diff --git a/cli/src/repl.rs b/cli/src/repl.rs
index cfd4811e5a..87f672704a 100644
--- a/cli/src/repl.rs
+++ b/cli/src/repl.rs
@@ -29,7 +29,7 @@ use roc_types::subs::{Content, Subs, VarStore, Variable};
 use roc_types::types::Type;
 use std::hash::Hash;
 use std::io::{self, Write};
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::str::from_utf8_unchecked;
 use target_lexicon::Triple;
 
@@ -158,33 +158,64 @@ pub fn repl_home() -> ModuleId {
     ModuleIds::default().get_or_insert(&"REPL".into())
 }
 
+fn promote_expr_to_module(src: &str) -> String {
+    let mut buffer = String::from("app Repl provides [ replOutput ] imports []\n\nreplOutput =\n");
+
+    for line in src.lines() {
+        // indent the body!
+        buffer.push_str("    ");
+        buffer.push_str(line);
+        buffer.push('\n');
+    }
+
+    buffer
+}
+
 fn gen(src: &[u8], target: Triple, opt_level: OptLevel) -> Result<ReplOutput, Fail> {
     use roc_reporting::report::{can_problem, type_problem, RocDocAllocator, DEFAULT_PALETTE};
 
-    // Look up the types and expressions of the `provided` values
-    let ptr_bytes = target.pointer_width().unwrap().bytes() as u32;
     let arena = Bump::new();
-    let CanExprOut {
-        loc_expr,
-        var_store,
-        var,
-        constraint,
-        home,
+
+    // SAFETY: we've already verified that this is valid UTF-8 during parsing.
+    let src_str: &str = unsafe { from_utf8_unchecked(src) };
+
+    let stdlib = roc_builtins::std::standard_stdlib();
+    let stdlib_mode = stdlib.mode;
+    let filename = PathBuf::from("REPL.roc");
+    let src_dir = Path::new("fake/test/path");
+
+    let module_src = promote_expr_to_module(src_str);
+
+    let exposed_types = MutMap::default();
+    let loaded = roc_load::file::load_and_monomorphize_from_str(
+        &arena,
+        filename,
+        &module_src,
+        stdlib,
+        src_dir,
+        exposed_types,
+    );
+
+    let loaded = loaded.expect("failed to load module");
+
+    use roc_load::file::MonomorphizedModule;
+    let MonomorphizedModule {
+        can_problems,
+        type_problems,
+        mono_problems,
+        mut procedures,
         interns,
-        problems: can_problems,
+        exposed_to_host,
+        mut subs,
+        module_id: home,
         ..
-    } = can_expr(src)?; // IMPORTANT: we must bail out here if there were UTF-8 errors!
+    } = loaded;
 
-    let subs = Subs::new(var_store.into());
-    let mut type_problems = Vec::new();
-    let (content, mut subs) = infer_expr(subs, &mut type_problems, &constraint, var);
+    let error_count = can_problems.len() + type_problems.len() + mono_problems.len();
 
-    let total_problems = can_problems.len() + type_problems.len();
-
-    if total_problems > 0 {
+    if error_count > 0 {
         // There were problems; report them and return.
-        // SAFETY: we've already verified that this is valid UTF-8 during parsing.
-        let src_lines: Vec<&str> = unsafe { from_utf8_unchecked(src).split('\n').collect() };
+        let src_lines: Vec<&str> = src_str.split('\n').collect();
 
         // Used for reporting where an error came from.
         //
@@ -197,7 +228,7 @@ fn gen(src: &[u8], target: Triple, opt_level: OptLevel) -> Result<ReplOutput, Fa
         // Report parsing and canonicalization problems
         let alloc = RocDocAllocator::new(&src_lines, home, &interns);
 
-        let mut lines = Vec::with_capacity(total_problems);
+        let mut lines = Vec::with_capacity(error_count);
 
         for problem in can_problems.into_iter() {
             let report = can_problem(&alloc, path.clone(), problem);
@@ -223,30 +254,41 @@ fn gen(src: &[u8], target: Triple, opt_level: OptLevel) -> Result<ReplOutput, Fa
         let module = arena.alloc(roc_gen::llvm::build::module_from_builtins(&context, "app"));
         let builder = context.create_builder();
 
-        // pretty-print the expr type string for later.
-        name_all_type_vars(var, &mut subs);
+        debug_assert_eq!(exposed_to_host.len(), 1);
+        let (main_fn_symbol, main_fn_var) = exposed_to_host.iter().nth(0).unwrap();
+        let main_fn_symbol = *main_fn_symbol;
+        let main_fn_var = *main_fn_var;
 
+        // pretty-print the expr type string for later.
+        name_all_type_vars(main_fn_var, &mut subs);
+        let content = subs.get(main_fn_var).content;
         let expr_type_str = content_to_string(content.clone(), &subs, home, &interns);
+
+        let (_, main_fn_layout) = procedures
+            .keys()
+            .find(|(s, _)| *s == main_fn_symbol)
+            .unwrap()
+            .clone();
+
+        let target = target_lexicon::Triple::host();
+        let ptr_bytes = target.pointer_width().unwrap().bytes() as u32;
+
+        let opt_level = if cfg!(debug_assertions) {
+            roc_gen::llvm::build::OptLevel::Normal
+        } else {
+            roc_gen::llvm::build::OptLevel::Optimize
+        };
+
+        let module = arena.alloc(module);
         let (module_pass, function_pass) =
             roc_gen::llvm::build::construct_optimization_passes(module, opt_level);
 
-        // Compute main_fn_type before moving subs to Env
-        let main_ret_layout = Layout::new(&arena, content.clone(), &subs).unwrap_or_else(|err| {
-            panic!(
-                "Code gen error in test: could not convert Content to main_layout. Err was {:?}",
-                err
-            )
-        });
         let execution_engine = module
             .create_jit_execution_engine(OptimizationLevel::None)
             .expect("Error creating JIT execution engine for test");
 
-        // Without calling this, we get a linker error when building this crate
-        // in --release mode and then trying to eval anything in the repl.
-        ExecutionEngine::link_in_mc_jit();
-
         // Compile and add all the Procs before adding main
-        let mut env = roc_gen::llvm::build::Env {
+        let env = roc_gen::llvm::build::Env {
             arena: &arena,
             builder: &builder,
             context: &context,
@@ -254,64 +296,18 @@ fn gen(src: &[u8], target: Triple, opt_level: OptLevel) -> Result<ReplOutput, Fa
             module,
             ptr_bytes,
             leak: false,
+            // important! we don't want any procedures to get the C calling convention
             exposed_to_host: MutSet::default(),
         };
-        let mut procs = Procs::default();
-        let mut ident_ids = env.interns.all_ident_ids.remove(&home).unwrap();
-        let mut layout_ids = LayoutIds::default();
 
-        // Populate Procs and get the low-level Expr from the canonical Expr
-        let mut mono_problems = Vec::new();
-        let mut mono_env = roc_mono::ir::Env {
-            arena: &arena,
-            subs: &mut subs,
-            problems: &mut mono_problems,
-            home,
-            ident_ids: &mut ident_ids,
-        };
-
-        let mut layout_cache = LayoutCache::default();
-        let main_body =
-            roc_mono::ir::Stmt::new(&mut mono_env, loc_expr.value, &mut procs, &mut layout_cache);
-
-        let param_map = roc_mono::borrow::ParamMap::default();
-        let main_body = roc_mono::inc_dec::visit_declaration(
-            mono_env.arena,
-            mono_env.arena.alloc(param_map),
-            mono_env.arena.alloc(main_body),
-        );
-        let mut headers = {
-            let num_headers = match &procs.pending_specializations {
-                Some(map) => map.len(),
-                None => 0,
-            };
-
-            Vec::with_capacity(num_headers)
-        };
-        let procs = roc_mono::ir::specialize_all(&mut mono_env, procs, &mut layout_cache);
-
-        assert_eq!(
-            procs.runtime_errors,
-            roc_collections::all::MutMap::default()
-        );
-
-        let (mut procs, param_map) = procs.get_specialized_procs_help(mono_env.arena);
-        let main_body = roc_mono::inc_dec::visit_declaration(
-            mono_env.arena,
-            param_map,
-            mono_env.arena.alloc(main_body),
-        );
-
-        // Put this module's ident_ids back in the interns, so we can use them in env.
-        // This must happen *after* building the headers, because otherwise there's
-        // a conflicting mutable borrow on ident_ids.
-        env.interns.all_ident_ids.insert(home, ident_ids);
+        let mut layout_ids = roc_gen::layout_id::LayoutIds::default();
+        let mut headers = Vec::with_capacity(procedures.len());
 
         // Add all the Proc headers to the module.
         // We have to do this in a separate pass first,
         // because their bodies may reference each other.
         let mut scope = roc_gen::llvm::build::Scope::default();
-        for ((symbol, layout), proc) in procs.drain() {
+        for ((symbol, layout), proc) in procedures.drain() {
             let fn_val = build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
 
             if proc.args.is_empty() {
@@ -325,34 +321,45 @@ fn gen(src: &[u8], target: Triple, opt_level: OptLevel) -> Result<ReplOutput, Fa
 
         // Build each proc using its header info.
         for (proc, fn_val) in headers {
-            // NOTE: This is here to be uncommented in case verification fails.
-            // (This approach means we don't have to defensively clone name here.)
-            //
-            // println!("\n\nBuilding and then verifying function {}\n\n", name);
+            let mut current_scope = scope.clone();
+
+            // only have top-level thunks for this proc's module in scope
+            // this retain is not needed for correctness, but will cause less confusion when debugging
+            let home = proc.name.module_id();
+            current_scope.retain_top_level_thunks_for_module(home);
+
             build_proc(&env, &mut layout_ids, scope.clone(), proc, fn_val);
 
             if fn_val.verify(true) {
                 function_pass.run_on(&fn_val);
             } else {
+                use roc_builtins::std::Mode;
+
+                let mode = match stdlib_mode {
+                    Mode::Uniqueness => "OPTIMIZED",
+                    Mode::Standard => "NON-OPTIMIZED",
+                };
+
                 eprintln!(
-                    "\n\nFunction {:?} failed LLVM verification in build. Its content was:\n",
-                    fn_val.get_name().to_str().unwrap()
+                    "\n\nFunction {:?} failed LLVM verification in {} build. Its content was:\n",
+                    fn_val.get_name().to_str().unwrap(),
+                    mode,
                 );
 
                 fn_val.print_to_stderr();
 
                 panic!(
-                    "The preceding code was from {:?}, which failed LLVM verification in build.",
-                    fn_val.get_name().to_str().unwrap()
+                    "The preceding code was from {:?}, which failed LLVM verification in {} build.",
+                    fn_val.get_name().to_str().unwrap(),
+                    mode,
                 );
             }
         }
-
-        let (main_fn_name, main_fn) = roc_gen::llvm::build::make_main_function(
+        let (main_fn_name, main_fn) = roc_gen::llvm::build::promote_to_main_function(
             &env,
             &mut layout_ids,
-            &main_ret_layout,
-            &main_body,
+            main_fn_symbol,
+            &main_fn_layout,
         );
 
         // Uncomment this to see the module's un-optimized LLVM instruction output:
@@ -379,7 +386,7 @@ fn gen(src: &[u8], target: Triple, opt_level: OptLevel) -> Result<ReplOutput, Fa
                 &arena,
                 execution_engine,
                 main_fn_name,
-                &main_ret_layout,
+                &main_fn_layout,
                 &content,
                 &env.interns,
                 home,
diff --git a/compiler/build/src/program.rs b/compiler/build/src/program.rs
index 445c6fcf1c..d8c4126daa 100644
--- a/compiler/build/src/program.rs
+++ b/compiler/build/src/program.rs
@@ -67,7 +67,7 @@ pub fn gen_from_mono_module(
         module,
         ptr_bytes,
         leak: false,
-        exposed_to_host: loaded.exposed_to_host,
+        exposed_to_host: loaded.exposed_to_host.keys().copied().collect(),
     };
 
     // Populate Procs further and get the low-level Expr from the canonical Expr
diff --git a/compiler/gen/tests/helpers/eval.rs b/compiler/gen/tests/helpers/eval.rs
index ced5d6ab94..34011ca7f8 100644
--- a/compiler/gen/tests/helpers/eval.rs
+++ b/compiler/gen/tests/helpers/eval.rs
@@ -134,8 +134,8 @@ pub fn helper<'a>(
         // important! we don't want any procedures to get the C calling convention
         exposed_to_host: MutSet::default(),
     };
-    let mut layout_ids = roc_gen::layout_id::LayoutIds::default();
 
+    let mut layout_ids = roc_gen::layout_id::LayoutIds::default();
     let mut headers = Vec::with_capacity(procedures.len());
 
     // Add all the Proc headers to the module.
diff --git a/compiler/load/src/file.rs b/compiler/load/src/file.rs
index 43b889e235..e7bc87225f 100644
--- a/compiler/load/src/file.rs
+++ b/compiler/load/src/file.rs
@@ -439,7 +439,7 @@ pub struct MonomorphizedModule<'a> {
     pub type_problems: Vec<solve::TypeError>,
     pub mono_problems: Vec<roc_mono::ir::MonoProblem>,
     pub procedures: MutMap<(Symbol, Layout<'a>), Proc<'a>>,
-    pub exposed_to_host: MutSet<Symbol>,
+    pub exposed_to_host: MutMap<Symbol, Variable>,
     pub src: Box<str>,
     pub timings: MutMap<ModuleId, ModuleTiming>,
 }
@@ -498,7 +498,7 @@ enum Msg<'a> {
     FinishedAllSpecialization {
         subs: Subs,
         problems: Vec<MonoProblem>,
-        exposed_to_host: MutSet<Symbol>,
+        exposed_to_host: MutMap<Symbol, Variable>,
         src: &'a str,
     },
 }
@@ -525,7 +525,7 @@ struct State<'a> {
     pub module_cache: ModuleCache<'a>,
     pub dependencies: Dependencies,
     pub procedures: MutMap<(Symbol, Layout<'a>), Proc<'a>>,
-    pub exposed_to_host: MutSet<Symbol>,
+    pub exposed_to_host: MutMap<Symbol, Variable>,
 
     /// This is the "final" list of IdentIds, after canonicalization and constraint gen
     /// have completed for a given module.
@@ -667,7 +667,7 @@ enum BuildTask<'a> {
         ident_ids: IdentIds,
         decls: Vec<Declaration>,
         finished_info: FinishedInfo<'a>,
-        exposed_to_host: MutSet<Symbol>,
+        exposed_to_host: MutMap<Symbol, Variable>,
     },
     MakeSpecializations {
         module_id: ModuleId,
@@ -1060,7 +1060,7 @@ where
                 module_cache: ModuleCache::default(),
                 dependencies: Dependencies::default(),
                 procedures: MutMap::default(),
-                exposed_to_host: MutSet::default(),
+                exposed_to_host: MutMap::default(),
                 exposed_types,
                 headers_parsed,
                 loading_started,
@@ -1283,7 +1283,7 @@ fn update<'a>(
             if module_id == state.root_id {
                 state
                     .exposed_to_host
-                    .extend(solved_module.exposed_vars_by_symbol.iter().map(|x| x.0));
+                    .extend(solved_module.exposed_vars_by_symbol.iter().copied());
             }
 
             if module_id == state.root_id && state.goal_phase == Phase::SolveTypes {
@@ -1480,7 +1480,7 @@ fn finish_specialization<'a>(
     mut state: State<'a>,
     subs: Subs,
     problems: Vec<MonoProblem>,
-    exposed_to_host: MutSet<Symbol>,
+    exposed_to_host: MutMap<Symbol, Variable>,
     src: &'a str,
 ) -> MonomorphizedModule<'a> {
     state.mono_problems.extend(problems);
@@ -2148,7 +2148,7 @@ fn build_pending_specializations<'a>(
     _module_timing: ModuleTiming,
     mut layout_cache: LayoutCache<'a>,
     // TODO remove
-    exposed_to_host: MutSet<Symbol>,
+    exposed_to_host: MutMap<Symbol, Variable>,
     finished_info: FinishedInfo<'a>,
 ) -> Msg<'a> {
     let mut procs = Procs::default();
@@ -2212,7 +2212,7 @@ fn add_def_to_module<'a>(
     procs: &mut Procs<'a>,
     mono_env: &mut roc_mono::ir::Env<'a, '_>,
     def: roc_can::def::Def,
-    exposed_to_host: &MutSet<Symbol>,
+    exposed_to_host: &MutMap<Symbol, Variable>,
     is_recursive: bool,
 ) {
     use roc_can::expr::Expr::*;
@@ -2220,7 +2220,7 @@ fn add_def_to_module<'a>(
 
     match def.loc_pattern.value {
         Identifier(symbol) => {
-            let is_exposed = exposed_to_host.contains(&symbol);
+            let is_exposed = exposed_to_host.contains_key(&symbol);
 
             match def.loc_expr.value {
                 Closure {