diff --git a/crates/fayalite/src/module.rs b/crates/fayalite/src/module.rs
index 87f86cc..1fcb529 100644
--- a/crates/fayalite/src/module.rs
+++ b/crates/fayalite/src/module.rs
@@ -34,6 +34,7 @@ use std::{
     collections::VecDeque,
     convert::Infallible,
     fmt,
+    future::IntoFuture,
     hash::{Hash, Hasher},
     iter::FusedIterator,
     marker::PhantomData,
@@ -1082,7 +1083,7 @@ pub struct ExternModuleBody<
 > {
     pub verilog_name: Interned<str>,
     pub parameters: P,
-    pub simulation: Option<ExternModuleSimulation<Bundle>>,
+    pub simulation: Option<ExternModuleSimulation>,
 }
 
 impl From<ExternModuleBody<Vec<ExternModuleParameter>>> for ExternModuleBody {
@@ -1767,12 +1768,8 @@ impl AssertValidityState {
             ModuleBody::Extern(ExternModuleBody {
                 verilog_name: _,
                 parameters: _,
-                simulation,
-            }) => {
-                if let Some(simulation) = simulation {
-                    simulation.check_io_ty(self.module.io_ty);
-                }
-            }
+                simulation: _,
+            }) => {}
             ModuleBody::Normal(NormalModuleBody { body }) => {
                 let body = self.make_block_index(body);
                 assert_eq!(body, 0);
@@ -2250,6 +2247,17 @@ impl ModuleBuilder {
         }
         *simulation = Some(ExternModuleSimulation::new(generator));
     }
+    #[track_caller]
+    pub fn extern_module_simulation_fn<
+        Args: fmt::Debug + Clone + Hash + Eq + Send + Sync + 'static,
+        Fut: IntoFuture<Output = ()> + 'static,
+    >(
+        &self,
+        args: Args,
+        f: fn(Args, crate::sim::ExternModuleSimulationState) -> Fut,
+    ) {
+        self.extern_module_simulation(crate::sim::SimGeneratorFn { args, f });
+    }
 }
 
 #[track_caller]
diff --git a/crates/fayalite/src/sim.rs b/crates/fayalite/src/sim.rs
index a5d7d13..b12e9a8 100644
--- a/crates/fayalite/src/sim.rs
+++ b/crates/fayalite/src/sim.rs
@@ -60,6 +60,7 @@ use std::{
     collections::BTreeSet,
     fmt,
     future::{Future, IntoFuture},
+    hash::Hash,
     marker::PhantomData,
     mem,
     ops::IndexMut,
@@ -1634,10 +1635,9 @@ impl<T> fmt::Debug for DebugOpaque<T> {
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 struct CompiledExternModule {
-    io_ty: Bundle,
     module_io_targets: Interned<[Target]>,
     module_io: Interned<[CompiledValue<CanonicalType>]>,
-    simulation: ExternModuleSimulation<Bundle>,
+    simulation: ExternModuleSimulation,
 }
 
 #[derive(Debug)]
@@ -4713,7 +4713,6 @@ impl Compiler {
                     );
                 };
                 self.extern_modules.push(CompiledExternModule {
-                    io_ty: module.leaf_module().io_ty(),
                     module_io_targets: module
                         .leaf_module()
                         .module_io()
@@ -6822,8 +6821,7 @@ impl Ord for WaitTarget {
 
 struct SimulationExternModuleState {
     module_state: SimulationModuleState,
-    io_ty: Bundle,
-    sim: ExternModuleSimulation<Bundle>,
+    sim: ExternModuleSimulation,
     running_generator: Option<Pin<Box<dyn Future<Output = ()> + 'static>>>,
     wait_target: Option<WaitTarget>,
 }
@@ -6832,14 +6830,12 @@ impl fmt::Debug for SimulationExternModuleState {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self {
             module_state,
-            io_ty,
             sim,
             running_generator,
             wait_target,
         } = self;
         f.debug_struct("SimulationExternModuleState")
             .field("module_state", module_state)
-            .field("io_ty", io_ty)
             .field("sim", sim)
             .field(
                 "running_generator",
@@ -7008,7 +7004,6 @@ impl SimulationImpl {
         let io_target = Target::from(compiled.io);
         let extern_modules = Box::from_iter(compiled.extern_modules.iter().map(
             |&CompiledExternModule {
-                 io_ty,
                  module_io_targets,
                  module_io,
                  simulation,
@@ -7020,7 +7015,6 @@ impl SimulationImpl {
                             .copied()
                             .zip(module_io.iter().copied()),
                     ),
-                    io_ty,
                     sim: simulation,
                     running_generator: None,
                     wait_target: Some(WaitTarget::Settle),
@@ -7337,12 +7331,10 @@ impl SimulationImpl {
                     extern_module.wait_target = None;
                     let mut generator = if !extern_module.module_state.did_initial_settle {
                         let sim = extern_module.sim;
-                        let io_ty = extern_module.io_ty;
                         drop(this);
                         Box::into_pin(sim.run(ExternModuleSimulationState {
                             sim_impl: this_ref.clone(),
                             module_index,
-                            io_ty,
                         }))
                     } else if let Some(generator) = extern_module.running_generator.take() {
                         drop(this);
@@ -8013,52 +8005,25 @@ impl<T: BundleType> Simulation<T> {
     }
 }
 
-pub struct ExternModuleSimulationState<T: BundleType> {
+pub struct ExternModuleSimulationState {
     sim_impl: Rc<RefCell<SimulationImpl>>,
     module_index: usize,
-    io_ty: T,
 }
 
-impl<T: BundleType> fmt::Debug for ExternModuleSimulationState<T> {
+impl fmt::Debug for ExternModuleSimulationState {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self {
             sim_impl: _,
             module_index,
-            io_ty,
         } = self;
         f.debug_struct("ExternModuleSimulationState")
             .field("sim_impl", &DebugAsDisplay("..."))
             .field("module_index", module_index)
-            .field("io_ty", io_ty)
             .finish()
     }
 }
 
-impl<T: BundleType> ExternModuleSimulationState<T> {
-    pub fn canonical(self) -> ExternModuleSimulationState<Bundle> {
-        let Self {
-            sim_impl,
-            module_index,
-            io_ty,
-        } = self;
-        ExternModuleSimulationState {
-            sim_impl,
-            module_index,
-            io_ty: Bundle::from_canonical(io_ty.canonical()),
-        }
-    }
-    pub fn from_canonical(sim: ExternModuleSimulationState<Bundle>) -> Self {
-        let ExternModuleSimulationState {
-            sim_impl,
-            module_index,
-            io_ty,
-        } = sim;
-        Self {
-            sim_impl,
-            module_index,
-            io_ty: T::from_canonical(io_ty.canonical()),
-        }
-    }
+impl ExternModuleSimulationState {
     pub async fn settle(&mut self) {
         SimulationImpl::yield_advance_time_or_settle(self.sim_impl.clone(), self.module_index, None)
             .await
@@ -8084,39 +8049,74 @@ impl<T: BundleType> ExternModuleSimulationState<T> {
     );
 }
 
-pub trait ExternModuleSimGenerator:
-    Clone + Eq + std::hash::Hash + Any + Send + Sync + fmt::Debug
-{
-    type IOType: BundleType;
+pub trait ExternModuleSimGenerator: Clone + Eq + Hash + Any + Send + Sync + fmt::Debug {
+    fn run<'a>(&'a self, sim: ExternModuleSimulationState) -> impl IntoFuture<Output = ()> + 'a;
+}
 
-    fn run<'a>(
-        &'a self,
-        sim: ExternModuleSimulationState<Self::IOType>,
-    ) -> impl IntoFuture<Output = ()> + 'a;
+pub struct SimGeneratorFn<Args, Fut> {
+    pub args: Args,
+    pub f: fn(Args, ExternModuleSimulationState) -> Fut,
+}
+
+impl<Args: fmt::Debug, Fut> fmt::Debug for SimGeneratorFn<Args, Fut> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self { args, f: _ } = self;
+        f.debug_struct("SimGeneratorFn")
+            .field("args", args)
+            .field("f", &DebugAsDisplay("..."))
+            .finish()
+    }
+}
+
+impl<Args: Hash, Fut> Hash for SimGeneratorFn<Args, Fut> {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        let Self { args, f } = self;
+        args.hash(state);
+        f.hash(state);
+    }
+}
+
+impl<Args: Eq, Fut> Eq for SimGeneratorFn<Args, Fut> {}
+
+impl<Args: PartialEq, Fut> PartialEq for SimGeneratorFn<Args, Fut> {
+    fn eq(&self, other: &Self) -> bool {
+        let Self { args, f } = self;
+        *args == other.args && *f == other.f
+    }
+}
+
+impl<Args: Clone, Fut> Clone for SimGeneratorFn<Args, Fut> {
+    fn clone(&self) -> Self {
+        Self {
+            args: self.args.clone(),
+            f: self.f,
+        }
+    }
+}
+
+impl<Args: Copy, Fut> Copy for SimGeneratorFn<Args, Fut> {}
+
+impl<
+        T: fmt::Debug + Clone + Eq + Hash + Send + Sync + 'static,
+        Fut: IntoFuture<Output = ()> + 'static,
+    > ExternModuleSimGenerator for SimGeneratorFn<T, Fut>
+{
+    fn run<'a>(&'a self, sim: ExternModuleSimulationState) -> impl IntoFuture<Output = ()> + 'a {
+        (self.f)(self.args.clone(), sim)
+    }
 }
 
 trait DynExternModuleSimGenerator: Any + Send + Sync + SupportsPtrEqWithTypeId + fmt::Debug {
-    fn dyn_run<'a>(
-        &'a self,
-        sim: ExternModuleSimulationState<Bundle>,
-    ) -> Box<dyn Future<Output = ()> + 'a>;
-    #[track_caller]
-    fn check_io_ty(&self, io_ty: Bundle);
+    fn dyn_run<'a>(&'a self, sim: ExternModuleSimulationState)
+        -> Box<dyn Future<Output = ()> + 'a>;
 }
 
 impl<T: ExternModuleSimGenerator> DynExternModuleSimGenerator for T {
     fn dyn_run<'a>(
         &'a self,
-        sim: ExternModuleSimulationState<Bundle>,
+        sim: ExternModuleSimulationState,
     ) -> Box<dyn Future<Output = ()> + 'a> {
-        Box::new(
-            self.run(ExternModuleSimulationState::from_canonical(sim))
-                .into_future(),
-        )
-    }
-    #[track_caller]
-    fn check_io_ty(&self, io_ty: Bundle) {
-        T::IOType::from_canonical(io_ty.canonical());
+        Box::new(self.run(sim).into_future())
     }
 }
 
@@ -8129,13 +8129,12 @@ impl InternedCompare for dyn DynExternModuleSimGenerator {
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
-pub struct ExternModuleSimulation<T: BundleType> {
+pub struct ExternModuleSimulation {
     generator: Interned<dyn DynExternModuleSimGenerator>,
     source_location: SourceLocation,
-    _phantom: PhantomData<T>,
 }
 
-impl<T: BundleType> ExternModuleSimulation<T> {
+impl ExternModuleSimulation {
     pub fn new_with_loc<G: ExternModuleSimGenerator>(
         source_location: SourceLocation,
         generator: G,
@@ -8146,48 +8145,13 @@ impl<T: BundleType> ExternModuleSimulation<T> {
                 |v| -> &dyn DynExternModuleSimGenerator { v },
             ),
             source_location,
-            _phantom: PhantomData,
         }
     }
     #[track_caller]
     pub fn new<G: ExternModuleSimGenerator>(generator: G) -> Self {
         Self::new_with_loc(SourceLocation::caller(), generator)
     }
-    pub fn canonical(self) -> ExternModuleSimulation<Bundle> {
-        let Self {
-            generator,
-            source_location,
-            _phantom: _,
-        } = self;
-        ExternModuleSimulation {
-            generator,
-            source_location,
-            _phantom: PhantomData,
-        }
-    }
-    pub fn from_canonical(v: ExternModuleSimulation<Bundle>) -> Self {
-        let ExternModuleSimulation {
-            generator,
-            source_location,
-            _phantom: _,
-        } = v;
-        Self {
-            generator,
-            source_location,
-            _phantom: PhantomData,
-        }
-    }
-}
-
-impl ExternModuleSimulation<Bundle> {
-    fn run(
-        &self,
-        sim: ExternModuleSimulationState<Bundle>,
-    ) -> Box<dyn Future<Output = ()> + 'static> {
+    fn run(&self, sim: ExternModuleSimulationState) -> Box<dyn Future<Output = ()> + 'static> {
         Interned::into_inner(self.generator).dyn_run(sim)
     }
-    #[track_caller]
-    pub fn check_io_ty(self, io_ty: Bundle) {
-        self.generator.check_io_ty(io_ty);
-    }
 }
diff --git a/crates/fayalite/tests/sim.rs b/crates/fayalite/tests/sim.rs
index 0265a7a..e516f22 100644
--- a/crates/fayalite/tests/sim.rs
+++ b/crates/fayalite/tests/sim.rs
@@ -5,14 +5,11 @@ use fayalite::{
     int::UIntValue,
     prelude::*,
     reset::ResetType,
-    sim::{
-        time::SimDuration, vcd::VcdWriterDecls, ExternModuleSimGenerator,
-        ExternModuleSimulationState, Simulation, ToSimValue,
-    },
+    sim::{time::SimDuration, vcd::VcdWriterDecls, Simulation, ToSimValue},
     ty::StaticType,
     util::RcWriter,
 };
-use std::{future::IntoFuture, num::NonZeroUsize};
+use std::num::NonZeroUsize;
 
 #[hdl_module(outline_generated)]
 pub fn connect_const() {
@@ -1453,33 +1450,17 @@ pub fn extern_module() {
     let i: Bool = m.input();
     #[hdl]
     let o: Bool = m.output();
-    #[derive(Clone, Eq, PartialEq, Hash, Debug)]
-    struct Sim {
-        i: Expr<Bool>,
-        o: Expr<Bool>,
-    }
-    impl ExternModuleSimGenerator for Sim {
-        type IOType = extern_module;
-
-        fn run<'a>(
-            &'a self,
-            mut sim: ExternModuleSimulationState<Self::IOType>,
-        ) -> impl IntoFuture<Output = ()> + 'a {
-            let Self { i, o } = *self;
-            async move {
-                sim.write(o, true).await;
-                sim.advance_time(SimDuration::from_nanos(500)).await;
-                let mut invert = false;
-                loop {
-                    sim.advance_time(SimDuration::from_micros(1)).await;
-                    let v = sim.read_bool(i).await;
-                    sim.write(o, v ^ invert).await;
-                    invert = !invert;
-                }
-            }
+    m.extern_module_simulation_fn((i, o), |(i, o), mut sim| async move {
+        sim.write(o, true).await;
+        sim.advance_time(SimDuration::from_nanos(500)).await;
+        let mut invert = false;
+        loop {
+            sim.advance_time(SimDuration::from_micros(1)).await;
+            let v = sim.read_bool(i).await;
+            sim.write(o, v ^ invert).await;
+            invert = !invert;
         }
-    }
-    m.extern_module_simulation(Sim { i, o });
+    });
 }
 
 #[test]
diff --git a/crates/fayalite/tests/sim/expected/extern_module.txt b/crates/fayalite/tests/sim/expected/extern_module.txt
index cb575a5..23af0b2 100644
--- a/crates/fayalite/tests/sim/expected/extern_module.txt
+++ b/crates/fayalite/tests/sim/expected/extern_module.txt
@@ -128,31 +128,27 @@ Simulation {
                 },
                 did_initial_settle: true,
             },
-            io_ty: Bundle {
-                #[hdl(flip)] /* offset = 0 */
-                i: Bool,
-                /* offset = 1 */
-                o: Bool,
-            },
             sim: ExternModuleSimulation {
-                generator: Sim {
-                    i: ModuleIO {
-                        name: extern_module::i,
-                        is_input: true,
-                        ty: Bool,
-                        ..
-                    },
-                    o: ModuleIO {
-                        name: extern_module::o,
-                        is_input: false,
-                        ty: Bool,
-                        ..
-                    },
+                generator: SimGeneratorFn {
+                    args: (
+                        ModuleIO {
+                            name: extern_module::i,
+                            is_input: true,
+                            ty: Bool,
+                            ..
+                        },
+                        ModuleIO {
+                            name: extern_module::o,
+                            is_input: false,
+                            ty: Bool,
+                            ..
+                        },
+                    ),
+                    f: ...,
                 },
                 source_location: SourceLocation(
                     module-XXXXXXXXXX.rs:4:1,
                 ),
-                _phantom: PhantomData<fayalite::bundle::Bundle>,
             },
             running_generator: Some(
                 ...,
diff --git a/crates/fayalite/visit_types.json b/crates/fayalite/visit_types.json
index 451dc90..ff2050a 100644
--- a/crates/fayalite/visit_types.json
+++ b/crates/fayalite/visit_types.json
@@ -1274,8 +1274,7 @@
         "ExternModuleSimulation": {
             "data": {
                 "$kind": "Opaque"
-            },
-            "generics": "<T: BundleType>"
+            }
         }
     }
 }
\ No newline at end of file