Merge pull request #3769 from roc-lang/expect-fx-codegen

expect fx codegen

Cargo.lock

@@ -4009,6 +4009,7 @@ dependencies = [
  "roc_std",
  "roc_target",
  "roc_types",
+ "signal-hook",
  "strip-ansi-escapes",
  "target-lexicon",
  "tempfile",

crates/cli/src/lib.rs

@@ -8,12 +8,10 @@ use roc_build::link::{LinkType, LinkingStrategy};
 use roc_collections::VecMap;
 use roc_error_macros::{internal_error, user_error};
 use roc_gen_llvm::llvm::build::LlvmBackendMode;
-use roc_gen_llvm::run_roc::RocCallResult;
-use roc_gen_llvm::run_roc_dylib;
 use roc_load::{ExecutionMode, Expectations, LoadConfig, LoadingProblem, Threading};
 use roc_module::symbol::{Interns, ModuleId};
 use roc_mono::ir::OptLevel;
-use roc_repl_expect::run::{expect_mono_module_to_dylib, roc_dev_expect};
+use roc_repl_expect::run::expect_mono_module_to_dylib;
 use roc_target::TargetInfo;
 use std::env;
 use std::ffi::{CString, OsStr};
@@ -320,8 +318,6 @@ pub enum FormatMode {
     CheckOnly,
 }
 
-const SHM_SIZE: i64 = 1024;
-
 pub fn test(matches: &ArgMatches, triple: Triple) -> io::Result<i32> {
     let start_time = Instant::now();
     let arena = Bump::new();
@@ -407,13 +403,6 @@ pub fn test(matches: &ArgMatches, triple: Triple) -> io::Result<i32> {
 
     let mut writer = std::io::stdout();
 
-    let mut shared_buffer = vec![0u8; SHM_SIZE as usize];
-
-    let set_shared_buffer = run_roc_dylib!(lib, "set_shared_buffer", (*mut u8, usize), ());
-    let mut result = RocCallResult::default();
-    let slice = (shared_buffer.as_mut_ptr(), shared_buffer.len());
-    unsafe { set_shared_buffer(slice, &mut result) };
-
     let (failed, passed) = roc_repl_expect::run::run_expects(
         &mut writer,
         roc_reporting::report::RenderTarget::ColorTerminal,
@@ -421,7 +410,6 @@ pub fn test(matches: &ArgMatches, triple: Triple) -> io::Result<i32> {
         interns,
         &lib,
         &mut expectations,
-        shared_buffer.as_mut_ptr(),
         expects,
     )
     .unwrap();
@@ -953,94 +941,13 @@ impl ExecutableFile {
 // with Expect
 #[cfg(target_family = "unix")]
 unsafe fn roc_run_native_debug(
-    executable: ExecutableFile,
+    _executable: ExecutableFile,
-    argv: &[*const c_char],
+    _argv: &[*const c_char],
-    envp: &[*const c_char],
+    _envp: &[*const c_char],
-    mut expectations: VecMap<ModuleId, Expectations>,
+    _expectations: VecMap<ModuleId, Expectations>,
-    interns: Interns,
+    _interns: Interns,
 ) {
-    use signal_hook::{consts::signal::SIGCHLD, consts::signal::SIGUSR1, iterator::Signals};
+    todo!()
-
-    let mut signals = Signals::new(&[SIGCHLD, SIGUSR1]).unwrap();
-
-    match libc::fork() {
-        0 => {
-            // we are the child
-
-            if executable.execve(argv, envp) < 0 {
-                // Get the current value of errno
-                let e = errno::errno();
-
-                // Extract the error code as an i32
-                let code = e.0;
-
-                // Display a human-friendly error message
-                println!("💥 Error {}: {}", code, e);
-            }
-        }
-        -1 => {
-            // something failed
-
-            // Get the current value of errno
-            let e = errno::errno();
-
-            // Extract the error code as an i32
-            let code = e.0;
-
-            // Display a human-friendly error message
-            println!("Error {}: {}", code, e);
-
-            process::exit(1)
-        }
-        1.. => {
-            let name = "/roc_expect_buffer"; // IMPORTANT: shared memory object names must begin with / and contain no other slashes!
-            let cstring = CString::new(name).unwrap();
-
-            let arena = &bumpalo::Bump::new();
-            let interns = arena.alloc(interns);
-
-            for sig in &mut signals {
-                match sig {
-                    SIGCHLD => {
-                        // clean up
-                        libc::shm_unlink(cstring.as_ptr().cast());
-
-                        // done!
-                        process::exit(0);
-                    }
-                    SIGUSR1 => {
-                        // this is the signal we use for an expect failure. Let's see what the child told us
-                        let shared_fd =
-                            libc::shm_open(cstring.as_ptr().cast(), libc::O_RDONLY, 0o666);
-
-                        libc::ftruncate(shared_fd, SHM_SIZE);
-
-                        let shared_ptr = libc::mmap(
-                            std::ptr::null_mut(),
-                            SHM_SIZE as usize,
-                            libc::PROT_READ,
-                            libc::MAP_SHARED,
-                            shared_fd,
-                            0,
-                        );
-
-                        let shared_memory_ptr: *mut u8 = shared_ptr.cast();
-
-                        roc_dev_expect(
-                            &mut std::io::stdout(),
-                            arena,
-                            &mut expectations,
-                            interns,
-                            shared_memory_ptr,
-                        )
-                        .unwrap();
-                    }
-                    _ => println!("received signal {}", sig),
-                }
-            }
-        }
-        _ => unreachable!(),
-    }
 }
 
 #[cfg(target_os = "linux")]

crates/compiler/alias_analysis/src/lib.rs

@@ -463,6 +463,7 @@ fn stmt_spec<'a>(
             builder.add_choice(block, &cases)
         }
         Expect { remainder, .. } => stmt_spec(builder, env, block, layout, remainder),
+        ExpectFx { remainder, .. } => stmt_spec(builder, env, block, layout, remainder),
         Ret(symbol) => Ok(env.symbols[symbol]),
         Refcounting(modify_rc, continuation) => match modify_rc {
             ModifyRc::Inc(symbol, _) => {

crates/compiler/builtins/bitcode/src/expect.zig

@@ -1,10 +1,5 @@
 const std = @import("std");
 
-extern fn shm_open(name: *const i8, oflag: c_int, mode: c_uint) c_int;
-extern fn mmap(addr: ?*anyopaque, length: c_uint, prot: c_int, flags: c_int, fd: c_int, offset: c_uint) *anyopaque;
-extern fn kill(pid: c_int, sig: c_int) c_int;
-extern fn getppid() c_int;
-
 const SIGUSR1: c_int = 10;
 
 const O_RDWR: c_int = 2;
@@ -26,9 +21,3 @@ pub fn setSharedBuffer(ptr: [*]u8, length: usize) callconv(.C) usize {
 pub fn expectFailedStart() callconv(.C) [*]u8 {
     return SHARED_BUFFER.ptr;
 }
-
-pub fn expectFailedFinalize() callconv(.C) void {
-    const parent_pid = getppid();
-
-    _ = kill(parent_pid, SIGUSR1);
-}

crates/compiler/builtins/bitcode/src/main.zig

@@ -168,7 +168,6 @@ comptime {
 
     if (builtin.target.cpu.arch != .wasm32) {
         exportUtilsFn(expect.expectFailedStart, "expect_failed_start");
-        exportUtilsFn(expect.expectFailedFinalize, "expect_failed_finalize");
 
         // sets the buffer used for expect failures
         @export(expect.setSharedBuffer, .{ .name = "set_shared_buffer", .linkage = .Weak });

crates/compiler/can/src/copy.rs

@@ -613,6 +613,16 @@ fn deep_copy_expr_help<C: CopyEnv>(env: &mut C, copied: &mut Vec<Variable>, expr
             lookups_in_cond: lookups_in_cond.to_vec(),
         },
 
+        ExpectFx {
+            loc_condition,
+            loc_continuation,
+            lookups_in_cond,
+        } => ExpectFx {
+            loc_condition: Box::new(loc_condition.map(|e| go_help!(e))),
+            loc_continuation: Box::new(loc_continuation.map(|e| go_help!(e))),
+            lookups_in_cond: lookups_in_cond.to_vec(),
+        },
+
         TypedHole(v) => TypedHole(sub!(*v)),
 
         RuntimeError(err) => RuntimeError(err.clone()),

crates/compiler/can/src/expr.rs

@@ -231,6 +231,13 @@ pub enum Expr {
         lookups_in_cond: Vec<(Symbol, Variable)>,
     },
 
+    // not parsed, but is generated when lowering toplevel effectful expects
+    ExpectFx {
+        loc_condition: Box<Loc<Expr>>,
+        loc_continuation: Box<Loc<Expr>>,
+        lookups_in_cond: Vec<(Symbol, Variable)>,
+    },
+
     /// Rendered as empty box in editor
     TypedHole(Variable),
 
@@ -277,6 +284,7 @@ impl Expr {
                 Category::OpaqueWrap(opaque_name)
             }
             Self::Expect { .. } => Category::Expect,
+            Self::ExpectFx { .. } => Category::Expect,
 
             // these nodes place no constraints on the expression's type
             Self::TypedHole(_) | Self::RuntimeError(..) => Category::Unknown,
@@ -1772,6 +1780,28 @@ pub fn inline_calls(var_store: &mut VarStore, scope: &mut Scope, expr: Expr) ->
             }
         }
 
+        ExpectFx {
+            loc_condition,
+            loc_continuation,
+            lookups_in_cond,
+        } => {
+            let loc_condition = Loc {
+                region: loc_condition.region,
+                value: inline_calls(var_store, scope, loc_condition.value),
+            };
+
+            let loc_continuation = Loc {
+                region: loc_continuation.region,
+                value: inline_calls(var_store, scope, loc_continuation.value),
+            };
+
+            ExpectFx {
+                loc_condition: Box::new(loc_condition),
+                loc_continuation: Box::new(loc_continuation),
+                lookups_in_cond,
+            }
+        }
+
         LetRec(defs, loc_expr, mark) => {
             let mut new_defs = Vec::with_capacity(defs.len());
 
@@ -2327,7 +2357,7 @@ impl Declarations {
     ) -> usize {
         let index = self.declarations.len();
 
-        self.declarations.push(DeclarationTag::Expectation);
+        self.declarations.push(DeclarationTag::ExpectationFx);
         self.variables.push(Variable::BOOL);
         self.symbols.push(Loc::at(preceding_comment, name));
         self.annotations.push(None);
@@ -2513,13 +2543,13 @@ impl Declarations {
                 }
                 Expectation => {
                     let loc_expr =
-                        toplevel_expect_to_inline_expect(self.expressions[index].clone());
+                        toplevel_expect_to_inline_expect_pure(self.expressions[index].clone());
 
                     collector.visit_expr(&loc_expr.value, loc_expr.region, var);
                 }
                 ExpectationFx => {
                     let loc_expr =
-                        toplevel_expect_to_inline_expect(self.expressions[index].clone());
+                        toplevel_expect_to_inline_expect_fx(self.expressions[index].clone());
 
                     collector.visit_expr(&loc_expr.value, loc_expr.region, var);
                 }
@@ -2658,6 +2688,9 @@ fn get_lookup_symbols(expr: &Expr, var_store: &mut VarStore) -> Vec<(Symbol, Var
             }
             Expr::Expect {
                 loc_continuation, ..
+            }
+            | Expr::ExpectFx {
+                loc_continuation, ..
             } => {
                 stack.push(&(*loc_continuation).value);
 
@@ -2705,7 +2738,15 @@ fn get_lookup_symbols(expr: &Expr, var_store: &mut VarStore) -> Vec<(Symbol, Var
 /// This is supposed to happen just before monomorphization:
 /// all type errors and such are generated from the user source,
 /// but this transformation means that we don't need special codegen for toplevel expects
-pub fn toplevel_expect_to_inline_expect(mut loc_expr: Loc<Expr>) -> Loc<Expr> {
+pub fn toplevel_expect_to_inline_expect_pure(loc_expr: Loc<Expr>) -> Loc<Expr> {
+    toplevel_expect_to_inline_expect_help(loc_expr, false)
+}
+
+pub fn toplevel_expect_to_inline_expect_fx(loc_expr: Loc<Expr>) -> Loc<Expr> {
+    toplevel_expect_to_inline_expect_help(loc_expr, true)
+}
+
+fn toplevel_expect_to_inline_expect_help(mut loc_expr: Loc<Expr>, has_effects: bool) -> Loc<Expr> {
     enum StoredDef {
         NonRecursive(Region, Box<Def>),
         Recursive(Region, Vec<Def>, IllegalCycleMark),
@@ -2735,10 +2776,18 @@ pub fn toplevel_expect_to_inline_expect(mut loc_expr: Loc<Expr>) -> Loc<Expr> {
     }
 
     let expect_region = loc_expr.region;
-    let expect = Expr::Expect {
+    let expect = if has_effects {
-        loc_condition: Box::new(loc_expr),
+        Expr::ExpectFx {
-        loc_continuation: Box::new(Loc::at_zero(Expr::EmptyRecord)),
+            loc_condition: Box::new(loc_expr),
-        lookups_in_cond,
+            loc_continuation: Box::new(Loc::at_zero(Expr::EmptyRecord)),
+            lookups_in_cond,
+        }
+    } else {
+        Expr::Expect {
+            loc_condition: Box::new(loc_expr),
+            loc_continuation: Box::new(Loc::at_zero(Expr::EmptyRecord)),
+            lookups_in_cond,
+        }
     };
 
     let mut loc_expr = Loc::at(expect_region, expect);
@@ -2764,14 +2813,21 @@ struct ExpectCollector {
 
 impl crate::traverse::Visitor for ExpectCollector {
     fn visit_expr(&mut self, expr: &Expr, _region: Region, var: Variable) {
-        if let Expr::Expect {
+        match expr {
-            lookups_in_cond,
+            Expr::Expect {
-            loc_condition,
+                lookups_in_cond,
-            ..
+                loc_condition,
-        } = expr
+                ..
-        {
+            }
-            self.expects
+            | Expr::ExpectFx {
-                .insert(loc_condition.region, lookups_in_cond.to_vec());
+                lookups_in_cond,
+                loc_condition,
+                ..
+            } => {
+                self.expects
+                    .insert(loc_condition.region, lookups_in_cond.to_vec());
+            }
+            _ => (),
         }
 
         walk_expr(self, expr, var)

crates/compiler/can/src/module.rs

@@ -964,6 +964,23 @@ fn fix_values_captured_in_closure_expr(
             );
         }
 
+        ExpectFx {
+            loc_condition,
+            loc_continuation,
+            lookups_in_cond: _,
+        } => {
+            fix_values_captured_in_closure_expr(
+                &mut loc_condition.value,
+                no_capture_symbols,
+                closure_captures,
+            );
+            fix_values_captured_in_closure_expr(
+                &mut loc_continuation.value,
+                no_capture_symbols,
+                closure_captures,
+            );
+        }
+
         Closure(ClosureData {
             captured_symbols,
             name,

crates/compiler/can/src/traverse.rs

@@ -276,6 +276,19 @@ pub fn walk_expr<V: Visitor>(visitor: &mut V, expr: &Expr, var: Variable) {
                 Variable::NULL,
             );
         }
+        Expr::ExpectFx {
+            loc_condition,
+            loc_continuation,
+            lookups_in_cond: _,
+        } => {
+            // TODO: what type does an expect have? bool
+            visitor.visit_expr(&loc_condition.value, loc_condition.region, Variable::NULL);
+            visitor.visit_expr(
+                &loc_continuation.value,
+                loc_continuation.region,
+                Variable::NULL,
+            );
+        }
         Expr::TypedHole(_) => { /* terminal */ }
         Expr::RuntimeError(..) => { /* terminal */ }
     }

crates/compiler/constrain/src/expr.rs

@@ -520,6 +520,53 @@ pub fn constrain_expr(
             constraints.exists_many(vars, all_constraints)
         }
 
+        ExpectFx {
+            loc_condition,
+            loc_continuation,
+            lookups_in_cond,
+        } => {
+            let expect_bool = |region| {
+                let bool_type = Type::Variable(Variable::BOOL);
+                Expected::ForReason(Reason::ExpectCondition, bool_type, region)
+            };
+
+            let cond_con = constrain_expr(
+                constraints,
+                env,
+                loc_condition.region,
+                &loc_condition.value,
+                expect_bool(loc_condition.region),
+            );
+
+            let continuation_con = constrain_expr(
+                constraints,
+                env,
+                loc_continuation.region,
+                &loc_continuation.value,
+                expected,
+            );
+
+            // + 2 for cond_con and continuation_con
+            let mut all_constraints = Vec::with_capacity(lookups_in_cond.len() + 2);
+
+            all_constraints.push(cond_con);
+            all_constraints.push(continuation_con);
+
+            let mut vars = Vec::with_capacity(lookups_in_cond.len());
+
+            for (symbol, var) in lookups_in_cond.iter() {
+                vars.push(*var);
+
+                all_constraints.push(constraints.lookup(
+                    *symbol,
+                    NoExpectation(Type::Variable(*var)),
+                    Region::zero(),
+                ));
+            }
+
+            constraints.exists_many(vars, all_constraints)
+        }
+
         If {
             cond_var,
             branch_var,

crates/compiler/gen_dev/src/lib.rs

@@ -1004,7 +1004,8 @@ trait Backend<'a> {
                 }
             }
 
-            Stmt::Expect { .. } => todo!("expect is not implemented in the wasm backend"),
+            Stmt::Expect { .. } => todo!("expect is not implemented in the dev backend"),
+            Stmt::ExpectFx { .. } => todo!("expect-fx is not implemented in the dev backend"),
 
             Stmt::RuntimeError(_) => {}
         }

crates/compiler/gen_llvm/src/llvm/build.rs

@@ -199,10 +199,6 @@ impl LlvmBackendMode {
             LlvmBackendMode::CliTest => true,
         }
     }
-
-    fn runs_expects_in_separate_process(self) -> bool {
-        false
-    }
 }
 
 pub struct Env<'a, 'ctx, 'env> {
@@ -2802,15 +2798,67 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
                             lookups,
                         );
 
-                        // NOTE: signals to the parent process that an expect failed
+                        bd.build_unconditional_branch(then_block);
-                        if env.mode.runs_expects_in_separate_process() {
+                    }
-                            let func = env
+                    roc_target::PtrWidth::Bytes4 => {
-                                .module
+                        // temporary WASM implementation
-                                .get_function(bitcode::UTILS_EXPECT_FAILED_FINALIZE)
+                        throw_exception(env, "An expectation failed!");
-                                .unwrap();
+                    }
+                }
+            } else {
+                bd.position_at_end(throw_block);
+                bd.build_unconditional_branch(then_block);
+            }
 
-                            bd.build_call(func, &[], "call_expect_finalize_failed");
+            bd.position_at_end(then_block);
-                        }
+
+            build_exp_stmt(
+                env,
+                layout_ids,
+                func_spec_solutions,
+                scope,
+                parent,
+                remainder,
+            )
+        }
+
+        ExpectFx {
+            condition: cond_symbol,
+            region,
+            lookups,
+            layouts: _,
+            remainder,
+        } => {
+            let bd = env.builder;
+            let context = env.context;
+
+            let (cond, _cond_layout) = load_symbol_and_layout(scope, cond_symbol);
+
+            let condition = bd.build_int_compare(
+                IntPredicate::EQ,
+                cond.into_int_value(),
+                context.bool_type().const_int(1, false),
+                "is_true",
+            );
+
+            let then_block = context.append_basic_block(parent, "then_block");
+            let throw_block = context.append_basic_block(parent, "throw_block");
+
+            bd.build_conditional_branch(condition, then_block, throw_block);
+
+            if env.mode.runs_expects() {
+                bd.position_at_end(throw_block);
+
+                match env.target_info.ptr_width() {
+                    roc_target::PtrWidth::Bytes8 => {
+                        clone_to_shared_memory(
+                            env,
+                            scope,
+                            layout_ids,
+                            *cond_symbol,
+                            *region,
+                            lookups,
+                        );
 
                         bd.build_unconditional_branch(then_block);
                     }

crates/compiler/gen_wasm/src/backend.rs

@@ -708,6 +708,7 @@ impl<'a> WasmBackend<'a> {
             Stmt::Refcounting(modify, following) => self.stmt_refcounting(modify, following),
 
             Stmt::Expect { .. } => todo!("expect is not implemented in the wasm backend"),
+            Stmt::ExpectFx { .. } => todo!("expect-fx is not implemented in the wasm backend"),
 
             Stmt::RuntimeError(msg) => self.stmt_runtime_error(msg),
         }

crates/compiler/load_internal/src/file.rs

@@ -707,6 +707,12 @@ struct LateSpecializationsModule<'a> {
     procs_base: ProcsBase<'a>,
 }
 
+#[derive(Debug, Default)]
+pub struct ToplevelExpects {
+    pub pure: VecMap<Symbol, Region>,
+    pub fx: VecMap<Symbol, Region>,
+}
+
 #[derive(Debug)]
 pub struct MonomorphizedModule<'a> {
     pub module_id: ModuleId,
@@ -716,7 +722,7 @@ pub struct MonomorphizedModule<'a> {
     pub can_problems: MutMap<ModuleId, Vec<roc_problem::can::Problem>>,
     pub type_problems: MutMap<ModuleId, Vec<TypeError>>,
     pub procedures: MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
-    pub toplevel_expects: VecMap<Symbol, Region>,
+    pub toplevel_expects: ToplevelExpects,
     pub entry_point: EntryPoint<'a>,
     pub exposed_to_host: ExposedToHost,
     pub sources: MutMap<ModuleId, (PathBuf, Box<str>)>,
@@ -821,7 +827,7 @@ enum Msg<'a> {
         solved_subs: Solved<Subs>,
         module_timing: ModuleTiming,
         abilities_store: AbilitiesStore,
-        toplevel_expects: VecMap<Symbol, Region>,
+        toplevel_expects: ToplevelExpects,
     },
     MadeSpecializations {
         module_id: ModuleId,
@@ -908,7 +914,7 @@ struct State<'a> {
     pub module_cache: ModuleCache<'a>,
     pub dependencies: Dependencies<'a>,
     pub procedures: MutMap<(Symbol, ProcLayout<'a>), Proc<'a>>,
-    pub toplevel_expects: VecMap<Symbol, Region>,
+    pub toplevel_expects: ToplevelExpects,
     pub exposed_to_host: ExposedToHost,
 
     /// This is the "final" list of IdentIds, after canonicalization and constraint gen
@@ -981,7 +987,7 @@ impl<'a> State<'a> {
             module_cache: ModuleCache::default(),
             dependencies,
             procedures: MutMap::default(),
-            toplevel_expects: VecMap::default(),
+            toplevel_expects: ToplevelExpects::default(),
             exposed_to_host: ExposedToHost::default(),
             exposed_types,
             arc_modules,
@@ -2527,7 +2533,8 @@ fn update<'a>(
 
             let subs = solved_subs.into_inner();
 
-            state.toplevel_expects.extend(toplevel_expects);
+            state.toplevel_expects.pure.extend(toplevel_expects.pure);
+            state.toplevel_expects.fx.extend(toplevel_expects.fx);
 
             state
                 .module_cache
@@ -4818,7 +4825,7 @@ fn build_pending_specializations<'a>(
     let find_specializations_start = Instant::now();
 
     let mut module_thunks = bumpalo::collections::Vec::new_in(arena);
-    let mut toplevel_expects = VecMap::default();
+    let mut toplevel_expects = ToplevelExpects::default();
 
     let mut procs_base = ProcsBase {
         partial_procs: BumpMap::default(),
@@ -5111,7 +5118,7 @@ fn build_pending_specializations<'a>(
                     );
                 }
 
-                let body = roc_can::expr::toplevel_expect_to_inline_expect(body);
+                let body = roc_can::expr::toplevel_expect_to_inline_expect_pure(body);
 
                 let proc = PartialProc {
                     annotation: expr_var,
@@ -5131,7 +5138,7 @@ fn build_pending_specializations<'a>(
                 let expr_region = declarations.expressions[index].region;
                 let region = Region::span_across(&name_region, &expr_region);
 
-                toplevel_expects.insert(symbol, region);
+                toplevel_expects.pure.insert(symbol, region);
                 procs_base.partial_procs.insert(symbol, proc);
             }
             ExpectationFx => {
@@ -5187,7 +5194,7 @@ fn build_pending_specializations<'a>(
                     );
                 }
 
-                let body = roc_can::expr::toplevel_expect_to_inline_expect(body);
+                let body = roc_can::expr::toplevel_expect_to_inline_expect_fx(body);
 
                 let proc = PartialProc {
                     annotation: expr_var,
@@ -5207,7 +5214,7 @@ fn build_pending_specializations<'a>(
                 let expr_region = declarations.expressions[index].region;
                 let region = Region::span_across(&name_region, &expr_region);
 
-                toplevel_expects.insert(symbol, region);
+                toplevel_expects.fx.insert(symbol, region);
                 procs_base.partial_procs.insert(symbol, proc);
             }
         }

crates/compiler/mono/src/borrow.rs

@@ -309,6 +309,7 @@ impl<'a> ParamMap<'a> {
                 }
 
                 Expect { remainder, .. } => stack.push(remainder),
+                ExpectFx { remainder, .. } => stack.push(remainder),
 
                 Switch {
                     branches,
@@ -820,6 +821,10 @@ impl<'a> BorrowInfState<'a> {
                 self.collect_stmt(param_map, remainder);
             }
 
+            ExpectFx { remainder, .. } => {
+                self.collect_stmt(param_map, remainder);
+            }
+
             Refcounting(_, _) => unreachable!("these have not been introduced yet"),
 
             Ret(_) | RuntimeError(_) => {
@@ -988,6 +993,7 @@ fn call_info_stmt<'a>(arena: &'a Bump, stmt: &Stmt<'a>, info: &mut CallInfo<'a>)
             }
 
             Expect { remainder, .. } => stack.push(remainder),
+            ExpectFx { remainder, .. } => stack.push(remainder),
 
             Refcounting(_, _) => unreachable!("these have not been introduced yet"),
 

crates/compiler/mono/src/inc_dec.rs

@@ -119,6 +119,17 @@ pub fn occurring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>)
                 stack.push(remainder);
             }
 
+            ExpectFx {
+                condition,
+                remainder,
+                lookups,
+                ..
+            } => {
+                result.insert(*condition);
+                result.extend(lookups.iter().copied());
+                stack.push(remainder);
+            }
+
             Jump(_, arguments) => {
                 result.extend(arguments.iter().copied());
             }
@@ -671,8 +682,8 @@ impl<'a> Context<'a> {
 
                     match ownership {
                         DataBorrowedFunctionOwns => {
-                            // the data is borrowed;
+                            // the data is borrowed; increment it to own the values so the function
-                            // increment it to own the values so the function can use them
+                            // can use them
                             let rc = Stmt::Refcounting(ModifyRc::Inc(argument, 1), stmt);
 
                             stmt = self.arena.alloc(rc);
@@ -690,8 +701,8 @@ impl<'a> Context<'a> {
             }};
         }
 
-        // incrementing/consuming the closure (if needed) is done by the zig implementation.
+        // incrementing/consuming the closure (if needed) is done by the zig implementation. We
-        // We don't want to touch the RC on the roc side, so treat these as borrowed.
+        // don't want to touch the RC on the roc side, so treat these as borrowed.
         const FUNCTION: bool = BORROWED;
         const CLOSURE_DATA: bool = BORROWED;
 
@@ -753,9 +764,9 @@ impl<'a> Context<'a> {
                 handle_ownerships_pre!(Stmt::Let(z, v, l, b), ownerships)
             }
             ListSortWith { xs } => {
-                // NOTE: we may apply the function to the same argument multiple times.
+                // NOTE: we may apply the function to the same argument multiple times. for that to
-                // for that to be valid, the function must borrow its argument. This is not
+                // be valid, the function must borrow its argument. This is not enforced at the
-                // enforced at the moment
+                // moment
                 //
                 // we also don't check that both arguments have the same ownership characteristics
                 let ownerships = [(xs, function_ps[0])];
@@ -768,7 +779,8 @@ impl<'a> Context<'a> {
 
                     match ownership {
                         DataOwnedFunctionOwns => {
-                            // if non-unique, elements have been consumed, must still consume the list itself
+                            // if non-unique, elements have been consumed, must still consume the
+                            // list itself
                             let rc = Stmt::Refcounting(ModifyRc::DecRef(xs), b);
 
                             let condition_stmt = branch_on_list_uniqueness(
@@ -914,8 +926,7 @@ impl<'a> Context<'a> {
             }
 
             EmptyArray | Literal(_) | Reset { .. } | RuntimeErrorFunction(_) => {
-                // EmptyArray is always stack-allocated
+                // EmptyArray is always stack-allocated function pointers are persistent
-                // function pointers are persistent
                 self.arena.alloc(Stmt::Let(z, v, l, b))
             }
         };
@@ -924,8 +935,7 @@ impl<'a> Context<'a> {
     }
 
     fn update_var_info(&self, symbol: Symbol, layout: &Layout<'a>, expr: &Expr<'a>) -> Self {
-        // is this value a constant?
+        // is this value a constant? TODO do function pointers also fall into this category?
-        // TODO do function pointers also fall into this category?
         let persistent = false;
 
         // must this value be consumed?
@@ -979,9 +989,7 @@ impl<'a> Context<'a> {
 
     // Add `dec` instructions for parameters that are
     //
-    //  - references
+    //  - references - not alive in `b` - not borrow.
-    //  - not alive in `b`
-    //  - not borrow.
     //
     // That is, we must make sure these parameters are consumed.
     fn add_dec_for_dead_params(
@@ -1021,9 +1029,9 @@ impl<'a> Context<'a> {
     ) -> (&'a Stmt<'a>, LiveVarSet) {
         use Stmt::*;
 
-        // let-chains can be very long, especially for large (list) literals
+        // let-chains can be very long, especially for large (list) literals in (rust) debug mode,
-        // in (rust) debug mode, this function can overflow the stack for such values
+        // this function can overflow the stack for such values so we have to write an explicit
-        // so we have to write an explicit loop.
+        // loop.
         {
             let mut cont = stmt;
             let mut triples = Vec::new_in(self.arena);
@@ -1199,6 +1207,30 @@ impl<'a> Context<'a> {
                 (expect, b_live_vars)
             }
 
+            ExpectFx {
+                remainder,
+                condition,
+                region,
+                lookups,
+                layouts,
+            } => {
+                let (b, mut b_live_vars) = self.visit_stmt(codegen, remainder);
+
+                let expect = self.arena.alloc(Stmt::ExpectFx {
+                    condition: *condition,
+                    region: *region,
+                    lookups,
+                    layouts,
+                    remainder: b,
+                });
+
+                let expect = self.add_inc_before_consume_all(lookups, expect, &b_live_vars);
+
+                b_live_vars.extend(lookups.iter().copied());
+
+                (expect, b_live_vars)
+            }
+
             RuntimeError(_) | Refcounting(_, _) => (stmt, MutSet::default()),
         }
     }
@@ -1313,6 +1345,17 @@ pub fn collect_stmt(
             collect_stmt(remainder, jp_live_vars, vars)
         }
 
+        ExpectFx {
+            condition,
+            remainder,
+            lookups,
+            ..
+        } => {
+            vars.insert(*condition);
+            vars.extend(lookups.iter().copied());
+            collect_stmt(remainder, jp_live_vars, vars)
+        }
+
         Join {
             id: j,
             parameters,

crates/compiler/mono/src/ir.rs

@@ -1513,6 +1513,14 @@ pub enum Stmt<'a> {
         /// what happens after the expect
         remainder: &'a Stmt<'a>,
     },
+    ExpectFx {
+        condition: Symbol,
+        region: Region,
+        lookups: &'a [Symbol],
+        layouts: &'a [Layout<'a>],
+        /// what happens after the expect
+        remainder: &'a Stmt<'a>,
+    },
     /// a join point `join f <params> = <continuation> in remainder`
     Join {
         id: JoinPointId,
@@ -2100,6 +2108,17 @@ impl<'a> Stmt<'a> {
                 .append(alloc.hardline())
                 .append(remainder.to_doc(alloc)),
 
+            ExpectFx {
+                condition,
+                remainder,
+                ..
+            } => alloc
+                .text("expect-fx ")
+                .append(symbol_to_doc(alloc, *condition))
+                .append(";")
+                .append(alloc.hardline())
+                .append(remainder.to_doc(alloc)),
+
             Ret(symbol) => alloc
                 .text("ret ")
                 .append(symbol_to_doc(alloc, *symbol))
@@ -4163,6 +4182,7 @@ pub fn with_hole<'a>(
         EmptyRecord => let_empty_struct(assigned, hole),
 
         Expect { .. } => unreachable!("I think this is unreachable"),
+        ExpectFx { .. } => unreachable!("I think this is unreachable"),
 
         If {
             cond_var,
@@ -6133,6 +6153,45 @@ pub fn from_can<'a>(
             stmt
         }
 
+        ExpectFx {
+            loc_condition,
+            loc_continuation,
+            lookups_in_cond,
+        } => {
+            let rest = from_can(env, variable, loc_continuation.value, procs, layout_cache);
+            let cond_symbol = env.unique_symbol();
+
+            let lookups = Vec::from_iter_in(lookups_in_cond.iter().map(|t| t.0), env.arena);
+
+            let mut layouts = Vec::with_capacity_in(lookups_in_cond.len(), env.arena);
+
+            for (_, var) in lookups_in_cond {
+                let res_layout = layout_cache.from_var(env.arena, var, env.subs);
+                let layout = return_on_layout_error!(env, res_layout, "Expect");
+                layouts.push(layout);
+            }
+
+            let mut stmt = Stmt::ExpectFx {
+                condition: cond_symbol,
+                region: loc_condition.region,
+                lookups: lookups.into_bump_slice(),
+                layouts: layouts.into_bump_slice(),
+                remainder: env.arena.alloc(rest),
+            };
+
+            stmt = with_hole(
+                env,
+                loc_condition.value,
+                variable,
+                procs,
+                layout_cache,
+                cond_symbol,
+                env.arena.alloc(stmt),
+            );
+
+            stmt
+        }
+
         LetRec(defs, cont, _cycle_mark) => {
             // because Roc is strict, only functions can be recursive!
             for def in defs.into_iter() {
@@ -6497,6 +6556,32 @@ fn substitute_in_stmt_help<'a>(
             Some(arena.alloc(expect))
         }
 
+        ExpectFx {
+            condition,
+            region,
+            lookups,
+            layouts,
+            remainder,
+        } => {
+            let new_remainder =
+                substitute_in_stmt_help(arena, remainder, subs).unwrap_or(remainder);
+
+            let new_lookups = Vec::from_iter_in(
+                lookups.iter().map(|s| substitute(subs, *s).unwrap_or(*s)),
+                arena,
+            );
+
+            let expect = ExpectFx {
+                condition: substitute(subs, *condition).unwrap_or(*condition),
+                region: *region,
+                lookups: new_lookups.into_bump_slice(),
+                layouts,
+                remainder: new_remainder,
+            };
+
+            Some(arena.alloc(expect))
+        }
+
         Jump(id, args) => {
             let mut did_change = false;
             let new_args = Vec::from_iter_in(

crates/compiler/mono/src/reset_reuse.rs

@@ -216,6 +216,31 @@ fn function_s<'a, 'i>(
             }
         }
 
+        ExpectFx {
+            condition,
+            region,
+            lookups,
+            layouts,
+            remainder,
+        } => {
+            let continuation: &Stmt = *remainder;
+            let new_continuation = function_s(env, w, c, continuation);
+
+            if std::ptr::eq(continuation, new_continuation) || continuation == new_continuation {
+                stmt
+            } else {
+                let new_refcounting = ExpectFx {
+                    condition: *condition,
+                    region: *region,
+                    lookups,
+                    layouts,
+                    remainder: new_continuation,
+                };
+
+                arena.alloc(new_refcounting)
+            }
+        }
+
         Ret(_) | Jump(_, _) | RuntimeError(_) => stmt,
     }
 }
@@ -446,6 +471,39 @@ fn function_d_main<'a, 'i>(
                 (arena.alloc(refcounting), found)
             }
         }
+        ExpectFx {
+            condition,
+            region,
+            lookups,
+            layouts,
+            remainder,
+        } => {
+            let (b, found) = function_d_main(env, x, c, remainder);
+
+            if found || *condition != x {
+                let refcounting = ExpectFx {
+                    condition: *condition,
+                    region: *region,
+                    lookups,
+                    layouts,
+                    remainder: b,
+                };
+
+                (arena.alloc(refcounting), found)
+            } else {
+                let b = try_function_s(env, x, c, b);
+
+                let refcounting = ExpectFx {
+                    condition: *condition,
+                    region: *region,
+                    lookups,
+                    layouts,
+                    remainder: b,
+                };
+
+                (arena.alloc(refcounting), found)
+            }
+        }
         Join {
             id,
             parameters,
@@ -618,6 +676,26 @@ fn function_r<'a, 'i>(env: &mut Env<'a, 'i>, stmt: &'a Stmt<'a>) -> &'a Stmt<'a>
             arena.alloc(expect)
         }
 
+        ExpectFx {
+            condition,
+            region,
+            lookups,
+            layouts,
+            remainder,
+        } => {
+            let b = function_r(env, remainder);
+
+            let expect = ExpectFx {
+                condition: *condition,
+                region: *region,
+                lookups,
+                layouts,
+                remainder: b,
+            };
+
+            arena.alloc(expect)
+        }
+
         Ret(_) | Jump(_, _) | RuntimeError(_) => {
             // terminals
             stmt
@@ -653,6 +731,11 @@ fn has_live_var<'a>(jp_live_vars: &JPLiveVarMap, stmt: &'a Stmt<'a>, needle: Sym
             remainder,
             ..
         } => *condition == needle || has_live_var(jp_live_vars, remainder, needle),
+        ExpectFx {
+            condition,
+            remainder,
+            ..
+        } => *condition == needle || has_live_var(jp_live_vars, remainder, needle),
         Join {
             id,
             parameters,

crates/compiler/mono/src/tail_recursion.rs

@@ -273,6 +273,30 @@ fn insert_jumps<'a>(
             None => None,
         },
 
+        ExpectFx {
+            condition,
+            region,
+            lookups,
+            layouts,
+            remainder,
+        } => match insert_jumps(
+            arena,
+            remainder,
+            goal_id,
+            needle,
+            needle_arguments,
+            needle_result,
+        ) {
+            Some(cont) => Some(arena.alloc(ExpectFx {
+                condition: *condition,
+                region: *region,
+                lookups,
+                layouts,
+                remainder: cont,
+            })),
+            None => None,
+        },
+
         Ret(_) => None,
         Jump(_, _) => None,
         RuntimeError(_) => None,

crates/compiler/parse/src/expr.rs

@@ -636,7 +636,7 @@ fn parse_defs_end<'a>(
         let parse_expect_vanilla =
             crate::parser::keyword_e(crate::keyword::EXPECT, EExpect::Expect);
         let parse_expect_fx = crate::parser::keyword_e(crate::keyword::EXPECT_FX, EExpect::Expect);
-        let parse_expect = either!(parse_expect_vanilla, parse_expect_fx);
+        let parse_expect = either!(parse_expect_fx, parse_expect_vanilla);
 
         match space0_after_e(
             crate::pattern::loc_pattern_help(min_indent),
@@ -682,11 +682,11 @@ fn parse_defs_end<'a>(
                         let preceding_comment = Region::new(spaces_before_current_start, start);
 
                         let value_def = match expect_flavor {
-                            Either::First(_) => ValueDef::Expect {
+                            Either::Second(_) => ValueDef::Expect {
                                 condition: arena.alloc(loc_def_expr),
                                 preceding_comment,
                             },
-                            Either::Second(_) => ValueDef::ExpectFx {
+                            Either::First(_) => ValueDef::ExpectFx {
                                 condition: arena.alloc(loc_def_expr),
                                 preceding_comment,
                             },

crates/compiler/test_derive/src/pretty_print.rs

@@ -257,6 +257,7 @@ fn expr<'a>(c: &Ctx, p: EPrec, f: &'a Arena<'a>, e: &'a Expr) -> DocBuilder<'a,
         ZeroArgumentTag { .. } => todo!(),
         OpaqueRef { .. } => todo!(),
         Expect { .. } => todo!(),
+        ExpectFx { .. } => todo!(),
         TypedHole(_) => todo!(),
         RuntimeError(_) => todo!(),
     }

crates/repl_expect/Cargo.toml

@@ -26,10 +26,11 @@ roc_build = { path = "../compiler/build" }
 
 libloading = "0.7.1"
 inkwell = { path = "../vendor/inkwell" }
+signal-hook = "0.3.14"
+libc = "0.2.106"
 
 [dev-dependencies]
 test_gen = { path = "../compiler/test_gen" }
-libc = "0.2.106"
 roc_build = { path = "../compiler/build", features = ["target-aarch64", "target-x86_64"]  }
 tempfile = "3.2.0"
 indoc = "1.0.7"

crates/repl_expect/src/lib.rs

@@ -141,6 +141,7 @@ mod test {
         const BUFFER_SIZE: usize = 1024;
 
         let mut shared_buffer = [0u8; BUFFER_SIZE];
+        let mut memory = crate::run::ExpectMemory::from_slice(&mut shared_buffer);
 
         // communicate the mmapped name to zig/roc
         let set_shared_buffer = run_roc_dylib!(lib, "set_shared_buffer", (*mut u8, usize), ());
@@ -148,15 +149,15 @@ mod test {
         unsafe { set_shared_buffer((shared_buffer.as_mut_ptr(), BUFFER_SIZE), &mut result) };
 
         let mut writer = Vec::with_capacity(1024);
-        let (_failed, _passed) = crate::run::run_expects(
+        let (_failed, _passed) = crate::run::run_expects_with_memory(
             &mut writer,
             RenderTarget::ColorTerminal,
             arena,
             interns,
             &lib,
             &mut expectations,
-            shared_buffer.as_mut_ptr(),
             expects,
+            &mut memory,
         )
         .unwrap();
 

crates/repl_expect/src/run.rs

@@ -1,9 +1,15 @@
+use std::os::unix::process::parent_id;
+
 use bumpalo::collections::Vec as BumpVec;
 use bumpalo::Bump;
 use inkwell::context::Context;
 use roc_build::link::llvm_module_to_dylib;
 use roc_collections::{MutSet, VecMap};
-use roc_gen_llvm::llvm::{build::LlvmBackendMode, externs::add_default_roc_externs};
+use roc_gen_llvm::{
+    llvm::{build::LlvmBackendMode, externs::add_default_roc_externs},
+    run_roc::RocCallResult,
+    run_roc_dylib,
+};
 use roc_load::{EntryPoint, Expectations, MonomorphizedModule};
 use roc_module::symbol::{Interns, ModuleId, Symbol};
 use roc_mono::ir::OptLevel;
@@ -12,6 +18,67 @@ use roc_reporting::{error::expect::Renderer, report::RenderTarget};
 use roc_target::TargetInfo;
 use target_lexicon::Triple;
 
+pub(crate) struct ExpectMemory<'a> {
+    ptr: *mut u8,
+    length: usize,
+    shm_name: Option<std::ffi::CString>,
+    _marker: std::marker::PhantomData<&'a ()>,
+}
+
+impl<'a> ExpectMemory<'a> {
+    const SHM_SIZE: usize = 1024;
+
+    #[cfg(test)]
+    pub(crate) fn from_slice(slice: &mut [u8]) -> Self {
+        Self {
+            ptr: slice.as_mut_ptr(),
+            length: slice.len(),
+            shm_name: None,
+            _marker: std::marker::PhantomData,
+        }
+    }
+
+    fn create_or_reuse_mmap(shm_name: &str) -> Self {
+        let cstring = std::ffi::CString::new(shm_name).unwrap();
+        Self::mmap_help(cstring, libc::O_RDWR | libc::O_CREAT)
+    }
+
+    fn reuse_mmap(&mut self) -> Option<Self> {
+        let shm_name = self.shm_name.as_ref()?.clone();
+        Some(Self::mmap_help(shm_name, libc::O_RDWR))
+    }
+
+    fn mmap_help(cstring: std::ffi::CString, shm_flags: i32) -> Self {
+        let ptr = unsafe {
+            let shared_fd = libc::shm_open(cstring.as_ptr().cast(), shm_flags, 0o666);
+
+            libc::ftruncate(shared_fd, Self::SHM_SIZE as _);
+
+            libc::mmap(
+                std::ptr::null_mut(),
+                Self::SHM_SIZE,
+                libc::PROT_WRITE | libc::PROT_READ,
+                libc::MAP_SHARED,
+                shared_fd,
+                0,
+            )
+        };
+
+        Self {
+            ptr: ptr.cast(),
+            length: Self::SHM_SIZE,
+            shm_name: Some(cstring),
+            _marker: std::marker::PhantomData,
+        }
+    }
+
+    fn set_shared_buffer(&mut self, lib: &libloading::Library) {
+        let set_shared_buffer = run_roc_dylib!(lib, "set_shared_buffer", (*mut u8, usize), ());
+        let mut result = RocCallResult::default();
+        unsafe { set_shared_buffer((self.ptr, self.length), &mut result) };
+    }
+}
+
 #[allow(clippy::too_many_arguments)]
 pub fn run_expects<W: std::io::Write>(
     writer: &mut W,
@@ -20,21 +87,66 @@ pub fn run_expects<W: std::io::Write>(
     interns: &Interns,
     lib: &libloading::Library,
     expectations: &mut VecMap<ModuleId, Expectations>,
-    shared_ptr: *mut u8,
+    expects: ExpectFunctions<'_>,
-    expects: bumpalo::collections::Vec<'_, ToplevelExpect<'_>>,
+) -> std::io::Result<(usize, usize)> {
+    let shm_name = format!("/roc_expect_buffer_{}", std::process::id());
+    let mut memory = ExpectMemory::create_or_reuse_mmap(&shm_name);
+
+    run_expects_with_memory(
+        writer,
+        render_target,
+        arena,
+        interns,
+        lib,
+        expectations,
+        expects,
+        &mut memory,
+    )
+}
+
+#[allow(clippy::too_many_arguments)]
+pub(crate) fn run_expects_with_memory<W: std::io::Write>(
+    writer: &mut W,
+    render_target: RenderTarget,
+    arena: &Bump,
+    interns: &Interns,
+    lib: &libloading::Library,
+    expectations: &mut VecMap<ModuleId, Expectations>,
+    expects: ExpectFunctions<'_>,
+    memory: &mut ExpectMemory,
 ) -> std::io::Result<(usize, usize)> {
     let mut failed = 0;
     let mut passed = 0;
 
-    for expect in expects {
+    for expect in expects.fx {
-        let result = run_expect(
+        let result = run_expect_fx(
             writer,
             render_target,
             arena,
             interns,
             lib,
             expectations,
-            shared_ptr,
+            memory,
+            expect,
+        )?;
+
+        match result {
+            true => passed += 1,
+            false => failed += 1,
+        }
+    }
+
+    memory.set_shared_buffer(lib);
+
+    for expect in expects.pure {
+        let result = run_expect_pure(
+            writer,
+            render_target,
+            arena,
+            interns,
+            lib,
+            expectations,
+            memory,
             expect,
         )?;
 
@@ -48,23 +160,23 @@ pub fn run_expects<W: std::io::Write>(
 }
 
 #[allow(clippy::too_many_arguments)]
-fn run_expect<W: std::io::Write>(
+fn run_expect_pure<W: std::io::Write>(
     writer: &mut W,
     render_target: RenderTarget,
     arena: &Bump,
     interns: &Interns,
     lib: &libloading::Library,
     expectations: &mut VecMap<ModuleId, Expectations>,
-    shared_ptr: *mut u8,
+    shared_memory: &mut ExpectMemory,
     expect: ToplevelExpect<'_>,
 ) -> std::io::Result<bool> {
     use roc_gen_llvm::try_run_jit_function;
 
-    let sequence = ExpectSequence::new(shared_ptr.cast());
+    let sequence = ExpectSequence::new(shared_memory.ptr.cast());
 
     let result: Result<(), String> = try_run_jit_function!(lib, expect.name, (), |v: ()| v);
 
-    let shared_memory_ptr: *const u8 = shared_ptr.cast();
+    let shared_memory_ptr: *const u8 = shared_memory.ptr.cast();
 
     if result.is_err() || sequence.count_failures() > 0 {
         let module_id = expect.symbol.module_id();
@@ -103,6 +215,104 @@ fn run_expect<W: std::io::Write>(
     }
 }
 
+#[allow(clippy::too_many_arguments)]
+fn run_expect_fx<W: std::io::Write>(
+    writer: &mut W,
+    render_target: RenderTarget,
+    arena: &Bump,
+    interns: &Interns,
+    lib: &libloading::Library,
+    expectations: &mut VecMap<ModuleId, Expectations>,
+    parent_memory: &mut ExpectMemory,
+    expect: ToplevelExpect<'_>,
+) -> std::io::Result<bool> {
+    use signal_hook::{consts::signal::SIGCHLD, consts::signal::SIGUSR1, iterator::Signals};
+
+    let mut signals = Signals::new(&[SIGCHLD, SIGUSR1]).unwrap();
+
+    match unsafe { libc::fork() } {
+        0 => unsafe {
+            // we are the child
+
+            use roc_gen_llvm::try_run_jit_function;
+
+            let mut child_memory = parent_memory.reuse_mmap().unwrap();
+
+            let sequence = ExpectSequence::new(child_memory.ptr);
+
+            child_memory.set_shared_buffer(lib);
+
+            let result: Result<(), String> = try_run_jit_function!(lib, expect.name, (), |v: ()| v);
+
+            if let Err(msg) = result {
+                panic!("roc panic {}", msg);
+            }
+
+            if sequence.count_failures() > 0 {
+                libc::kill(parent_id() as _, SIGUSR1);
+            }
+
+            std::process::exit(0)
+        },
+        -1 => {
+            // something failed
+
+            // Display a human-friendly error message
+            println!("Error {:?}", std::io::Error::last_os_error());
+
+            std::process::exit(1)
+        }
+        1.. => {
+            let mut has_succeeded = true;
+
+            for sig in &mut signals {
+                match sig {
+                    SIGCHLD => {
+                        // done!
+                        return Ok(has_succeeded);
+                    }
+                    SIGUSR1 => {
+                        // this is the signal we use for an expect failure. Let's see what the child told us
+                        has_succeeded = false;
+
+                        let frame =
+                            ExpectFrame::at_offset(parent_memory.ptr, ExpectSequence::START_OFFSET);
+                        let module_id = frame.module_id;
+
+                        let data = expectations.get_mut(&module_id).unwrap();
+                        let filename = data.path.to_owned();
+                        let source = std::fs::read_to_string(&data.path).unwrap();
+
+                        let renderer = Renderer::new(
+                            arena,
+                            interns,
+                            render_target,
+                            module_id,
+                            filename,
+                            &source,
+                        );
+
+                        render_expect_failure(
+                            writer,
+                            &renderer,
+                            arena,
+                            None,
+                            expectations,
+                            interns,
+                            parent_memory.ptr,
+                            ExpectSequence::START_OFFSET,
+                        )?;
+                    }
+                    _ => println!("received signal {}", sig),
+                }
+            }
+
+            Ok(true)
+        }
+        _ => unreachable!(),
+    }
+}
+
 pub fn roc_dev_expect(
     writer: &mut impl std::io::Write,
     arena: &Bump,
@@ -205,8 +415,8 @@ impl ExpectSequence {
     fn new(ptr: *mut u8) -> Self {
         unsafe {
             let ptr = ptr as *mut usize;
-            *ptr.add(Self::COUNT_INDEX) = 0;
+            std::ptr::write_unaligned(ptr.add(Self::COUNT_INDEX), 0);
-            *ptr.add(Self::OFFSET_INDEX) = Self::START_OFFSET;
+            std::ptr::write_unaligned(ptr.add(Self::OFFSET_INDEX), Self::START_OFFSET);
         }
 
         Self {
@@ -251,13 +461,19 @@ pub struct ToplevelExpect<'a> {
     pub region: Region,
 }
 
+#[derive(Debug)]
+pub struct ExpectFunctions<'a> {
+    pub pure: BumpVec<'a, ToplevelExpect<'a>>,
+    pub fx: BumpVec<'a, ToplevelExpect<'a>>,
+}
+
 pub fn expect_mono_module_to_dylib<'a>(
     arena: &'a Bump,
     target: Triple,
     loaded: MonomorphizedModule<'a>,
     opt_level: OptLevel,
     mode: LlvmBackendMode,
-) -> Result<(libloading::Library, BumpVec<'a, ToplevelExpect<'a>>), libloading::Error> {
+) -> Result<(libloading::Library, ExpectFunctions<'a>), libloading::Error> {
     let target_info = TargetInfo::from(&target);
 
     let MonomorphizedModule {
@@ -306,18 +522,25 @@ pub fn expect_mono_module_to_dylib<'a>(
         EntryPoint::Test => None,
     };
 
+    let capacity = toplevel_expects.pure.len() + toplevel_expects.fx.len();
+    let mut expect_symbols = BumpVec::with_capacity_in(capacity, env.arena);
+
+    expect_symbols.extend(toplevel_expects.pure.keys().copied());
+    expect_symbols.extend(toplevel_expects.fx.keys().copied());
+
     let expect_names = roc_gen_llvm::llvm::build::build_procedures_expose_expects(
         &env,
         opt_level,
-        toplevel_expects.unzip_slices().0,
+        &expect_symbols,
         procedures,
         opt_entry_point,
     );
 
-    let expects = bumpalo::collections::Vec::from_iter_in(
+    let expects_fx = bumpalo::collections::Vec::from_iter_in(
         toplevel_expects
+            .fx
             .into_iter()
-            .zip(expect_names.into_iter())
+            .zip(expect_names.iter().skip(toplevel_expects.pure.len()))
             .map(|((symbol, region), name)| ToplevelExpect {
                 symbol,
                 region,
@@ -326,6 +549,24 @@ pub fn expect_mono_module_to_dylib<'a>(
         env.arena,
     );
 
+    let expects_pure = bumpalo::collections::Vec::from_iter_in(
+        toplevel_expects
+            .pure
+            .into_iter()
+            .zip(expect_names.iter())
+            .map(|((symbol, region), name)| ToplevelExpect {
+                symbol,
+                region,
+                name,
+            }),
+        env.arena,
+    );
+
+    let expects = ExpectFunctions {
+        pure: expects_pure,
+        fx: expects_fx,
+    };
+
     env.dibuilder.finalize();
 
     // we don't use the debug info, and it causes weird errors.