diff --git a/ir/BUILD b/ir/BUILD
index b96669c8..339404ee 100644
--- a/ir/BUILD
+++ b/ir/BUILD
@@ -105,6 +105,7 @@ cc_library(
         "//parser:parse_tree",
         "//parser:parse_tree_node_kind_xmacro",
         "//type",
+        "@asoffer_jasmin//jasmin:execute",
         "@asoffer_jasmin//jasmin:value_stack",
         "@asoffer_nth//nth/debug",
         "@asoffer_nth//nth/debug/log",
diff --git a/ir/builtin_module.cc b/ir/builtin_module.cc
index 3532dd7d..bf1214d3 100644
--- a/ir/builtin_module.cc
+++ b/ir/builtin_module.cc
@@ -30,6 +30,12 @@ nth::NoDestructor<IrFunction> Foreign([] {
   return f;
 }());
 
+nth::NoDestructor<IrFunction> ForeignType([] {
+  IrFunction f(1, 1);
+  f.append<jasmin::Return>();
+  return f;
+}());
+
 Module BuiltinModule(GlobalFunctionRegistry& registry) {
   uint32_t next_id = 0;
 
@@ -47,10 +53,11 @@ Module BuiltinModule(GlobalFunctionRegistry& registry) {
   registry.Register(FunctionId(ModuleId::Builtin(), LocalFunctionId(next_id++)),
                     &*PrintFn);
 
-  m.Insert(resources.IdentifierIndex("foreign"),
-           {.qualified_type = type::QualifiedType(
-                type::Qualifier::Constant(), type::GenericFunction(nullptr)),
-            .value = {}});
+  m.Insert(
+      resources.IdentifierIndex("foreign"),
+      {.qualified_type = type::QualifiedType(
+           type::Qualifier::Constant(), type::GenericFunction(&*ForeignType)),
+       .value = {}});
   registry.Register(FunctionId(ModuleId::Builtin(), LocalFunctionId(next_id++)),
                     &*Foreign);
 
diff --git a/ir/emit.cc b/ir/emit.cc
index 5d9fc31e..bbb40f8e 100644
--- a/ir/emit.cc
+++ b/ir/emit.cc
@@ -179,6 +179,7 @@ void EmitContext::Push(jasmin::Value v, type::Type t) {
 void EmitIr(nth::interval<ParseTree::Node::Index> node_range, EmitContext& context) {
   ParseTree::Node::Index start = node_range.lower_bound();
   for (auto const& [range, value_stack] : context.constants) {
+    if (range.lower_bound() < start) { continue; }
     EmitNonConstant(nth::interval(start, range.lower_bound()), context);
     // TODO: This type is wrong.
     for (jasmin::Value const& v : value_stack) { context.Push(v, type::Bool); }
@@ -188,15 +189,14 @@ void EmitIr(nth::interval<ParseTree::Node::Index> node_range, EmitContext& conte
   context.function_stack.back()->append<jasmin::Return>();
 }
 
-void Evaluate(nth::interval<ParseTree::Node::Index> subtree,
-              ParseTree const& tree,
-              DependentModules const& modules NTH_ATTRIBUTE(lifetimebound),
-              jasmin::ValueStack& value_stack) {
+void EmitContext::Evaluate(nth::interval<ParseTree::Node::Index> subtree,
+                           jasmin::ValueStack& value_stack) {
   IrFunction f(0, 1);
-  EmitContext context(tree, modules, f);
-  EmitIr(subtree, context);
-  context.function_stack.back()->append<jasmin::Return>();
+  function_stack.push_back(&f);
+  EmitIr(subtree, *this);
+  f.append<jasmin::Return>();
   jasmin::Execute(f, value_stack);
+  function_stack.pop_back();
 }
 
 }  // namespace ic
diff --git a/ir/emit.h b/ir/emit.h
index 57fdd8db..0721e21d 100644
--- a/ir/emit.h
+++ b/ir/emit.h
@@ -32,6 +32,9 @@ struct EmitContext {
 
   void Push(jasmin::Value v, type::Type);
 
+  void Evaluate(nth::interval<ParseTree::Node::Index> subtree,
+                jasmin::ValueStack& value_stack);
+
   ParseTree::Node const& Node(ParseTree::Node::Index index) {
     return tree[index];
   }
@@ -69,11 +72,6 @@ struct EmitContext {
 void EmitIr(nth::interval<ParseTree::Node::Index> node_range,
             EmitContext& context);
 
-void Evaluate(nth::interval<ParseTree::Node::Index> subtree,
-              ParseTree const& tree,
-              DependentModules const& modules NTH_ATTRIBUTE(lifetimebound),
-              jasmin::ValueStack& value_stack);
-
 }  // namespace ic
 
 #endif  // ICARUS_IR_EMIT_H
diff --git a/ir/ir.cc b/ir/ir.cc
index 840c69c2..8ab3ead1 100644
--- a/ir/ir.cc
+++ b/ir/ir.cc
@@ -2,6 +2,7 @@
 
 #include "common/string.h"
 #include "ir/module_id.h"
+#include "jasmin/execute.h"
 #include "nth/debug/debug.h"
 #include "nth/debug/log/log.h"
 #include "type/type.h"
@@ -199,6 +200,22 @@ void HandleParseTreeNodeCallExpression(ParseTree::Node::Index index,
     } else {
       NTH_UNIMPLEMENTED();
     }
+  } else if (invocable_type.type().kind() ==
+             type::Type::Kind::GenericFunction) {
+    jasmin::ValueStack value_stack;
+    for (auto iter = context.ChildIndices(index).begin();
+         context.Node(*iter).kind !=
+         ParseTree::Node::Kind::InvocationArgumentStart;
+         ++iter) {
+      context.emit.Evaluate(context.emit.tree.subtree_range(*iter),
+                            value_stack);
+    }
+
+    jasmin::Execute(*static_cast<IrFunction const*>(
+                        invocable_type.type().AsGenericFunction().data()),
+                    value_stack);
+    context.type_stack.push_back(type::QualifiedType(
+        type::Qualifier::Constant(), value_stack.pop<type::Type>()));
   } else {
     NTH_UNIMPLEMENTED("{}") <<= {node};
   }
diff --git a/ir/ir.h b/ir/ir.h
index f51f2f7f..750cd40b 100644
--- a/ir/ir.h
+++ b/ir/ir.h
@@ -21,6 +21,9 @@ struct IrContext {
     return emit.tree[index];
   }
 
+  auto ChildIndices(ParseTree::Node::Index index) {
+    return emit.tree.child_indices(index);
+  }
   auto Children(ParseTree::Node::Index index) {
     return emit.tree.children(index);
   }
@@ -31,12 +34,12 @@ struct IrContext {
   std::optional<T> EvaluateAs(ParseTree::Node::Index subtree_root_index) {
     T result;
     nth::interval range   = emit.tree.subtree_range(subtree_root_index);
-    auto [iter, inserted] = emit.constants.try_emplace(range);
+    jasmin::ValueStack value_stack;
+    emit.Evaluate(range, value_stack);
+    auto [iter, inserted] = emit.constants.try_emplace(range, std::move(value_stack));
     NTH_REQUIRE((v.harden), inserted);
-    jasmin::ValueStack& value_stack = iter->second;
-    Evaluate(range, emit.tree, emit.modules, value_stack);
     if (IcarusDeserializeValue(
-            std::span(value_stack.begin(), value_stack.end()), result)) {
+            std::span(iter->second.begin(), iter->second.end()), result)) {
       return result;
     } else {
       return std::nullopt;
diff --git a/type/type.cc b/type/type.cc
index b827e8f6..ad0cb399 100644
--- a/type/type.cc
+++ b/type/type.cc
@@ -104,6 +104,10 @@ std::vector<Type> const& FunctionType::returns() const {
   return ic::type::returns->from_index(functions->from_index(data()).second);
 }
 
+void const* GenericFunctionType::data() const {
+  return generic_function_types->from_index(BasicType::data());
+}
+
 size_t JasminSize(Type t) {
   switch (t.kind()) {
     case Type::Kind::Primitive: return 1;
diff --git a/type/type.h b/type/type.h
index bbcee767..d6b3f98b 100644
--- a/type/type.h
+++ b/type/type.h
@@ -3,6 +3,9 @@
 
 #include <cstdint>
 #include <cstring>
+#include <span>
+#include <string_view>
+#include <utility>
 #include <vector>
 
 namespace ic::type {
@@ -208,6 +211,24 @@ struct FunctionType : internal_type::BasicType {
   ParametersType parameters() const;
   std::vector<Type> const& returns() const;
 
+  friend void NthPrint(auto& p, auto&fmt, FunctionType f) {
+    std::string_view separator = "(";
+    for (auto const& param : *f.parameters()) {
+      p.write(std::exchange(separator, ", "));
+      fmt(p, param.name);
+      p.write(": ");
+      fmt(p, param.type);
+    }
+    std::span returns = f.returns();
+    p.write(returns.size() != 1 ? ") -> (" : ") -> ");
+    separator = "";
+    for (auto const& r: f.returns()) {
+      p.write(std::exchange(separator, ", "));
+      fmt(p, r);
+    }
+    if (returns.size() != 1) { p.write(")"); }
+  }
+
  private:
   friend Type;
   friend FunctionType Function(ParametersType, std::vector<Type>&&);
@@ -293,6 +314,8 @@ struct SliceType : internal_type::BasicType {
 SliceType Slice(Type t);
 
 struct GenericFunctionType : internal_type::BasicType {
+  void const* data() const;
+
  private:
   friend Type;
   friend GenericFunctionType GenericFunction(void const* fn);
diff --git a/type/type_system.proto b/type/type_system.proto
new file mode 100644
index 00000000..1de308ef
--- /dev/null
+++ b/type/type_system.proto
@@ -0,0 +1,30 @@
+syntax = "proto3";
+
+package type;
+
+message TypeProto {
+  enum Kind {
+    UNKNOWN = 0;
+    PRIMITIVE = 1;
+    POINTER = 2;
+    BUFFER_POINTER = 3;
+    SLICE = 4;
+    FUNCTION = 5;
+    PATTERN = 6;
+  }
+  Kind kind = 1;
+  uint32 index = 2;
+}
+
+message ParameterTypeProto {
+  TypeProto type = 1;
+}
+
+message FunctionTypeProto {
+  repeated ParameterTypeProto parameters = 1;
+  repeated TypeProto returns = 2;
+}
+
+message TypeSystem {
+  repeated FunctionTypeProto functions = 1;
+}