graph, visiting, visited) {
+                return Some(cycle);
+            }
+        }
+    }
+    visiting.remove(name);
+    visited.insert(name.clone());
+    None
+}
+
+/// Return an error if any function in the call graph participates in a cycle
+/// (direct self-recursion or mutual recursion).
+fn check_for_cycles(
+    func_items: &[&parse::Function],
+    graph: &HashMap<FunctionName, HashSet<FunctionName>>,
+) -> Result<(), RichError> {
+    let mut visiting: HashSet<FunctionName> = HashSet::new();
+    let mut visited: HashSet<FunctionName> = HashSet::new();
+    for func in func_items {
+        if !visited.contains(func.name()) {
+            if let Some(cycle_name) =
+                dfs_find_cycle(func.name(), graph, &mut visiting, &mut visited)
+            {
+                let offending = func_items
+                    .iter()
+                    .find(|f| f.name() == &cycle_name)
+                    .copied()
+                    .unwrap_or(func);
+                return Err(Error::FunctionRecursive(cycle_name)).with_span(offending);
+            }
+        }
+    }
+    Ok(())
+}
+
+/// DFS post-order visitor for topological sort.
+fn topo_dfs<'a>(
+    name: &FunctionName,
+    graph: &HashMap<FunctionName, HashSet<FunctionName>>,
+    func_map: &HashMap<&FunctionName, &'a parse::Function>,
+    visited: &mut HashSet<FunctionName>,
+    result: &mut Vec<&'a parse::Function>,
+) {
+    if visited.contains(name) {
+        return;
+    }
+    visited.insert(name.clone());
+    if let Some(deps) = graph.get(name) {
+        for dep in deps {
+            topo_dfs(dep, graph, func_map, visited, result);
+        }
+    }
+    if let Some(&func) = func_map.get(name) {
+        result.push(func);
+    }
+}
+
+/// Return the non-main functions in an order where every callee appears before
+/// its callers. Assumes no cycles (call `check_for_cycles` first).
+fn topological_sort<'a>(
+    func_items: &[&'a parse::Function],
+    graph: &HashMap<FunctionName, HashSet<FunctionName>>,
+) -> Vec<&'a parse::Function> {
+    let func_map: HashMap<&FunctionName, &'a parse::Function> =
+        func_items.iter().map(|f| (f.name(), *f)).collect();
+    let mut visited: HashSet<FunctionName> = HashSet::new();
+    let mut result: Vec<&'a parse::Function> = Vec::new();
+    for func in func_items {
+        topo_dfs(func.name(), graph, &func_map, &mut visited, &mut result);
+    }
+    result
+}
+
 impl AbstractSyntaxTree for Item {
     type From = parse::Item;
 
@@ -1579,3 +1764,193 @@ impl AsRef<Span> for ModuleAssignment {
         &self.span
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::TemplateProgram;
+
+    fn compile_ok(src: &str) {
+        TemplateProgram::new(src).unwrap_or_else(|e| panic!("Expected success:\n{e}"));
+    }
+
+    fn compile_err(src: &str, expected_msg: &str) {
+        match TemplateProgram::new(src) {
+            Ok(_) => {
+                panic!("Expected error containing '{expected_msg}', but compilation succeeded")
+            }
+            Err(e) => assert!(
+                e.contains(expected_msg),
+                "Error message did not contain '{expected_msg}':\n{e}"
+            ),
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // Forward references
+    // -----------------------------------------------------------------------
+
+    #[test]
+    fn forward_reference_simple() {
+        // `always_true` is called in `main` but defined after it.
+        compile_ok(
+            r#"fn main() {
+    let x: bool = always_true();
+    assert!(x);
+}
+
+fn always_true() -> bool {
+    true
+}"#,
+        );
+    }
+
+    #[test]
+    fn forward_reference_chain() {
+        // `main` calls `outer`, which calls `inner`, both defined after main.
+        compile_ok(
+            r#"fn main() {
+    let x: bool = outer();
+    assert!(x);
+}
+
+fn outer() -> bool {
+    inner()
+}
+
+fn inner() -> bool {
+    true
+}"#,
+        );
+    }
+
+    #[test]
+    fn forward_reference_array_fold() {
+        // `summer` is referenced in `array_fold` before it is defined.
+        compile_ok(
+            r#"fn main() {
+    let arr: [u32; 3] = [1, 2, 3];
+    let _total: u32 = array_fold::<summer, 3>(arr, 0);
+}
+
+fn summer(elt: u32, acc: u32) -> u32 {
+    let (_, result): (bool, u32) = jet::add_32(elt, acc);
+    result
+}"#,
+        );
+    }
+
+    // -----------------------------------------------------------------------
+    // Recursion detection
+    // -----------------------------------------------------------------------
+
+    #[test]
+    fn direct_self_recursion_rejected() {
+        compile_err(
+            r#"fn foo(n: u32) -> u32 {
+    foo(n)
+}
+
+fn main() {
+    let _x: u32 = foo(1);
+}"#,
+            "recursive call cycle",
+        );
+    }
+
+    #[test]
+    fn mutual_recursion_two_functions_rejected() {
+        // foo → bar → foo
+        compile_err(
+            r#"fn foo(n: u32) -> u32 {
+    bar(n)
+}
+
+fn bar(n: u32) -> u32 {
+    foo(n)
+}
+
+fn main() {
+    let _x: u32 = foo(1);
+}"#,
+            "recursive call cycle",
+        );
+    }
+
+    #[test]
+    fn mutual_recursion_three_functions_rejected() {
+        // a → b → c → a
+        compile_err(
+            r#"fn a(n: u32) -> u32 {
+    b(n)
+}
+
+fn b(n: u32) -> u32 {
+    c(n)
+}
+
+fn c(n: u32) -> u32 {
+    a(n)
+}
+
+fn main() {
+    let _x: u32 = a(1);
+}"#,
+            "recursive call cycle",
+        );
+    }
+
+    #[test]
+    fn call_to_main_rejected() {
+        compile_err(
+            r#"fn foo() -> bool {
+    main()
+}
+
+fn main() {
+    assert!(foo());
+}"#,
+            "Function `main` cannot be called",
+        );
+    }
+
+    #[test]
+    fn duplicate_helper_function_rejected() {
+        compile_err(
+            r#"fn foo() -> bool {
+    true
+}
+
+fn foo() -> bool {
+    false
+}
+
+fn main() {
+    assert!(foo());
+}"#,
+            "Function `foo` was defined multiple times",
+        );
+    }
+
+    #[test]
+    fn non_recursive_chain_ok() {
+        // top → mid → bottom, defined in reverse order, no cycle.
+        compile_ok(
+            r#"fn main() {
+    let x: bool = top();
+    assert!(x);
+}
+
+fn top() -> bool {
+    mid()
+}
+
+fn mid() -> bool {
+    bottom()
+}
+
+fn bottom() -> bool {
+    true
+}"#,
+        );
+    }
+}
diff --git a/src/error.rs b/src/error.rs
index b4d5a4b..86ec13e 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -426,8 +426,10 @@ pub enum Error {
     MainNoInputs,
     MainNoOutput,
     MainRequired,
+    MainNotCallable,
     FunctionRedefined(FunctionName),
     FunctionUndefined(FunctionName),
+    FunctionRecursive(FunctionName),
     InvalidNumberOfArguments(usize, usize),
     FunctionNotFoldable(FunctionName),
     FunctionNotLoopable(FunctionName),
@@ -516,6 +518,10 @@ impl fmt::Display for Error {
                 f,
                 "Main function is required"
             ),
+            Error::MainNotCallable => write!(
+                f,
+                "Function `main` cannot be called"
+            ),
             Error::FunctionRedefined(name) => write!(
                 f,
                 "Function `{name}` was defined multiple times"
@@ -524,6 +530,10 @@ impl fmt::Display for Error {
                 f,
                 "Function `{name}` was called but not defined"
             ),
+            Error::FunctionRecursive(name) => write!(
+                f,
+                "Function `{name}` is part of a recursive call cycle, which is not allowed"
+            ),
             Error::InvalidNumberOfArguments(expected, found) => write!(
                 f,
                 "Expected {expected} arguments, found {found} arguments"