Merge branch 'trunk' into NumBytesTo

cli/src/repl/eval.rs

@@ -352,8 +352,6 @@ fn jit_to_ast_help<'a>(
         | Layout::RecursivePointer => {
             todo!("add support for rendering recursive tag unions in the REPL")
         }
-
-        Layout::Closure(_, _, _) => Err(ToAstProblem::FunctionLayout),
     }
 }
 

compiler/gen_llvm/src/llvm/bitcode.rs

@@ -271,33 +271,11 @@ fn build_transform_caller_help<'a, 'ctx, 'env>(
     }
 
     match closure_data_layout {
-        Layout::Closure(_, lambda_set, _) => {
+        Layout::Struct(&[]) => {
-            if let Layout::Struct(&[]) = lambda_set.runtime_representation() {
+            // nothing to add
-                // do nothing
-            } else {
-                let closure_type =
-                    basic_type_from_layout(env, &lambda_set.runtime_representation())
-                        .ptr_type(AddressSpace::Generic);
-
-                let closure_cast = env
-                    .builder
-                    .build_bitcast(closure_ptr, closure_type, "load_opaque")
-                    .into_pointer_value();
-
-                let closure_data = env.builder.build_load(closure_cast, "load_opaque");
-
-                arguments_cast.push(closure_data);
-            }
         }
-        Layout::Struct([Layout::Closure(_, lambda_set, _)]) => {
+        other => {
-            // a case required for Set.walk; may be able to remove when we can define builtins in
+            let closure_type = basic_type_from_layout(env, &other).ptr_type(AddressSpace::Generic);
-            // terms of other builtins in the right way (using their function symbols instead of
-            // hacking with lowlevel ops).
-            let closure_type = basic_type_from_layout(
-                env,
-                &Layout::Struct(&[lambda_set.runtime_representation()]),
-            )
-            .ptr_type(AddressSpace::Generic);
 
             let closure_cast = env
                 .builder
@@ -308,13 +286,6 @@ fn build_transform_caller_help<'a, 'ctx, 'env>(
 
             arguments_cast.push(closure_data);
         }
-        Layout::Struct([]) => {
-            // do nothing, should try to remove this case later
-        }
-        Layout::Struct(_) => {
-            // do nothing, should try to remove this case later
-        }
-        other => unreachable!("layout is not valid for a closure: {:?}", other),
     }
 
     let call = {
@@ -625,26 +596,23 @@ pub fn build_compare_wrapper<'a, 'ctx, 'env>(
             let default = [value1, value2];
 
             let arguments_cast = match closure_data_layout {
-                Layout::Closure(_, lambda_set, _) => {
+                Layout::Struct(&[]) => {
-                    if let Layout::Struct(&[]) = lambda_set.runtime_representation() {
+                    // nothing to add
-                        &default
+                    &default
-                    } else {
+                }
-                        let closure_type =
+                other => {
-                            basic_type_from_layout(env, &lambda_set.runtime_representation())
+                    let closure_type =
-                                .ptr_type(AddressSpace::Generic);
+                        basic_type_from_layout(env, &other).ptr_type(AddressSpace::Generic);
-
+
-                        let closure_cast = env
+                    let closure_cast = env
-                            .builder
+                        .builder
-                            .build_bitcast(closure_ptr, closure_type, "load_opaque")
+                        .build_bitcast(closure_ptr, closure_type, "load_opaque")
-                            .into_pointer_value();
+                        .into_pointer_value();
-
+
-                        let closure_data = env.builder.build_load(closure_cast, "load_opaque");
+                    let closure_data = env.builder.build_load(closure_cast, "load_opaque");
-
+
-                        env.arena.alloc([value1, value2, closure_data]) as &[_]
+                    env.arena.alloc([value1, value2, closure_data]) as &[_]
-                    }
                 }
-                Layout::Struct([]) => &default,
-                other => unreachable!("layout is not valid for a closure: {:?}", other),
             };
 
             let call = env.builder.build_call(

compiler/gen_llvm/src/llvm/build.rs

@@ -1082,14 +1082,6 @@ pub fn build_exp_expr<'a, 'ctx, 'env>(
                         )
                         .unwrap()
                 }
-                (StructValue(argument), Layout::Closure(_, _, _)) => env
-                    .builder
-                    .build_extract_value(
-                        argument,
-                        *index as u32,
-                        env.arena.alloc(format!("closure_field_access_{}_", index)),
-                    )
-                    .unwrap(),
                 (
                     PointerValue(argument),
                     Layout::Union(UnionLayout::NonNullableUnwrapped(fields)),
@@ -3626,18 +3618,6 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
     lambda_set: LambdaSet<'a>,
     result: &Layout<'a>,
 ) {
-    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.as_str(&env.interns)
-    );
-
     let mut argument_types = Vec::with_capacity_in(arguments.len() + 3, env.arena);
 
     for layout in arguments {
@@ -3654,6 +3634,9 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
     };
     argument_types.push(closure_argument_type.into());
 
+    let context = &env.context;
+    let builder = env.builder;
+
     let result_type = basic_type_from_layout(env, result);
 
     let roc_call_result_type =
@@ -3662,6 +3645,15 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
     let output_type = { roc_call_result_type.ptr_type(AddressSpace::Generic) };
     argument_types.push(output_type.into());
 
+    // STEP 1: build function header
+
+    // e.g. `roc__main_1_Fx_caller`
+    let function_name = format!(
+        "roc__{}_{}_caller",
+        def_name,
+        alias_symbol.as_str(&env.interns)
+    );
+
     let function_type = context.void_type().fn_type(&argument_types, false);
 
     let function_value = add_func(
@@ -3680,9 +3672,15 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
 
     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 closure_data = builder.build_load(closure_data_ptr, "load_closure_data");
+    let closure_data = if let Some(closure_data_ptr) = parameters.pop() {
+        let closure_data =
+            builder.build_load(closure_data_ptr.into_pointer_value(), "load_closure_data");
+
+        env.arena.alloc([closure_data]) as &[_]
+    } else {
+        &[]
+    };
 
     let mut parameters = parameters;
 
@@ -3696,7 +3694,7 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
         function_value,
         evaluator,
         evaluator.get_call_conventions(),
-        &[closure_data],
+        closure_data,
         result_type,
     );
 
@@ -3714,8 +3712,12 @@ pub fn build_closure_caller<'a, 'ctx, 'env>(
     );
 
     // STEP 4: build a {} -> u64 function that gives the size of the closure
-    let layout = lambda_set.runtime_representation();
+    build_host_exposed_alias_size(
-    build_host_exposed_alias_size(env, def_name, alias_symbol, layout);
+        env,
+        def_name,
+        alias_symbol,
+        lambda_set.runtime_representation(),
+    );
 }
 
 fn build_host_exposed_alias_size<'a, 'ctx, 'env>(

compiler/gen_llvm/src/llvm/build_hash.rs

@@ -88,10 +88,6 @@ fn build_hash_layout<'a, 'ctx, 'env>(
                 )
             }
         },
-
-        Layout::Closure(_, _, _) => {
-            unreachable!("the type system will guarantee these are never hashed")
-        }
     }
 }
 

compiler/gen_llvm/src/llvm/compare.rs

@@ -198,10 +198,6 @@ fn build_eq<'a, 'ctx, 'env>(
                 )
             }
         },
-
-        Layout::Closure(_, _, _) => {
-            unreachable!("the type system will guarantee these are never compared")
-        }
     }
 }
 
@@ -340,10 +336,6 @@ fn build_neq<'a, 'ctx, 'env>(
         Layout::RecursivePointer => {
             unreachable!("recursion pointers should never be compared directly")
         }
-
-        Layout::Closure(_, _, _) => {
-            unreachable!("the type system will guarantee these are never compared")
-        }
     }
 }
 

compiler/gen_llvm/src/llvm/convert.rs

@@ -26,10 +26,6 @@ pub fn basic_type_from_layout<'a, 'ctx, 'env>(
     use Layout::*;
 
     match layout {
-        Closure(_args, closure_layout, _ret_layout) => {
-            let closure_data_layout = closure_layout.runtime_representation();
-            basic_type_from_layout(env, &closure_data_layout)
-        }
         Struct(sorted_fields) => basic_type_from_record(env, sorted_fields),
         Union(union_layout) => {
             use UnionLayout::*;

compiler/gen_llvm/src/llvm/refcounting.rs

@@ -659,23 +659,6 @@ fn modify_refcount_layout_build_function<'a, 'ctx, 'env>(
             Some(function)
         }
 
-        Closure(_, lambda_set, _) => {
-            if lambda_set.contains_refcounted() {
-                let function = modify_refcount_layout_build_function(
-                    env,
-                    parent,
-                    layout_ids,
-                    mode,
-                    when_recursive,
-                    &lambda_set.runtime_representation(),
-                )?;
-
-                Some(function)
-            } else {
-                None
-            }
-        }
-
         Struct(layouts) => {
             let function = modify_refcount_struct(env, layout_ids, layouts, mode, when_recursive);
 

compiler/load/src/file.rs

@@ -4060,7 +4060,7 @@ fn add_def_to_module<'a>(
                             pattern_vars.push(*var);
                         }
 
-                        let layout = match layout_cache.from_var(
+                        let layout = match layout_cache.raw_from_var(
                             mono_env.arena,
                             annotation,
                             mono_env.subs,
@@ -4085,7 +4085,7 @@ fn add_def_to_module<'a>(
 
                         procs.insert_exposed(
                             symbol,
-                            ProcLayout::from_layout(mono_env.arena, layout),
+                            ProcLayout::from_raw(mono_env.arena, layout),
                             mono_env.arena,
                             mono_env.subs,
                             def.annotation,

compiler/mono/src/alias_analysis.rs

@@ -64,24 +64,10 @@ where
         let mut hasher = DefaultHasher::new();
 
         for layout in argument_layouts {
-            match layout {
+            layout.hash(&mut hasher);
-                Layout::Closure(_, lambda_set, _) => {
-                    lambda_set.runtime_representation().hash(&mut hasher);
-                }
-                _ => {
-                    layout.hash(&mut hasher);
-                }
-            }
         }
 
-        match return_layout {
+        return_layout.hash(&mut hasher);
-            Layout::Closure(_, lambda_set, _) => {
-                lambda_set.runtime_representation().hash(&mut hasher);
-            }
-            _ => {
-                return_layout.hash(&mut hasher);
-            }
-        }
 
         hasher.finish()
     };
@@ -1258,11 +1244,6 @@ fn layout_spec_help(
                 }
             },
         },
-        Closure(_, lambda_set, _) => layout_spec_help(
-            builder,
-            &lambda_set.runtime_representation(),
-            when_recursive,
-        ),
     }
 }
 

compiler/mono/src/borrow.rs

@@ -10,10 +10,7 @@ pub const OWNED: bool = false;
 pub const BORROWED: bool = true;
 
 fn should_borrow_layout(layout: &Layout) -> bool {
-    match layout {
+    layout.is_refcounted()
-        Layout::Closure(_, _, _) => false,
-        _ => layout.is_refcounted(),
-    }
 }
 
 pub fn infer_borrow<'a>(

compiler/mono/src/expand_rc.rs

@@ -160,17 +160,9 @@ impl<'a, 'i> Env<'a, 'i> {
     fn try_insert_struct_info(&mut self, symbol: Symbol, layout: &Layout<'a>) {
         use Layout::*;
 
-        match layout {
+        if let Struct(fields) = layout {
-            Struct(fields) => {
+            self.constructor_map.insert(symbol, 0);
-                self.constructor_map.insert(symbol, 0);
+            self.layout_map.insert(symbol, Layout::Struct(fields));
-                self.layout_map.insert(symbol, Layout::Struct(fields));
-            }
-            Closure(_, lambda_set, _) => {
-                self.constructor_map.insert(symbol, 0);
-                self.layout_map
-                    .insert(symbol, lambda_set.runtime_representation());
-            }
-            _ => {}
         }
     }
 
@@ -244,10 +236,6 @@ fn layout_for_constructor<'a>(
             debug_assert_eq!(constructor, 0);
             HasFields(fields)
         }
-        Closure(_arguments, _lambda_set, _result) => {
-            // HasFields(fields)
-            ConstructorLayout::Unknown
-        }
         other => unreachable!("weird layout {:?}", other),
     }
 }
@@ -368,20 +356,10 @@ pub fn expand_and_cancel_proc<'a>(
     let mut introduced = Vec::new_in(env.arena);
 
     for (layout, symbol) in arguments {
-        match layout {
+        if let Layout::Struct(fields) = layout {
-            Layout::Struct(fields) => {
+            env.insert_struct_info(*symbol, fields);
-                env.insert_struct_info(*symbol, fields);
 
-                introduced.push(*symbol);
+            introduced.push(*symbol);
-            }
-            Layout::Closure(_arguments, _lambda_set, _result) => {
-                // TODO can this be improved again?
-                // let fpointer = Layout::FunctionPointer(arguments, result);
-                // let fields = env.arena.alloc([fpointer, *closure_layout.layout]);
-                // env.insert_struct_info(*symbol, fields);
-                // introduced.push(*symbol);
-            }
-            _ => {}
         }
     }
 

compiler/mono/src/inc_dec.rs

@@ -271,10 +271,20 @@ impl<'a> Context<'a> {
     fn get_var_info(&self, symbol: Symbol) -> VarInfo {
         match self.vars.get(&symbol) {
             Some(info) => *info,
-            None => panic!(
+            None => {
-                "Symbol {:?} {} has no info in {:?}",
+                eprintln!(
-                symbol, symbol, self.vars
+                    "Symbol {:?} {} has no info in self.vars",
-            ),
+                    symbol,
+                    symbol, // self.vars
+                );
+
+                VarInfo {
+                    persistent: true,
+                    reference: false,
+                    consume: false,
+                    reset: false,
+                }
+            }
         }
     }
 

compiler/mono/src/ir.rs

@@ -546,11 +546,11 @@ impl<'a> Procs<'a> {
         // anonymous functions cannot reference themselves, therefore cannot be tail-recursive
         let is_self_recursive = false;
 
-        let layout = layout_cache
+        let raw_layout = layout_cache
-            .from_var(env.arena, annotation, env.subs)
+            .raw_from_var(env.arena, annotation, env.subs)
             .unwrap_or_else(|err| panic!("TODO turn fn_var into a RuntimeError {:?}", err));
 
-        let top_level = ProcLayout::from_layout(env.arena, layout);
+        let top_level = ProcLayout::from_raw(env.arena, raw_layout);
 
         match patterns_to_when(env, layout_cache, loc_args, ret_var, loc_body) {
             Ok((_, pattern_symbols, body)) => {
@@ -617,7 +617,11 @@ impl<'a> Procs<'a> {
                                 Ok((proc, layout)) => {
                                     let top_level = ProcLayout::from_raw(env.arena, layout);
 
-                                    debug_assert_eq!(outside_layout, top_level);
+                                    debug_assert_eq!(
+                                        outside_layout, top_level,
+                                        "different raw layouts for {:?}",
+                                        proc.name
+                                    );
 
                                     if self.module_thunks.contains(&proc.name) {
                                         debug_assert!(top_level.arguments.is_empty());
@@ -2028,7 +2032,9 @@ fn specialize_external<'a>(
 
                     aliases.insert(*symbol, (name, top_level, layout));
                 }
-                RawFunctionLayout::ZeroArgumentThunk(_) => unreachable!("so far"),
+                RawFunctionLayout::ZeroArgumentThunk(_) => {
+                    unreachable!("so far");
+                }
             }
         }
 
@@ -2096,7 +2102,7 @@ fn specialize_external<'a>(
 
                     match closure_layout.layout_for_member(proc_name) {
                         ClosureRepresentation::Union {
-                            tag_layout: field_layouts,
+                            alphabetic_order_fields: field_layouts,
                             union_layout,
                             tag_id,
                             ..
@@ -2104,7 +2110,23 @@ fn specialize_external<'a>(
                             debug_assert!(matches!(union_layout, UnionLayout::NonRecursive(_)));
                             debug_assert_eq!(field_layouts.len(), captured.len());
 
-                            for (index, (symbol, _variable)) in captured.iter().enumerate() {
+                            // captured variables are in symbol-alphabetic order, but now we want
+                            // them ordered by their alignment requirements
+                            let mut combined = Vec::from_iter_in(
+                                captured.iter().map(|(x, _)| x).zip(field_layouts.iter()),
+                                env.arena,
+                            );
+
+                            let ptr_bytes = env.ptr_bytes;
+
+                            combined.sort_by(|(_, layout1), (_, layout2)| {
+                                let size1 = layout1.alignment_bytes(ptr_bytes);
+                                let size2 = layout2.alignment_bytes(ptr_bytes);
+
+                                size2.cmp(&size1)
+                            });
+
+                            for (index, (symbol, layout)) in combined.iter().enumerate() {
                                 let expr = Expr::UnionAtIndex {
                                     tag_id,
                                     structure: Symbol::ARG_CLOSURE,
@@ -2112,52 +2134,65 @@ fn specialize_external<'a>(
                                     union_layout,
                                 };
 
-                                let layout = field_layouts[index];
-
                                 specialized_body = Stmt::Let(
-                                    *symbol,
+                                    **symbol,
                                     expr,
-                                    layout,
+                                    **layout,
                                     env.arena.alloc(specialized_body),
                                 );
                             }
                         }
-                        ClosureRepresentation::Other(layout) => match layout {
+                        ClosureRepresentation::AlphabeticOrderStruct(field_layouts) => {
-                            Layout::Struct(field_layouts) => {
+                            // captured variables are in symbol-alphabetic order, but now we want
-                                debug_assert_eq!(
+                            // them ordered by their alignment requirements
-                                    captured.len(),
+                            let mut combined = Vec::from_iter_in(
-                                    field_layouts.len(),
+                                captured.iter().map(|(x, _)| x).zip(field_layouts.iter()),
-                                    "{:?} captures {:?} but has layout {:?}",
+                                env.arena,
-                                    proc_name,
+                            );
-                                    &captured,
+
-                                    &field_layouts
+                            let ptr_bytes = env.ptr_bytes;
+
+                            combined.sort_by(|(_, layout1), (_, layout2)| {
+                                let size1 = layout1.alignment_bytes(ptr_bytes);
+                                let size2 = layout2.alignment_bytes(ptr_bytes);
+
+                                size2.cmp(&size1)
+                            });
+
+                            debug_assert_eq!(
+                                captured.len(),
+                                field_layouts.len(),
+                                "{:?} captures {:?} but has layout {:?}",
+                                proc_name,
+                                &captured,
+                                &field_layouts
+                            );
+
+                            for (index, (symbol, layout)) in combined.iter().enumerate() {
+                                let expr = Expr::StructAtIndex {
+                                    index: index as _,
+                                    field_layouts,
+                                    structure: Symbol::ARG_CLOSURE,
+                                };
+
+                                specialized_body = Stmt::Let(
+                                    **symbol,
+                                    expr,
+                                    **layout,
+                                    env.arena.alloc(specialized_body),
                                 );
-
-                                for (index, (symbol, _variable)) in captured.iter().enumerate() {
-                                    let expr = Expr::StructAtIndex {
-                                        index: index as _,
-                                        field_layouts,
-                                        structure: Symbol::ARG_CLOSURE,
-                                    };
-
-                                    let layout = field_layouts[index];
-
-                                    specialized_body = Stmt::Let(
-                                        *symbol,
-                                        expr,
-                                        layout,
-                                        env.arena.alloc(specialized_body),
-                                    );
-                                }
-                                //                                    let symbol = captured[0].0;
-                                //
-                                //                                    substitute_in_exprs(
-                                //                                        env.arena,
-                                //                                        &mut specialized_body,
-                                //                                        symbol,
-                                //                                        Symbol::ARG_CLOSURE,
-                                //                                    );
                             }
+                            //                                    let symbol = captured[0].0;
+                            //
+                            //                                    substitute_in_exprs(
+                            //                                        env.arena,
+                            //                                        &mut specialized_body,
+                            //                                        symbol,
+                            //                                        Symbol::ARG_CLOSURE,
+                            //                                    );
+                        }
+
+                        ClosureRepresentation::Other(layout) => match layout {
                             Layout::Builtin(Builtin::Int1) => {
                                 // just ignore this value
                                 // IDEA don't pass this value in the future
@@ -2467,19 +2502,19 @@ fn specialize_solved_type<'a>(
     let fn_var = introduce_solved_type_to_subs(env, &solved_type);
 
     // for debugging only
-    let attempted_layout = layout_cache
+    let raw = layout_cache
-        .from_var(env.arena, fn_var, env.subs)
+        .raw_from_var(env.arena, fn_var, env.subs)
         .unwrap_or_else(|err| panic!("TODO handle invalid function {:?}", err));
 
-    let raw = match attempted_layout {
+    let raw = if procs.module_thunks.contains(&proc_name) {
-        Layout::Closure(a, lambda_set, c) => {
+        match raw {
-            if procs.module_thunks.contains(&proc_name) {
+            RawFunctionLayout::Function(_, lambda_set, _) => {
                 RawFunctionLayout::ZeroArgumentThunk(lambda_set.runtime_representation())
-            } else {
-                RawFunctionLayout::Function(a, lambda_set, c)
             }
+            _ => raw,
         }
-        _ => RawFunctionLayout::ZeroArgumentThunk(attempted_layout),
+    } else {
+        raw
     };
 
     // make sure rigid variables in the annotation are converted to flex variables
@@ -2506,12 +2541,12 @@ fn specialize_solved_type<'a>(
     match specialized {
         Ok(proc) => {
             // when successful, the layout after unification should be the layout before unification
-            debug_assert_eq!(
+            //            debug_assert_eq!(
-                attempted_layout,
+            //                attempted_layout,
-                layout_cache
+            //                layout_cache
-                    .from_var(env.arena, fn_var, env.subs)
+            //                    .from_var(env.arena, fn_var, env.subs)
-                    .unwrap_or_else(|err| panic!("TODO handle invalid function {:?}", err))
+            //                    .unwrap_or_else(|err| panic!("TODO handle invalid function {:?}", err))
-            );
+            //            );
 
             env.subs.rollback_to(snapshot);
             layout_cache.rollback_to(cache_snapshot);
@@ -2541,39 +2576,20 @@ impl<'a> ProcLayout<'a> {
         let mut arguments = Vec::with_capacity_in(old_arguments.len(), arena);
 
         for old in old_arguments {
-            match old {
+            let other = old;
-                Layout::Closure(_, lambda_set, _) => {
+            arguments.push(*other);
-                    let repr = lambda_set.runtime_representation();
-                    arguments.push(repr)
-                }
-                other => arguments.push(*other),
-            }
         }
 
-        let new_result = match result {
+        let other = result;
-            Layout::Closure(_, lambda_set, _) => lambda_set.runtime_representation(),
+        let new_result = other;
-            other => other,
-        };
 
         ProcLayout {
             arguments: arguments.into_bump_slice(),
             result: new_result,
         }
     }
-    pub fn from_layout(arena: &'a Bump, layout: Layout<'a>) -> Self {
-        match layout {
-            Layout::Closure(arguments, lambda_set, result) => {
-                let arguments = lambda_set.extend_argument_list(arena, arguments);
-                ProcLayout::new(arena, arguments, *result)
-            }
-            _ => ProcLayout {
-                arguments: &[],
-                result: layout,
-            },
-        }
-    }
 
-    fn from_raw(arena: &'a Bump, raw: RawFunctionLayout<'a>) -> Self {
+    pub fn from_raw(arena: &'a Bump, raw: RawFunctionLayout<'a>) -> Self {
         match raw {
             RawFunctionLayout::Function(arguments, lambda_set, result) => {
                 let arguments = lambda_set.extend_argument_list(arena, arguments);
@@ -2666,11 +2682,13 @@ macro_rules! match_on_closure_argument {
         let arg_layouts = top_level.arguments;
         let ret_layout = top_level.result;
 
+
         match closure_data_layout {
             RawFunctionLayout::Function(_, lambda_set, _) =>  {
                 lowlevel_match_on_lambda_set(
                     $env,
                     lambda_set,
+                    $op,
                     $closure_data_symbol,
                     |top_level_function, closure_data, closure_env_layout, specialization_id| self::Call {
                         call_type: CallType::HigherOrderLowLevel {
@@ -4054,10 +4072,27 @@ fn construct_closure_data<'a>(
     match lambda_set.layout_for_member(name) {
         ClosureRepresentation::Union {
             tag_id,
-            tag_layout: _,
+            alphabetic_order_fields: field_layouts,
             tag_name,
             union_layout,
         } => {
+            // captured variables are in symbol-alphabetic order, but now we want
+            // them ordered by their alignment requirements
+            let mut combined =
+                Vec::from_iter_in(symbols.iter().zip(field_layouts.iter()), env.arena);
+
+            let ptr_bytes = env.ptr_bytes;
+
+            combined.sort_by(|(_, layout1), (_, layout2)| {
+                let size1 = layout1.alignment_bytes(ptr_bytes);
+                let size2 = layout2.alignment_bytes(ptr_bytes);
+
+                size2.cmp(&size1)
+            });
+
+            let symbols =
+                Vec::from_iter_in(combined.iter().map(|(a, _)| **a), env.arena).into_bump_slice();
+
             let expr = Expr::Tag {
                 tag_id,
                 tag_layout: union_layout,
@@ -4072,9 +4107,33 @@ fn construct_closure_data<'a>(
                 env.arena.alloc(hole),
             )
         }
-        ClosureRepresentation::Other(Layout::Struct(field_layouts)) => {
+        ClosureRepresentation::AlphabeticOrderStruct(field_layouts) => {
             debug_assert_eq!(field_layouts.len(), symbols.len());
 
+            // captured variables are in symbol-alphabetic order, but now we want
+            // them ordered by their alignment requirements
+            let mut combined =
+                Vec::from_iter_in(symbols.iter().zip(field_layouts.iter()), env.arena);
+
+            let ptr_bytes = env.ptr_bytes;
+
+            combined.sort_by(|(_, layout1), (_, layout2)| {
+                let size1 = layout1.alignment_bytes(ptr_bytes);
+                let size2 = layout2.alignment_bytes(ptr_bytes);
+
+                size2.cmp(&size1)
+            });
+
+            let symbols =
+                Vec::from_iter_in(combined.iter().map(|(a, _)| **a), env.arena).into_bump_slice();
+            let field_layouts =
+                Vec::from_iter_in(combined.iter().map(|(_, b)| **b), env.arena).into_bump_slice();
+
+            debug_assert_eq!(
+                Layout::Struct(field_layouts),
+                lambda_set.runtime_representation()
+            );
+
             let expr = Expr::Struct(symbols);
 
             Stmt::Let(assigned, expr, lambda_set.runtime_representation(), hole)
@@ -5952,12 +6011,21 @@ fn reuse_function_symbol<'a>(
         None => {
             match arg_var {
                 Some(arg_var) if env.is_imported_symbol(original) => {
-                    let layout = layout_cache
+                    let raw = layout_cache
-                        .from_var(env.arena, arg_var, env.subs)
+                        .raw_from_var(env.arena, arg_var, env.subs)
                         .expect("creating layout does not fail");
 
                     if procs.imported_module_thunks.contains(&original) {
-                        let top_level = ProcLayout::new(env.arena, &[], layout);
+                        let layout = match raw {
+                            RawFunctionLayout::ZeroArgumentThunk(layout) => layout,
+                            RawFunctionLayout::Function(_, lambda_set, _) => {
+                                lambda_set.runtime_representation()
+                            }
+                        };
+
+                        let raw = RawFunctionLayout::ZeroArgumentThunk(layout);
+                        let top_level = ProcLayout::from_raw(env.arena, raw);
+
                         procs.insert_passed_by_name(
                             env,
                             arg_var,
@@ -5968,7 +6036,7 @@ fn reuse_function_symbol<'a>(
 
                         force_thunk(env, original, layout, symbol, env.arena.alloc(result))
                     } else {
-                        let top_level = ProcLayout::from_layout(env.arena, layout);
+                        let top_level = ProcLayout::from_raw(env.arena, raw);
                         procs.insert_passed_by_name(
                             env,
                             arg_var,
@@ -6011,7 +6079,7 @@ fn reuse_function_symbol<'a>(
             let captured = partial_proc.captured_symbols.clone();
 
             match res_layout {
-                RawFunctionLayout::Function(argument_layouts, lambda_set, ret_layout) => {
+                RawFunctionLayout::Function(_, lambda_set, _) => {
                     // define the function pointer
                     let function_ptr_layout = ProcLayout::from_raw(env.arena, res_layout);
 
@@ -6045,7 +6113,11 @@ fn reuse_function_symbol<'a>(
                         )
                     } else if procs.module_thunks.contains(&original) {
                         // this is a 0-argument thunk
-                        let layout = Layout::Closure(argument_layouts, lambda_set, ret_layout);
+
+                        // TODO suspicious
+                        // let layout = Layout::Closure(argument_layouts, lambda_set, ret_layout);
+                        // panic!("suspicious");
+                        let layout = lambda_set.runtime_representation();
                         let top_level = ProcLayout::new(env.arena, &[], layout);
                         procs.insert_passed_by_name(
                             env,
@@ -6651,8 +6723,12 @@ fn call_by_name_module_thunk<'a>(
 
                         match specialize(env, procs, proc_name, layout_cache, pending, partial_proc)
                         {
-                            Ok((proc, layout)) => {
+                            Ok((proc, raw_layout)) => {
-                                debug_assert!(layout.is_zero_argument_thunk());
+                                debug_assert!(
+                                    raw_layout.is_zero_argument_thunk(),
+                                    "but actually {:?}",
+                                    raw_layout
+                                );
 
                                 let was_present =
                                     procs.specialized.remove(&(proc_name, top_level_layout));
@@ -7613,6 +7689,7 @@ pub fn num_argument_to_int_or_float(
 fn lowlevel_match_on_lambda_set<'a, ToLowLevelCall>(
     env: &mut Env<'a, '_>,
     lambda_set: LambdaSet<'a>,
+    op: LowLevel,
     closure_data_symbol: Symbol,
     to_lowlevel_call: ToLowLevelCall,
     return_layout: Layout<'a>,
@@ -7652,19 +7729,29 @@ where
                 env.arena.alloc(result),
             )
         }
-        Layout::Struct(_) => {
+        Layout::Struct(_) => match lambda_set.set.get(0) {
-            let function_symbol = lambda_set.set[0].0;
+            Some((function_symbol, _)) => {
+                let call_spec_id = env.next_call_specialization_id();
+                let call = to_lowlevel_call(
+                    *function_symbol,
+                    closure_data_symbol,
+                    lambda_set.is_represented(),
+                    call_spec_id,
+                );
 
-            let call_spec_id = env.next_call_specialization_id();
+                build_call(env, call, assigned, return_layout, env.arena.alloc(hole))
-            let call = to_lowlevel_call(
+            }
-                function_symbol,
+            None => {
-                closure_data_symbol,
+                eprintln!(
-                lambda_set.is_represented(),
+                    "a function passed to `{:?}` LowLevel call has an empty lambda set!
-                call_spec_id,
+                     The most likely reason is that some symbol you use is not in scope.
-            );
+                    ",
+                    op
+                );
 
-            build_call(env, call, assigned, return_layout, env.arena.alloc(hole))
+                hole.clone()
-        }
+            }
+        },
         Layout::Builtin(Builtin::Int1) => {
             let closure_tag_id_symbol = closure_data_symbol;
 

compiler/mono/src/layout.rs

@@ -29,10 +29,154 @@ pub enum RawFunctionLayout<'a> {
     ZeroArgumentThunk(Layout<'a>),
 }
 
-impl RawFunctionLayout<'_> {
+impl<'a> RawFunctionLayout<'a> {
     pub fn is_zero_argument_thunk(&self) -> bool {
         matches!(self, RawFunctionLayout::ZeroArgumentThunk(_))
     }
+
+    fn new_help<'b>(
+        env: &mut Env<'a, 'b>,
+        var: Variable,
+        content: Content,
+    ) -> Result<Self, LayoutProblem> {
+        use roc_types::subs::Content::*;
+        match content {
+            FlexVar(_) | RigidVar(_) => Err(LayoutProblem::UnresolvedTypeVar(var)),
+            RecursionVar { structure, .. } => {
+                let structure_content = env.subs.get_content_without_compacting(structure);
+                Self::new_help(env, structure, structure_content.clone())
+            }
+            Structure(flat_type) => Self::layout_from_flat_type(env, flat_type),
+
+            // Ints
+            Alias(Symbol::NUM_I128, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int128)))
+            }
+            Alias(Symbol::NUM_I64, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int64)))
+            }
+            Alias(Symbol::NUM_I32, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int32)))
+            }
+            Alias(Symbol::NUM_I16, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int16)))
+            }
+            Alias(Symbol::NUM_I8, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int8)))
+            }
+
+            // I think unsigned and signed use the same layout
+            Alias(Symbol::NUM_U128, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int128)))
+            }
+            Alias(Symbol::NUM_U64, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int64)))
+            }
+            Alias(Symbol::NUM_U32, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int32)))
+            }
+            Alias(Symbol::NUM_U16, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int16)))
+            }
+            Alias(Symbol::NUM_U8, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Int8)))
+            }
+
+            Alias(Symbol::NUM_NAT, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Usize)))
+            }
+
+            // Floats
+            Alias(Symbol::NUM_F64, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Float64)))
+            }
+            Alias(Symbol::NUM_F32, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Self::ZeroArgumentThunk(Layout::Builtin(Builtin::Float32)))
+            }
+
+            Alias(symbol, _, _) if symbol.is_builtin() => Ok(Self::ZeroArgumentThunk(
+                Layout::new_help(env, var, content)?,
+            )),
+
+            Alias(_, _, var) => Self::from_var(env, var),
+            Error => Err(LayoutProblem::Erroneous),
+        }
+    }
+
+    fn layout_from_flat_type(
+        env: &mut Env<'a, '_>,
+        flat_type: FlatType,
+    ) -> Result<Self, LayoutProblem> {
+        use roc_types::subs::FlatType::*;
+
+        let arena = env.arena;
+
+        match flat_type {
+            Func(args, closure_var, ret_var) => {
+                let mut fn_args = Vec::with_capacity_in(args.len(), arena);
+
+                for index in args.into_iter() {
+                    let arg_var = env.subs[index];
+                    fn_args.push(Layout::from_var(env, arg_var)?);
+                }
+
+                let ret = Layout::from_var(env, ret_var)?;
+
+                let fn_args = fn_args.into_bump_slice();
+                let ret = arena.alloc(ret);
+
+                let lambda_set = LambdaSet::from_var(env.arena, env.subs, closure_var)?;
+
+                Ok(Self::Function(fn_args, lambda_set, ret))
+            }
+            TagUnion(tags, ext) if tags.is_newtype_wrapper(env.subs) => {
+                debug_assert!(ext_var_is_empty_tag_union(env.subs, ext));
+                let slice_index = tags.variables().into_iter().next().unwrap();
+                let slice = env.subs[slice_index];
+                let var_index = slice.into_iter().next().unwrap();
+                let var = env.subs[var_index];
+
+                Self::from_var(env, var)
+            }
+            Record(fields, ext) if fields.len() == 1 => {
+                debug_assert!(ext_var_is_empty_record(env.subs, ext));
+
+                let var_index = fields.iter_variables().next().unwrap();
+                let var = env.subs[var_index];
+
+                Self::from_var(env, var)
+            }
+            _ => {
+                let layout = layout_from_flat_type(env, flat_type)?;
+                Ok(Self::ZeroArgumentThunk(layout))
+            }
+        }
+    }
+
+    /// Returns Err(()) if given an error, or Ok(Layout) if given a non-erroneous Structure.
+    /// Panics if given a FlexVar or RigidVar, since those should have been
+    /// monomorphized away already!
+    fn from_var(env: &mut Env<'a, '_>, var: Variable) -> Result<Self, LayoutProblem> {
+        if env.is_seen(var) {
+            unreachable!("The initial variable of a signature cannot be seen already")
+        } else {
+            let content = env.subs.get_content_without_compacting(var);
+            Self::new_help(env, var, content.clone())
+        }
+    }
 }
 
 /// Types for code gen must be monomorphic. No type variables allowed!
@@ -45,9 +189,6 @@ pub enum Layout<'a> {
     Struct(&'a [Layout<'a>]),
     Union(UnionLayout<'a>),
     RecursivePointer,
-
-    /// A function. The types of its arguments, then the type of its return value.
-    Closure(&'a [Layout<'a>], LambdaSet<'a>, &'a Layout<'a>),
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
@@ -259,11 +400,16 @@ pub struct LambdaSet<'a> {
 pub enum ClosureRepresentation<'a> {
     /// the closure is represented as a union. Includes the tag ID!
     Union {
-        tag_layout: &'a [Layout<'a>],
+        alphabetic_order_fields: &'a [Layout<'a>],
         tag_name: TagName,
         tag_id: u8,
         union_layout: UnionLayout<'a>,
     },
+    /// The closure is represented as a struct. The layouts are sorted
+    /// alphabetically by the identifier that is captured.
+    ///
+    /// We MUST sort these according to their stack size before code gen!
+    AlphabeticOrderStruct(&'a [Layout<'a>]),
     /// the representation is anything but a union
     Other(Layout<'a>),
 }
@@ -292,16 +438,19 @@ impl<'a> LambdaSet<'a> {
                 // here we rely on the fact that a union in a closure would be stored in a one-element record.
                 // a closure representation that is itself union must be a of the shape `Closure1 ... | Closure2 ...`
                 match union {
-                    UnionLayout::NonRecursive(tags) => {
+                    UnionLayout::NonRecursive(_) => {
-                        let index = self
+                        // get the fields from the set, where they are sorted in alphabetic order
+                        // (and not yet sorted by their alignment)
+                        let (index, (_, fields)) = self
                             .set
                             .iter()
-                            .position(|(s, _)| *s == function_symbol)
+                            .enumerate()
+                            .find(|(_, (s, _))| *s == function_symbol)
                             .unwrap();
 
                         ClosureRepresentation::Union {
                             tag_id: index as u8,
-                            tag_layout: tags[index],
+                            alphabetic_order_fields: fields,
                             tag_name: TagName::Closure(function_symbol),
                             union_layout: *union,
                         }
@@ -318,6 +467,17 @@ impl<'a> LambdaSet<'a> {
                     } => todo!("recursive closures"),
                 }
             }
+            Layout::Struct(_) => {
+                // get the fields from the set, where they are sorted in alphabetic order
+                // (and not yet sorted by their alignment)
+                let (_, fields) = self
+                    .set
+                    .iter()
+                    .find(|(s, _)| *s == function_symbol)
+                    .unwrap();
+
+                ClosureRepresentation::AlphabeticOrderStruct(fields)
+            }
             _ => ClosureRepresentation::Other(*self.representation),
         }
     }
@@ -587,6 +747,12 @@ impl<'a> Layout<'a> {
                 Ok(Layout::Builtin(Builtin::Float32))
             }
 
+            // Nat
+            Alias(Symbol::NUM_NAT, args, _) => {
+                debug_assert!(args.is_empty());
+                Ok(Layout::Builtin(Builtin::Usize))
+            }
+
             Alias(_, _, var) => Self::from_var(env, var),
             Error => Err(LayoutProblem::Erroneous),
         }
@@ -628,7 +794,6 @@ impl<'a> Layout<'a> {
                     }
                 }
             }
-            Closure(_, closure_layout, _) => closure_layout.safe_to_memcpy(),
             RecursivePointer => {
                 // We cannot memcpy pointers, because then we would have the same pointer in multiple places!
                 false
@@ -693,7 +858,6 @@ impl<'a> Layout<'a> {
                     | NonNullableUnwrapped(_) => pointer_size,
                 }
             }
-            Closure(_, lambda_set, _) => lambda_set.stack_size(pointer_size),
             RecursivePointer => pointer_size,
         }
     }
@@ -725,9 +889,6 @@ impl<'a> Layout<'a> {
             }
             Layout::Builtin(builtin) => builtin.alignment_bytes(pointer_size),
             Layout::RecursivePointer => pointer_size,
-            Layout::Closure(_, captured, _) => {
-                pointer_size.max(captured.alignment_bytes(pointer_size))
-            }
         }
     }
 
@@ -778,8 +939,6 @@ impl<'a> Layout<'a> {
                 }
             }
             RecursivePointer => true,
-
-            Closure(_, closure_layout, _) => closure_layout.contains_refcounted(),
         }
     }
 
@@ -803,20 +962,6 @@ impl<'a> Layout<'a> {
             }
             Union(union_layout) => union_layout.to_doc(alloc, parens),
             RecursivePointer => alloc.text("*self"),
-            Closure(args, closure_layout, result) => {
-                let args_doc = args.iter().map(|x| x.to_doc(alloc, Parens::InFunction));
-
-                let bom = closure_layout
-                    .representation
-                    .to_doc(alloc, Parens::NotNeeded);
-
-                alloc
-                    .intersperse(args_doc, ", ")
-                    .append(alloc.text(" {| "))
-                    .append(bom)
-                    .append(" |} -> ")
-                    .append(result.to_doc(alloc, Parens::InFunction))
-            }
         }
     }
 }
@@ -873,10 +1018,7 @@ impl<'a> LayoutCache<'a> {
             seen: Vec::new_in(arena),
         };
 
-        Layout::from_var(&mut env, var).map(|l| match l {
+        RawFunctionLayout::from_var(&mut env, var)
-            Layout::Closure(a, b, c) => RawFunctionLayout::Function(a, b, c),
-            other => RawFunctionLayout::ZeroArgumentThunk(other),
-        })
     }
 
     pub fn snapshot(&mut self) -> SnapshotKeyPlaceholder {
@@ -1130,22 +1272,10 @@ fn layout_from_flat_type<'a>(
                 }
             }
         }
-        Func(args, closure_var, ret_var) => {
+        Func(_, closure_var, _) => {
-            let mut fn_args = Vec::with_capacity_in(args.len(), arena);
-
-            for index in args.into_iter() {
-                let arg_var = env.subs[index];
-                fn_args.push(Layout::from_var(env, arg_var)?);
-            }
-
-            let ret = Layout::from_var(env, ret_var)?;
-
-            let fn_args = fn_args.into_bump_slice();
-            let ret = arena.alloc(ret);
-
             let lambda_set = LambdaSet::from_var(env.arena, env.subs, closure_var)?;
 
-            Ok(Layout::Closure(fn_args, lambda_set, ret))
+            Ok(lambda_set.runtime_representation())
         }
         Record(fields, ext_var) => {
             // extract any values from the ext_var
@@ -2083,6 +2213,20 @@ fn layout_from_tag_union<'a>(arena: &'a Bump, tags: UnionTags, subs: &Subs) -> L
     }
 }
 
+#[cfg(debug_assertions)]
+fn ext_var_is_empty_record(subs: &Subs, ext_var: Variable) -> bool {
+    // the ext_var is empty
+    let fields = roc_types::types::gather_fields(subs, RecordFields::empty(), ext_var);
+
+    fields.fields.is_empty()
+}
+
+#[cfg(not(debug_assertions))]
+fn ext_var_is_empty_record(subs: &Subs, ext_var: Variable) -> bool {
+    // This should only ever be used in debug_assert! macros
+    unreachable!();
+}
+
 #[cfg(debug_assertions)]
 fn ext_var_is_empty_tag_union(subs: &Subs, ext_var: Variable) -> bool {
     // the ext_var is empty

compiler/test_gen/src/gen_primitives.rs

@@ -2657,3 +2657,85 @@ fn lambda_set_enum_byte_byte() {
         i64
     );
 }
+
+#[test]
+fn list_walk_until() {
+    // see https://github.com/rtfeldman/roc/issues/1576
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [ main ] to "./platform"
+
+
+            satisfyA : {} -> List {}
+            satisfyA = \_ -> []
+
+            oneOfResult =
+                List.walkUntil [ satisfyA ] (\_, _ -> Stop []) []
+
+            main =
+                when oneOfResult is
+                    _ -> 32
+            "#
+        ),
+        32,
+        i64
+    );
+}
+
+#[test]
+#[ignore]
+fn int_literal_not_specialized() {
+    // see https://github.com/rtfeldman/roc/issues/1600
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [ main ] to "./platform"
+
+
+            satisfy : (U8 -> Bool) -> Str
+            satisfy = \_ -> "foo"
+
+
+            main : I64
+            main =
+                p1 = (\u -> u == 97)
+
+                satisfyA = satisfy p1
+
+                when satisfyA is
+                    _ -> 32
+            "#
+        ),
+        32,
+        i64
+    );
+}
+
+#[test]
+#[ignore]
+fn unresolved_tvar_when_capture_is_unused() {
+    // see https://github.com/rtfeldman/roc/issues/1585
+    assert_evals_to!(
+        indoc!(
+            r#"
+                app "test" provides [ main ] to "./platform"
+
+                main : I64
+                main =
+                    r : Bool
+                    r = False
+
+                    # underscore does not change the problem, maybe it's type-related? We don 't really know what `Green` refers to below
+                    p1 = (\x -> r == (1 == 1))
+                    oneOfResult = List.map [p1] (\p -> p Green)
+
+                    when oneOfResult is
+                        _ -> 32
+
+            "#
+        ),
+        32,
+        i64
+    );
+}

compiler/types/src/subs.rs

@@ -337,7 +337,9 @@ fn subs_fmt_content(this: &Content, subs: &Subs, f: &mut fmt::Formatter) -> fmt:
         } => write!(f, "Recursion({:?}, {:?})", structure, opt_name),
         Content::Structure(flat_type) => subs_fmt_flat_type(flat_type, subs, f),
         Content::Alias(name, arguments, actual) => {
-            write!(f, "Alias({:?}, {:?}, {:?})", name, arguments, actual)
+            let slice = subs.get_subs_slice(*arguments.variables().as_subs_slice());
+
+            write!(f, "Alias({:?}, {:?}, {:?})", name, slice, actual)
         }
         Content::Error => write!(f, "Error"),
     }
@@ -354,7 +356,21 @@ fn subs_fmt_flat_type(this: &FlatType, subs: &Subs, f: &mut fmt::Formatter) -> f
             let slice = subs.get_subs_slice(*arguments.as_subs_slice());
             write!(f, "Func({:?}, {:?}, {:?})", slice, lambda_set, result)
         }
-        FlatType::Record(_, _) => todo!(),
+        FlatType::Record(fields, ext) => {
+            write!(f, "{{ ")?;
+
+            let (it, new_ext) = fields.sorted_iterator_and_ext(subs, *ext);
+            for (name, content) in it {
+                let separator = match content {
+                    RecordField::Optional(_) => '?',
+                    RecordField::Required(_) => ':',
+                    RecordField::Demanded(_) => ':',
+                };
+                write!(f, "{:?} {} {:?}, ", name, separator, content)?;
+            }
+
+            write!(f, "}}<{:?}>", new_ext)
+        }
         FlatType::TagUnion(tags, ext) => {
             write!(f, "[ ")?;