diff --git a/compiler/build/src/program.rs b/compiler/build/src/program.rs
index 2f22ebc32d..6797088b4a 100644
--- a/compiler/build/src/program.rs
+++ b/compiler/build/src/program.rs
@@ -5,7 +5,6 @@ use inkwell::passes::PassManager;
 use inkwell::types::BasicType;
 use inkwell::OptimizationLevel;
 use roc_collections::all::ImMap;
-use roc_collections::all::MutMap;
 use roc_gen::layout_id::LayoutIds;
 use roc_gen::llvm::build::{
     build_proc, build_proc_header, get_call_conventions, module_from_builtins, OptLevel,
@@ -14,7 +13,7 @@ use roc_gen::llvm::convert::basic_type_from_layout;
 use roc_load::file::LoadedModule;
 use roc_module::symbol::Symbol;
 use roc_mono::expr::{Env, Expr, PartialProc, Procs};
-use roc_mono::layout::Layout;
+use roc_mono::layout::{Layout, LayoutCache};
 
 use inkwell::targets::{
     CodeModel, FileType, InitializationConfig, RelocMode, Target, TargetTriple,
@@ -23,6 +22,7 @@ use std::path::{Path, PathBuf};
 use target_lexicon::{Architecture, OperatingSystem, Triple, Vendor};
 
 // TODO this should probably use more helper functions
+// TODO make this polymorphic in the llvm functions so it can be reused for another backend.
 #[allow(clippy::cognitive_complexity)]
 pub fn build(
     arena: &Bump,
@@ -225,24 +225,27 @@ pub fn build(
     // Populate Procs further and get the low-level Expr from the canonical Expr
     let main_body = Expr::new(&mut mono_env, loc_expr.value, &mut procs);
 
-    // Put this module's ident_ids back in the interns, so we can use them in env.
-    env.interns.all_ident_ids.insert(home, ident_ids);
+    let mut headers = Vec::with_capacity(procs.pending_specializations.len());
+    let mut layout_cache = LayoutCache::default();
 
-    let mut headers = Vec::with_capacity(procs.len());
-    let (mut proc_map, runtime_errors) = procs.into_map();
+    let (mut specializations, runtime_errors) =
+        roc_mono::expr::specialize_all(&mut mono_env, procs, &mut layout_cache);
 
     assert_eq!(runtime_errors, roc_collections::all::MutSet::default());
 
+    // 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);
+
     // 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.
-    for (symbol, mut procs_by_layout) in proc_map.drain() {
-        for (layout, proc) in procs_by_layout.drain() {
-            let (fn_val, arg_basic_types) =
-                build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
+    for (symbol, layout, proc) in specializations.drain(..) {
+        let (fn_val, arg_basic_types) =
+            build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
 
-            headers.push((proc, fn_val, arg_basic_types));
-        }
+        headers.push((proc, fn_val, arg_basic_types));
     }
 
     // Build each proc using its header info.
diff --git a/compiler/gen/tests/helpers/eval.rs b/compiler/gen/tests/helpers/eval.rs
index 6a42c8d117..a882c2a8f1 100644
--- a/compiler/gen/tests/helpers/eval.rs
+++ b/compiler/gen/tests/helpers/eval.rs
@@ -75,23 +75,27 @@ macro_rules! assert_llvm_evals_to {
         };
         let main_body = Expr::new(&mut mono_env, loc_expr.value, &mut procs);
 
-        // Put this module's ident_ids back in the interns, so we can use them in Env.
-        env.interns.all_ident_ids.insert(home, ident_ids);
+        let mut headers = Vec::with_capacity(procs.pending_specializations.len());
+        let mut layout_cache = roc_mono::layout::LayoutCache::default();
 
-        let mut headers = Vec::with_capacity(procs.len());
-        let (mut proc_map, runtime_errors) = procs.into_map();
+        let (mut specializations, runtime_errors) =
+            roc_mono::expr::specialize_all(&mut mono_env, procs, &mut layout_cache);
 
         assert_eq!(runtime_errors, roc_collections::all::MutSet::default());
 
+        // 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);
+
         // 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.
-        for (symbol, mut procs_by_layout) in proc_map.drain() {
-            for (layout, proc) in procs_by_layout.drain() {
-                let (fn_val, arg_basic_types) = build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
+        for (symbol, layout, proc) in specializations.drain(..) {
+            let (fn_val, arg_basic_types) =
+                build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
 
-                headers.push((proc, fn_val, arg_basic_types));
-            }
+            headers.push((proc, fn_val, arg_basic_types));
         }
 
         // Build each proc using its header info.
@@ -106,7 +110,9 @@ macro_rules! assert_llvm_evals_to {
                 fpm.run_on(&fn_val);
             } else {
                 // NOTE: If this fails, uncomment the above println to debug.
-                panic!("Non-main function failed LLVM verification. Uncomment the above println to debug!");
+                panic!(
+                    "Non-main function failed LLVM verification. Uncomment the above println to debug!"
+                );
             }
         }
 
@@ -240,23 +246,27 @@ macro_rules! assert_opt_evals_to {
         };
         let main_body = Expr::new(&mut mono_env, loc_expr.value, &mut procs);
 
-        // Put this module's ident_ids back in the interns, so we can use them in Env.
-        env.interns.all_ident_ids.insert(home, ident_ids);
+        let mut headers = Vec::with_capacity(procs.pending_specializations.len());
+        let mut layout_cache = roc_mono::layout::LayoutCache::default();
 
-        let mut headers = Vec::with_capacity(procs.len());
-        let (mut proc_map, runtime_errors) = procs.into_map();
+        let (mut specializations, runtime_errors) =
+            roc_mono::expr::specialize_all(&mut mono_env, procs, &mut layout_cache);
 
         assert_eq!(runtime_errors, roc_collections::all::MutSet::default());
 
+        // 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);
+
         // 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.
-        for (symbol, mut procs_by_layout) in proc_map.drain() {
-            for (layout, proc) in procs_by_layout.drain() {
-                let (fn_val, arg_basic_types) = build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
+        for (symbol, layout, proc) in specializations.drain(..) {
+            let (fn_val, arg_basic_types) =
+                build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
 
-                headers.push((proc, fn_val, arg_basic_types));
-            }
+            headers.push((proc, fn_val, arg_basic_types));
         }
 
         // Build each proc using its header info.
@@ -271,7 +281,9 @@ macro_rules! assert_opt_evals_to {
                 fpm.run_on(&fn_val);
             } else {
                 // NOTE: If this fails, uncomment the above println to debug.
-                panic!("Non-main function failed LLVM verification. Uncomment the above println to debug!");
+                panic!(
+                    "Non-main function failed LLVM verification. Uncomment the above println to debug!"
+                );
             }
         }
 
@@ -398,20 +410,27 @@ macro_rules! emit_expr {
         };
         let main_body = Expr::new(&mut mono_env, loc_expr.value, &mut procs);
 
-        // Put this module's ident_ids back in the interns, so we can use them in Env.
-        env.interns.all_ident_ids.insert(home, ident_ids);
+        let mut headers = Vec::with_capacity(procs.pending_specializations.len());
+        let mut layout_cache = roc_mono::layout::LayoutCache::default();
 
-        let mut headers = Vec::with_capacity(procs.len());
+        let (mut specializations, runtime_errors) =
+            roc_mono::expr::specialize_all(&mut mono_env, procs, &mut layout_cache);
+
+        assert_eq!(runtime_errors, roc_collections::all::MutSet::default());
+
+        // 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);
 
         // 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.
-        for (symbol, opt_proc) in procs.as_map().into_iter() {
-            if let Some(proc) = opt_proc {
-                let (fn_val, arg_basic_types) = build_proc_header(&env, symbol, &proc);
+        for (symbol, layout, proc) in specializations.drain(..) {
+            let (fn_val, arg_basic_types) =
+                build_proc_header(&env, &mut layout_ids, symbol, &layout, &proc);
 
-                headers.push((proc, fn_val, arg_basic_types));
-            }
+            headers.push((proc, fn_val, arg_basic_types));
         }
 
         // Build each proc using its header info.
@@ -420,13 +439,15 @@ macro_rules! emit_expr {
             // (This approach means we don't have to defensively clone name here.)
             //
             // println!("\n\nBuilding and then verifying function {}\n\n", name);
-            build_proc(&env, proc, &procs, fn_val, arg_basic_types);
+            build_proc(&env, &mut layout_ids, proc, fn_val, arg_basic_types);
 
             if fn_val.verify(true) {
                 fpm.run_on(&fn_val);
             } else {
                 // NOTE: If this fails, uncomment the above println to debug.
-                panic!("Non-main function failed LLVM verification. Uncomment the above println to debug!");
+                panic!(
+                    "Non-main function failed LLVM verification. Uncomment the above println to debug!"
+                );
             }
         }
 
diff --git a/compiler/module/src/symbol.rs b/compiler/module/src/symbol.rs
index 735a43de61..bca6e35f57 100644
--- a/compiler/module/src/symbol.rs
+++ b/compiler/module/src/symbol.rs
@@ -58,7 +58,7 @@ impl Symbol {
             .get_name(self.module_id())
             .unwrap_or_else(|| {
                 panic!(
-                    "module_string could not find IdentIds for {:?} in interns {:?}",
+                    "module_string could not find IdentIds for module {:?} in {:?}",
                     self.module_id(),
                     interns
                 )
@@ -71,7 +71,7 @@ impl Symbol {
             .get(&self.module_id())
             .unwrap_or_else(|| {
                 panic!(
-                    "ident_string could not find IdentIds for {:?} in interns {:?}",
+                    "ident_string could not find IdentIds for module {:?} in {:?}",
                     self.module_id(),
                     interns
                 )
diff --git a/compiler/mono/src/expr.rs b/compiler/mono/src/expr.rs
index 21a50dc9e6..fdb9129704 100644
--- a/compiler/mono/src/expr.rs
+++ b/compiler/mono/src/expr.rs
@@ -1432,9 +1432,17 @@ pub fn specialize_all<'a>(
     env: &mut Env<'a, '_>,
     mut procs: Procs<'a>,
     layout_cache: &mut LayoutCache<'a>,
-) {
+) -> (Vec<'a, (Symbol, Layout<'a>, Proc<'a>)>, MutSet<Symbol>) {
+    let mut answer = Vec::with_capacity_in(procs.pending_specializations.len(), env.arena);
+    let mut runtime_errors = MutSet::default();
     let mut is_finished = procs.pending_specializations.is_empty();
 
+    // TODO replace this synchronous loop with a work-stealing queue which
+    // processes each entry in pending_specializations in parallel, one
+    // module at a time (because the &mut env will need exclusive access to
+    // that module's IdentIds; the only reason Env is &mut in specialize is
+    // that we need to generate unique symbols and register them in them module's
+    // IdentIds).
     while !is_finished {
         let Procs {
             partial_procs,
@@ -1457,10 +1465,10 @@ pub fn specialize_all<'a>(
 
                 match specialize(env, &mut procs, name, layout_cache, pending, partial_proc) {
                     Ok(proc) => {
-                        // TODO stuff
+                        answer.push((name, layout, proc));
                     }
                     Err(()) => {
-                        // TODO runtime error
+                        runtime_errors.insert(name);
                     }
                 }
             }
@@ -1468,6 +1476,8 @@ pub fn specialize_all<'a>(
 
         is_finished = procs.pending_specializations.is_empty();
     }
+
+    (answer, runtime_errors)
 }
 
 fn specialize<'a>(