diff --git a/compiler/gen/src/llvm/build.rs b/compiler/gen/src/llvm/build.rs
index 2cd97e0a58..94fc5202b3 100644
--- a/compiler/gen/src/llvm/build.rs
+++ b/compiler/gen/src/llvm/build.rs
@@ -2173,15 +2173,24 @@ pub fn build_proc_header<'a, 'ctx, 'env>(
             for (name, layout) in aliases {
                 match layout {
                     Layout::Closure(arguments, closure, result) => {
+                        // define closure size and return value size, e.g.
+                        //
+                        // * roc__mainForHost_1_Update_size() -> i64
+                        // * roc__mainForHost_1_Update_result_size() -> i64
                         build_closure_caller(env, &fn_name, *name, arguments, closure, result)
                     }
-                    Layout::FunctionPointer(_arguments, _result) => {
-                        // TODO should this be considered a closure of size 0?
-                        // or do we let the host call it directly?
-                        // then we have no RocCallResult wrapping though
+                    Layout::FunctionPointer(arguments, result) => {
+                        // define function size (equal to pointer size) and return value size, e.g.
+                        //
+                        // * roc__mainForHost_1_Update_size() -> i64
+                        // * roc__mainForHost_1_Update_result_size() -> i64
+                        build_function_caller(env, &fn_name, *name, arguments, result)
                     }
                     _ => {
-                        // TODO
+                        // for anything else we only define the size symbol, e.g.
+                        //
+                        // * roc__mainForHost_1_Model_size() -> i64
+                        build_host_exposed_alias_size(env, &fn_name, *name, layout)
                     }
                 }
             }
@@ -2301,23 +2310,187 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
 
     let closure_data = builder.build_load(closure_data_ptr, "load_closure_data");
 
-    let mut arguments = parameters;
-    arguments.push(closure_data);
+    let mut parameters = parameters;
+    parameters.push(closure_data);
 
-    let result = invoke_and_catch(env, function_value, function_ptr, &arguments, result_type);
+    let call_result = invoke_and_catch(env, function_value, function_ptr, &parameters, result_type);
 
-    builder.build_store(output, result);
+    builder.build_store(output, call_result);
 
     builder.build_return(None);
 
     // STEP 3: build a {} -> u64 function that gives the size of the return type
-    let size_function_type = env.context.i64_type().fn_type(&[], false);
-    let size_function_name: String = format!(
-        "roc_{}_{}_size",
+    build_host_exposed_alias_size_help(
+        env,
+        def_name,
+        alias_symbol,
+        Some("result"),
+        roc_call_result_type.into(),
+    );
+
+    // STEP 4: build a {} -> u64 function that gives the size of the closure
+    let layout = Layout::Closure(arguments, closure.clone(), result);
+    build_host_exposed_alias_size(env, def_name, alias_symbol, &layout);
+}
+
+pub fn build_function_caller<'a, 'ctx, 'env>(
+    env: &'a Env<'a, 'ctx, 'env>,
+    def_name: &str,
+    alias_symbol: Symbol,
+    arguments: &[Layout<'a>],
+    result: &Layout<'a>,
+) {
+    use inkwell::types::BasicType;
+
+    let arena = env.arena;
+    let context = &env.context;
+    let builder = env.builder;
+
+    // STEP 1: build function header
+
+    // e.g. `roc__main_1_Fx_caller`
+    let function_name = format!(
+        "roc_{}_{}_caller",
         def_name,
         alias_symbol.ident_string(&env.interns)
     );
 
+    let mut argument_types = Vec::with_capacity_in(arguments.len() + 3, env.arena);
+
+    for layout in arguments {
+        argument_types.push(basic_type_from_layout(
+            arena,
+            context,
+            layout,
+            env.ptr_bytes,
+        ));
+    }
+
+    let function_pointer_type = {
+        let mut args = Vec::new_in(env.arena);
+        args.extend(arguments.iter().cloned());
+
+        // pretend the closure layout is empty
+        args.push(Layout::Struct(&[]));
+
+        let function_layout = Layout::FunctionPointer(&args, result);
+
+        // this is already a (function) pointer type
+        basic_type_from_layout(arena, context, &function_layout, env.ptr_bytes)
+    };
+    argument_types.push(function_pointer_type);
+
+    let closure_argument_type = {
+        let basic_type =
+            basic_type_from_layout(arena, context, &Layout::Struct(&[]), env.ptr_bytes);
+
+        basic_type.ptr_type(AddressSpace::Generic)
+    };
+    argument_types.push(closure_argument_type.into());
+
+    let result_type = basic_type_from_layout(arena, context, result, env.ptr_bytes);
+
+    let roc_call_result_type =
+        context.struct_type(&[context.i64_type().into(), result_type], false);
+
+    let output_type = { roc_call_result_type.ptr_type(AddressSpace::Generic) };
+    argument_types.push(output_type.into());
+
+    let function_type = context.void_type().fn_type(&argument_types, false);
+
+    let function_value = env.module.add_function(
+        function_name.as_str(),
+        function_type,
+        Some(Linkage::External),
+    );
+
+    function_value.set_call_conventions(C_CALL_CONV);
+
+    // STEP 2: build function body
+
+    let entry = context.append_basic_block(function_value, "entry");
+
+    builder.position_at_end(entry);
+
+    let mut parameters = function_value.get_params();
+    let output = parameters.pop().unwrap().into_pointer_value();
+    let _closure_data_ptr = parameters.pop().unwrap().into_pointer_value();
+    let function_ptr = parameters.pop().unwrap().into_pointer_value();
+
+    // let closure_data = context.struct_type(&[], false).const_zero().into();
+
+    let actual_function_type = basic_type_from_layout(
+        arena,
+        context,
+        &Layout::FunctionPointer(arguments, result),
+        env.ptr_bytes,
+    );
+
+    let function_ptr = builder
+        .build_bitcast(function_ptr, actual_function_type, "cast")
+        .into_pointer_value();
+
+    let call_result = invoke_and_catch(env, function_value, function_ptr, &parameters, result_type);
+
+    builder.build_store(output, call_result);
+
+    builder.build_return(None);
+
+    // STEP 3: build a {} -> u64 function that gives the size of the return type
+    build_host_exposed_alias_size_help(
+        env,
+        def_name,
+        alias_symbol,
+        Some("result"),
+        roc_call_result_type.into(),
+    );
+
+    // STEP 4: build a {} -> u64 function that gives the size of the function
+    let layout = Layout::FunctionPointer(arguments, result);
+    build_host_exposed_alias_size(env, def_name, alias_symbol, &layout);
+}
+
+fn build_host_exposed_alias_size<'a, 'ctx, 'env>(
+    env: &'a Env<'a, 'ctx, 'env>,
+    def_name: &str,
+    alias_symbol: Symbol,
+    layout: &Layout<'a>,
+) {
+    build_host_exposed_alias_size_help(
+        env,
+        def_name,
+        alias_symbol,
+        None,
+        basic_type_from_layout(env.arena, env.context, layout, env.ptr_bytes),
+    )
+}
+
+fn build_host_exposed_alias_size_help<'a, 'ctx, 'env>(
+    env: &'a Env<'a, 'ctx, 'env>,
+    def_name: &str,
+    alias_symbol: Symbol,
+    opt_label: Option<&str>,
+    basic_type: BasicTypeEnum<'ctx>,
+) {
+    let builder = env.builder;
+    let context = env.context;
+
+    let size_function_type = env.context.i64_type().fn_type(&[], false);
+    let size_function_name: String = if let Some(label) = opt_label {
+        format!(
+            "roc_{}_{}_{}_size",
+            def_name,
+            alias_symbol.ident_string(&env.interns),
+            label
+        )
+    } else {
+        format!(
+            "roc_{}_{}_size",
+            def_name,
+            alias_symbol.ident_string(&env.interns)
+        )
+    };
+
     let size_function = env.module.add_function(
         size_function_name.as_str(),
         size_function_type,
@@ -2328,7 +2501,8 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
 
     builder.position_at_end(entry);
 
-    let size: BasicValueEnum = roc_call_result_type.size_of().unwrap().into();
+    use inkwell::types::BasicType;
+    let size: BasicValueEnum = basic_type.size_of().unwrap().into();
     builder.build_return(Some(&size));
 }
 
diff --git a/compiler/gen/src/llvm/build_list.rs b/compiler/gen/src/llvm/build_list.rs
index 4049f38e05..6b7b3e99e1 100644
--- a/compiler/gen/src/llvm/build_list.rs
+++ b/compiler/gen/src/llvm/build_list.rs
@@ -2067,7 +2067,7 @@ pub fn allocate_list<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
     inplace: InPlace,
     elem_layout: &Layout<'a>,
-    length: IntValue<'ctx>,
+    number_of_elements: IntValue<'ctx>,
 ) -> PointerValue<'ctx> {
     let builder = env.builder;
     let ctx = env.context;
@@ -2075,10 +2075,11 @@ pub fn allocate_list<'a, 'ctx, 'env>(
     let len_type = env.ptr_int();
     let elem_bytes = elem_layout.stack_size(env.ptr_bytes) as u64;
     let bytes_per_element = len_type.const_int(elem_bytes, false);
-    let number_of_data_bytes = builder.build_int_mul(bytes_per_element, length, "data_length");
+    let number_of_data_bytes =
+        builder.build_int_mul(bytes_per_element, number_of_elements, "data_length");
 
     let rc1 = match inplace {
-        InPlace::InPlace => length,
+        InPlace::InPlace => number_of_elements,
         InPlace::Clone => {
             // the refcount of a new list is initially 1
             // we assume that the list is indeed used (dead variables are eliminated)
diff --git a/compiler/gen/src/llvm/convert.rs b/compiler/gen/src/llvm/convert.rs
index cb7842b859..5bcc9b4c78 100644
--- a/compiler/gen/src/llvm/convert.rs
+++ b/compiler/gen/src/llvm/convert.rs
@@ -3,6 +3,7 @@ use bumpalo::Bump;
 use inkwell::context::Context;
 use inkwell::types::BasicTypeEnum::{self, *};
 use inkwell::types::{ArrayType, BasicType, FunctionType, IntType, PointerType, StructType};
+use inkwell::values::IntValue;
 use inkwell::AddressSpace;
 use roc_mono::layout::Layout;
 
diff --git a/compiler/gen/src/llvm/refcounting.rs b/compiler/gen/src/llvm/refcounting.rs
index 0ef67b4960..4c360d7c53 100644
--- a/compiler/gen/src/llvm/refcounting.rs
+++ b/compiler/gen/src/llvm/refcounting.rs
@@ -59,13 +59,10 @@ impl<'ctx> PointerToRefcount<'ctx> {
         let builder = env.builder;
         // pointer to usize
         let refcount_type = ptr_int(env.context, env.ptr_bytes);
+        let refcount_ptr_type = refcount_type.ptr_type(AddressSpace::Generic);
 
-        let ptr_as_usize_ptr = cast_basic_basic(
-            builder,
-            data_ptr.into(),
-            refcount_type.ptr_type(AddressSpace::Generic).into(),
-        )
-        .into_pointer_value();
+        let ptr_as_usize_ptr = cast_basic_basic(builder, data_ptr.into(), refcount_ptr_type.into())
+            .into_pointer_value();
 
         // get a pointer to index -1
         let index_intvalue = refcount_type.const_int((-1 as i64) as u64, false);
@@ -431,6 +428,24 @@ pub fn increment_refcount_layout<'a, 'ctx, 'env>(
         RecursiveUnion(tags) => {
             build_inc_union(env, layout_ids, tags, value);
         }
+        Closure(_, closure_layout, _) => {
+            if closure_layout.contains_refcounted() {
+                let wrapper_struct = value.into_struct_value();
+
+                let field_ptr = env
+                    .builder
+                    .build_extract_value(wrapper_struct, 1, "increment_closure_data")
+                    .unwrap();
+
+                increment_refcount_layout(
+                    env,
+                    parent,
+                    layout_ids,
+                    field_ptr,
+                    &closure_layout.as_block_of_memory_layout(),
+                )
+            }
+        }
         _ => {}
     }
 }
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index 9e471d41d1..dbacb9fe64 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -587,7 +587,10 @@ impl<'a> Procs<'a> {
                 let symbol = name;
 
                 // TODO should pending_procs hold a Rc<Proc>?
-                let partial_proc = self.partial_procs.get(&symbol).unwrap().clone();
+                let partial_proc = match self.partial_procs.get(&symbol) {
+                    Some(p) => p.clone(),
+                    None => panic!("no partial_proc for {:?}", symbol),
+                };
 
                 // Mark this proc as in-progress, so if we're dealing with
                 // mutually recursive functions, we don't loop forever.