diff --git a/spec/src/run.rs b/spec/src/run.rs
index 55dfa09..c58be84 100644
--- a/spec/src/run.rs
+++ b/spec/src/run.rs
@@ -21,16 +21,17 @@ use parity_wasm::interpreter::{
     TableInstance,
     ModuleInstance,
     HostFunc,
+	MemoryRef,
 };
 
 struct SpecModule {
     default_host_callback: Rc<HostFunc<()>>,
     table: Rc<TableInstance<()>>,
-    memory: Rc<MemoryInstance>,
-    global_i32: Rc<GlobalInstance>,
-    global_i64: Rc<GlobalInstance>,
-    global_f32: Rc<GlobalInstance>,
-    global_f64: Rc<GlobalInstance>,
+    memory: MemoryRef,
+    global_i32: GlobalRef,
+    global_i64: GlobalRef,
+    global_f32: GlobalRef,
+    global_f64: GlobalRef,
 }
 
 impl SpecModule {
@@ -73,7 +74,7 @@ impl ImportResolver<()> for SpecModule {
         &self,
         field_name: &str,
         global_type: &GlobalType,
-    ) -> Result<Rc<GlobalInstance>, InterpreterError> {
+    ) -> Result<GlobalRef, InterpreterError> {
         if field_name == "global" {
             return match global_type.content_type() {
                 ValueType::I32 => {
@@ -98,7 +99,7 @@ impl ImportResolver<()> for SpecModule {
         &self,
         field_name: &str,
         _memory_type: &MemoryType,
-    ) -> Result<Rc<MemoryInstance>, InterpreterError> {
+    ) -> Result<MemoryRef, InterpreterError> {
         if field_name == "memory" {
             return Ok(Rc::clone(&self.memory));
         }
diff --git a/src/interpreter/func.rs b/src/interpreter/func.rs
index 4442e4b..b4754e8 100644
--- a/src/interpreter/func.rs
+++ b/src/interpreter/func.rs
@@ -3,6 +3,7 @@ use std::fmt;
 use std::collections::HashMap;
 use std::borrow::Cow;
 use elements::{FunctionType, Local, Opcodes};
+use interpreter::module::FuncRef;
 use interpreter::{Error, ModuleInstance};
 use interpreter::runner::{prepare_function_args, FunctionContext, Interpreter};
 use interpreter::host::HostFunc;
@@ -83,7 +84,7 @@ impl FuncInstance {
 	}
 
 	pub fn invoke<'a, 'b: 'a>(
-		func: Rc<FuncInstance>,
+		func: FuncRef,
 		args: Cow<[RuntimeValue]>,
 		state: &'a mut HostState<'b>,
 	) -> Result<Option<RuntimeValue>, Error> {
diff --git a/src/interpreter/host.rs b/src/interpreter/host.rs
index 780a9bf..47819dc 100644
--- a/src/interpreter/host.rs
+++ b/src/interpreter/host.rs
@@ -1,8 +1,11 @@
+use interpreter::module::GlobalRef;
 use std::rc::Rc;
 use std::collections::HashMap;
 use std::collections::hash_map::Entry;
 use elements::{FunctionType, GlobalType, MemoryType, TableType, ValueType};
-use interpreter::module::{ExternVal, ModuleInstance};
+use interpreter::module::{ExternVal, ModuleInstance, FuncRef};
+use interpreter::module::MemoryRef;
+use interpreter::module::TableRef;
 use interpreter::func::FuncInstance;
 use interpreter::global::GlobalInstance;
 use interpreter::memory::MemoryInstance;
@@ -222,33 +225,33 @@ impl HostModuleBuilder {
 		self
 	}
 
-	pub fn insert_func<N: Into<String>>(&mut self, name: N, func: Rc<FuncInstance>) {
+	pub fn insert_func<N: Into<String>>(&mut self, name: N, func: FuncRef) {
 		self.insert(name, ExternVal::Func(func));
 	}
 
-	pub fn insert_global<N: Into<String>>(&mut self, name: N, global: Rc<GlobalInstance>) {
+	pub fn insert_global<N: Into<String>>(&mut self, name: N, global: GlobalRef) {
 		self.insert(name, ExternVal::Global(global));
 	}
 
-	pub fn insert_memory<N: Into<String>>(&mut self, name: N, memory: Rc<MemoryInstance>) {
+	pub fn insert_memory<N: Into<String>>(&mut self, name: N, memory: MemoryRef) {
 		self.insert(name, ExternVal::Memory(memory));
 	}
 
-	pub fn insert_table<N: Into<String>>(&mut self, name: N, table: Rc<TableInstance>) {
+	pub fn insert_table<N: Into<String>>(&mut self, name: N, table: TableRef) {
 		self.insert(name, ExternVal::Table(table));
 	}
 
-	pub fn with_global<N: Into<String>>(mut self, name: N, global: Rc<GlobalInstance>) -> Self {
+	pub fn with_global<N: Into<String>>(mut self, name: N, global: GlobalRef) -> Self {
 		self.insert_global(name, global);
 		self
 	}
 
-	pub fn with_memory<N: Into<String>>(mut self, name: N, memory: Rc<MemoryInstance>) -> Self {
+	pub fn with_memory<N: Into<String>>(mut self, name: N, memory: MemoryRef) -> Self {
 		self.insert_memory(name, memory);
 		self
 	}
 
-	pub fn with_table<N: Into<String>>(mut self, name: N, table: Rc<TableInstance>) -> Self {
+	pub fn with_table<N: Into<String>>(mut self, name: N, table: TableRef) -> Self {
 		self.insert_table(name, table);
 		self
 	}
@@ -281,7 +284,7 @@ impl ImportResolver for HostModule {
 		&self,
 		field_name: &str,
 		func_type: &FunctionType,
-	) -> Result<Rc<FuncInstance>, Error> {
+	) -> Result<FuncRef, Error> {
 		self.internal_instance.resolve_func(field_name, func_type)
 	}
 
@@ -289,7 +292,7 @@ impl ImportResolver for HostModule {
 		&self,
 		field_name: &str,
 		global_type: &GlobalType,
-	) -> Result<Rc<GlobalInstance>, Error> {
+	) -> Result<GlobalRef, Error> {
 		self.internal_instance
 			.resolve_global(field_name, global_type)
 	}
@@ -298,7 +301,7 @@ impl ImportResolver for HostModule {
 		&self,
 		field_name: &str,
 		memory_type: &MemoryType,
-	) -> Result<Rc<MemoryInstance>, Error> {
+	) -> Result<MemoryRef, Error> {
 		self.internal_instance
 			.resolve_memory(field_name, memory_type)
 	}
@@ -307,7 +310,7 @@ impl ImportResolver for HostModule {
 		&self,
 		field_name: &str,
 		table_type: &TableType,
-	) -> Result<Rc<TableInstance>, Error> {
+	) -> Result<TableRef, Error> {
 		self.internal_instance.resolve_table(field_name, table_type)
 	}
 }
@@ -382,3 +385,20 @@ impl IntoReturnVal for () {
 		None
 	}
 }
+
+trait Externals {
+	fn invoke_index(
+		&mut self,
+		index: u32,
+		args: &[RuntimeValue],
+	) -> Result<Option<RuntimeValue>, Error>;
+
+	// TODO: or check signature?
+	fn signature(&self, index: usize) -> &FunctionType;
+
+	fn memory_by_index(&self, index: usize) -> &MemoryInstance;
+	fn table_by_index(&self, index: usize) -> &TableInstance;
+	fn global_by_index(&self, index: usize) -> &GlobalInstance;
+}
+
+
diff --git a/src/interpreter/imports.rs b/src/interpreter/imports.rs
index 1717069..f22a4d8 100644
--- a/src/interpreter/imports.rs
+++ b/src/interpreter/imports.rs
@@ -1,10 +1,9 @@
-use std::rc::Rc;
+use interpreter::module::GlobalRef;
 use std::collections::HashMap;
 use elements::{FunctionType, GlobalType, MemoryType, TableType};
-use interpreter::func::FuncInstance;
-use interpreter::global::GlobalInstance;
-use interpreter::memory::MemoryInstance;
-use interpreter::table::TableInstance;
+use interpreter::module::MemoryRef;
+use interpreter::module::FuncRef;
+use interpreter::module::TableRef;
 use interpreter::Error;
 
 pub struct Imports<'a> {
@@ -45,23 +44,23 @@ pub trait ImportResolver {
 		&self,
 		field_name: &str,
 		func_type: &FunctionType,
-	) -> Result<Rc<FuncInstance>, Error>;
+	) -> Result<FuncRef, Error>;
 
 	fn resolve_global(
 		&self,
 		field_name: &str,
 		global_type: &GlobalType,
-	) -> Result<Rc<GlobalInstance>, Error>;
+	) -> Result<GlobalRef, Error>;
 
 	fn resolve_memory(
 		&self,
 		field_name: &str,
 		memory_type: &MemoryType,
-	) -> Result<Rc<MemoryInstance>, Error>;
+	) -> Result<MemoryRef, Error>;
 
 	fn resolve_table(
 		&self,
 		field_name: &str,
 		table_type: &TableType,
-	) -> Result<Rc<TableInstance>, Error>;
+	) -> Result<TableRef, Error>;
 }
diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index 062b309..b3a8575 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -157,7 +157,7 @@ pub use self::program::ProgramInstance;
 pub use self::value::RuntimeValue;
 pub use self::host::{HostModule, HostModuleBuilder, HostFunc, IntoReturnVal, FromArg};
 pub use self::imports::{ImportResolver, Imports};
-pub use self::module::ModuleInstance;
+pub use self::module::{ModuleInstance, FuncRef, MemoryRef, GlobalRef, TableRef};
 pub use self::global::GlobalInstance;
 pub use self::func::FuncInstance;
 pub use self::state::{HostState, StateKey};
diff --git a/src/interpreter/module.rs b/src/interpreter/module.rs
index ef09303..f64feb0 100644
--- a/src/interpreter/module.rs
+++ b/src/interpreter/module.rs
@@ -13,11 +13,16 @@ use interpreter::state::HostState;
 use validation::validate_module;
 use common::{DEFAULT_MEMORY_INDEX, DEFAULT_TABLE_INDEX};
 
+pub type FuncRef = Rc<FuncInstance>;
+pub type TableRef = Rc<TableInstance>;
+pub type MemoryRef = Rc<MemoryInstance>;
+pub type GlobalRef = Rc<GlobalInstance>;
+
 pub enum ExternVal {
-	Func(Rc<FuncInstance>),
-	Table(Rc<TableInstance>),
-	Memory(Rc<MemoryInstance>),
-	Global(Rc<GlobalInstance>),
+	Func(FuncRef),
+	Table(TableRef),
+	Memory(MemoryRef),
+	Global(GlobalRef),
 }
 
 impl Clone for ExternVal {
@@ -47,28 +52,28 @@ impl fmt::Debug for ExternVal {
 }
 
 impl ExternVal {
-	pub fn as_func(&self) -> Option<Rc<FuncInstance>> {
+	pub fn as_func(&self) -> Option<FuncRef> {
 		match *self {
 			ExternVal::Func(ref func) => Some(Rc::clone(func)),
 			_ => None,
 		}
 	}
 
-	pub fn as_table(&self) -> Option<Rc<TableInstance>> {
+	pub fn as_table(&self) -> Option<TableRef> {
 		match *self {
 			ExternVal::Table(ref table) => Some(Rc::clone(table)),
 			_ => None,
 		}
 	}
 
-	pub fn as_memory(&self) -> Option<Rc<MemoryInstance>> {
+	pub fn as_memory(&self) -> Option<MemoryRef> {
 		match *self {
 			ExternVal::Memory(ref memory) => Some(Rc::clone(memory)),
 			_ => None,
 		}
 	}
 
-	pub fn as_global(&self) -> Option<Rc<GlobalInstance>> {
+	pub fn as_global(&self) -> Option<GlobalRef> {
 		match *self {
 			ExternVal::Global(ref global) => Some(Rc::clone(global)),
 			_ => None,
@@ -79,10 +84,10 @@ impl ExternVal {
 #[derive(Debug)]
 pub struct ModuleInstance {
 	types: RefCell<Vec<Rc<FunctionType>>>,
-	tables: RefCell<Vec<Rc<TableInstance>>>,
-	funcs: RefCell<Vec<Rc<FuncInstance>>>,
-	memories: RefCell<Vec<Rc<MemoryInstance>>>,
-	globals: RefCell<Vec<Rc<GlobalInstance>>>,
+	tables: RefCell<Vec<TableRef>>,
+	funcs: RefCell<Vec<FuncRef>>,
+	memories: RefCell<Vec<MemoryRef>>,
+	globals: RefCell<Vec<GlobalRef>>,
 	exports: RefCell<HashMap<String, ExternVal>>,
 }
 
@@ -106,19 +111,19 @@ impl ModuleInstance {
 		instance
 	}
 
-	pub fn memory_by_index(&self, idx: u32) -> Option<Rc<MemoryInstance>> {
+	pub fn memory_by_index(&self, idx: u32) -> Option<MemoryRef> {
 		self.memories.borrow().get(idx as usize).cloned()
 	}
 
-	pub fn table_by_index(&self, idx: u32) -> Option<Rc<TableInstance>> {
+	pub fn table_by_index(&self, idx: u32) -> Option<TableRef> {
 		self.tables.borrow().get(idx as usize).cloned()
 	}
 
-	pub fn global_by_index(&self, idx: u32) -> Option<Rc<GlobalInstance>> {
+	pub fn global_by_index(&self, idx: u32) -> Option<GlobalRef> {
 		self.globals.borrow().get(idx as usize).cloned()
 	}
 
-	pub fn func_by_index(&self, idx: u32) -> Option<Rc<FuncInstance>> {
+	pub fn func_by_index(&self, idx: u32) -> Option<FuncRef> {
 		self.funcs.borrow().get(idx as usize).cloned()
 	}
 
@@ -130,7 +135,7 @@ impl ModuleInstance {
 		self.exports.borrow().get(name).cloned()
 	}
 
-	fn push_func(&self, func: Rc<FuncInstance>) {
+	fn push_func(&self, func: FuncRef) {
 		self.funcs.borrow_mut().push(func);
 	}
 
@@ -138,15 +143,15 @@ impl ModuleInstance {
 		self.types.borrow_mut().push(func_type)
 	}
 
-	fn push_memory(&self, memory: Rc<MemoryInstance>) {
+	fn push_memory(&self, memory: MemoryRef) {
 		self.memories.borrow_mut().push(memory)
 	}
 
-	fn push_table(&self, table: Rc<TableInstance>) {
+	fn push_table(&self, table: TableRef) {
 		self.tables.borrow_mut().push(table)
 	}
 
-	fn push_global(&self, global: Rc<GlobalInstance>) {
+	fn push_global(&self, global: GlobalRef) {
 		self.globals.borrow_mut().push(global)
 	}
 
@@ -498,7 +503,7 @@ impl ImportResolver for ModuleInstance {
 		&self,
 		field_name: &str,
 		_func_type: &FunctionType,
-	) -> Result<Rc<FuncInstance>, Error> {
+	) -> Result<FuncRef, Error> {
 		Ok(self.export_by_name(field_name)
 			.ok_or_else(|| {
 				Error::Validation(format!("Export {} not found", field_name))
@@ -513,7 +518,7 @@ impl ImportResolver for ModuleInstance {
 		&self,
 		field_name: &str,
 		_global_type: &GlobalType,
-	) -> Result<Rc<GlobalInstance>, Error> {
+	) -> Result<GlobalRef, Error> {
 		Ok(self.export_by_name(field_name)
 			.ok_or_else(|| {
 				Error::Validation(format!("Export {} not found", field_name))
@@ -528,7 +533,7 @@ impl ImportResolver for ModuleInstance {
 		&self,
 		field_name: &str,
 		_memory_type: &MemoryType,
-	) -> Result<Rc<MemoryInstance>, Error> {
+	) -> Result<MemoryRef, Error> {
 		Ok(self.export_by_name(field_name)
 			.ok_or_else(|| {
 				Error::Validation(format!("Export {} not found", field_name))
@@ -543,7 +548,7 @@ impl ImportResolver for ModuleInstance {
 		&self,
 		field_name: &str,
 		_table_type: &TableType,
-	) -> Result<Rc<TableInstance>, Error> {
+	) -> Result<TableRef, Error> {
 		Ok(self.export_by_name(field_name)
 			.ok_or_else(|| {
 				Error::Validation(format!("Export {} not found", field_name))
@@ -559,17 +564,17 @@ fn alloc_func_type(func_type: FunctionType) -> Rc<FunctionType> {
 	Rc::new(func_type)
 }
 
-fn alloc_table(table_type: &TableType) -> Result<Rc<TableInstance>, Error> {
+fn alloc_table(table_type: &TableType) -> Result<TableRef, Error> {
 	let table = TableInstance::new(table_type)?;
 	Ok(Rc::new(table))
 }
 
-fn alloc_memory(mem_type: &MemoryType) -> Result<Rc<MemoryInstance>, Error> {
+fn alloc_memory(mem_type: &MemoryType) -> Result<MemoryRef, Error> {
 	let memory = MemoryInstance::new(&mem_type)?;
 	Ok(Rc::new(memory))
 }
 
-fn alloc_global(global_type: GlobalType, val: RuntimeValue) -> Rc<GlobalInstance> {
+fn alloc_global(global_type: GlobalType, val: RuntimeValue) -> GlobalRef {
 	let global = GlobalInstance::new(val, global_type.is_mutable());
 	Rc::new(global)
 }
diff --git a/src/interpreter/program.rs b/src/interpreter/program.rs
index 66fb8f4..ac0e125 100644
--- a/src/interpreter/program.rs
+++ b/src/interpreter/program.rs
@@ -3,7 +3,7 @@ use std::collections::HashMap;
 use std::borrow::Cow;
 use elements::Module;
 use interpreter::Error;
-use interpreter::module::ModuleInstance;
+use interpreter::module::{ModuleInstance, FuncRef};
 use interpreter::func::FuncInstance;
 use interpreter::host::HostModule;
 use interpreter::value::RuntimeValue;
@@ -98,7 +98,7 @@ impl ProgramInstance {
 
 	pub fn invoke_func<'a>(
 		&mut self,
-		func_instance: Rc<FuncInstance>,
+		func_instance: FuncRef,
 		args: &[RuntimeValue],
 		state: &'a mut HostState<'a>,
 	) -> Result<Option<RuntimeValue>, Error> {
diff --git a/src/interpreter/runner.rs b/src/interpreter/runner.rs
index 752ad48..44e8fba 100644
--- a/src/interpreter/runner.rs
+++ b/src/interpreter/runner.rs
@@ -8,6 +8,7 @@ use std::collections::{HashMap, VecDeque};
 use elements::{Opcode, BlockType, Local, FunctionType};
 use interpreter::Error;
 use interpreter::module::ModuleInstance;
+use interpreter::module::FuncRef;
 use interpreter::func::FuncInstance;
 use interpreter::value::{
 	RuntimeValue, TryInto, WrapInto, TryTruncateInto, ExtendInto,
@@ -27,7 +28,7 @@ pub struct FunctionContext {
 	/// Is context initialized.
 	pub is_initialized: bool,
 	/// Internal function reference.
-	pub function: Rc<FuncInstance>,
+	pub function: FuncRef,
 	pub module: Rc<ModuleInstance>,
 	/// Function return type.
 	pub return_type: BlockType,
@@ -48,7 +49,7 @@ pub enum InstructionOutcome {
 	/// Branch to given frame.
 	Branch(usize),
 	/// Execute function call.
-	ExecuteCall(Rc<FuncInstance>),
+	ExecuteCall(FuncRef),
 	/// End current frame.
 	End,
 	/// Return from current function block.
@@ -60,7 +61,7 @@ enum RunResult {
 	/// Function has returned (optional) value.
 	Return(Option<RuntimeValue>),
 	/// Function is calling other function.
-	NestedCall(Rc<FuncInstance>),
+	NestedCall(FuncRef),
 }
 
 impl<'a, 'b: 'a> Interpreter<'a, 'b> {
@@ -998,7 +999,7 @@ impl<'a, 'b: 'a> Interpreter<'a, 'b> {
 }
 
 impl FunctionContext {
-	pub fn new<'store>(function: Rc<FuncInstance>, value_stack_limit: usize, frame_stack_limit: usize, function_type: &FunctionType, args: Vec<RuntimeValue>) -> Self {
+	pub fn new<'store>(function: FuncRef, value_stack_limit: usize, frame_stack_limit: usize, function_type: &FunctionType, args: Vec<RuntimeValue>) -> Self {
 		let module = match *function {
 			FuncInstance::Internal { ref module, .. } => Rc::clone(module),
 			FuncInstance::Host { .. } => panic!("Host functions can't be called as internally defined functions; Thus FunctionContext can be created only with internally defined functions; qed"),
@@ -1015,7 +1016,7 @@ impl FunctionContext {
 		}
 	}
 
-	pub fn nested(&mut self, function: Rc<FuncInstance>) -> Result<Self, Error> {
+	pub fn nested(&mut self, function: FuncRef) -> Result<Self, Error> {
 		let (function_locals, module, function_return_type) = {
 			let module = match *function {
 				FuncInstance::Internal { ref module, .. } => Rc::clone(module),
diff --git a/src/interpreter/table.rs b/src/interpreter/table.rs
index 4308d61..8ebab55 100644
--- a/src/interpreter/table.rs
+++ b/src/interpreter/table.rs
@@ -1,18 +1,17 @@
 use std::u32;
 use std::fmt;
-use std::rc::Rc;
 use std::cell::RefCell;
 use elements::{ResizableLimits, TableType};
 use interpreter::Error;
+use interpreter::module::FuncRef;
 use interpreter::module::check_limits;
-use interpreter::func::FuncInstance;
 
 /// Table instance.
 pub struct TableInstance {
 	/// Table limits.
 	limits: ResizableLimits,
 	/// Table memory buffer.
-	buffer: RefCell<Vec<Option<Rc<FuncInstance>>>>,
+	buffer: RefCell<Vec<Option<FuncRef>>>,
 }
 
 impl fmt::Debug for TableInstance {
@@ -40,7 +39,7 @@ impl TableInstance {
 	}
 
 	/// Get the specific value in the table
-	pub fn get(&self, offset: u32) -> Result<Rc<FuncInstance>, Error> {
+	pub fn get(&self, offset: u32) -> Result<FuncRef, Error> {
 		let buffer = self.buffer.borrow();
 		let buffer_len = buffer.len();
 		let table_elem = buffer.get(offset as usize).cloned().ok_or(
@@ -57,7 +56,7 @@ impl TableInstance {
 	}
 
 	/// Set the table element to the specified function.
-	pub fn set(&self, offset: u32, value: Rc<FuncInstance>) -> Result<(), Error> {
+	pub fn set(&self, offset: u32, value: FuncRef) -> Result<(), Error> {
 		let mut buffer = self.buffer.borrow_mut();
 		let buffer_len = buffer.len();
 		let table_elem = buffer.get_mut(offset as usize).ok_or(Error::Table(format!(
diff --git a/src/interpreter/tests/basics.rs b/src/interpreter/tests/basics.rs
index d7e9b45..7c0e3fd 100644
--- a/src/interpreter/tests/basics.rs
+++ b/src/interpreter/tests/basics.rs
@@ -118,7 +118,7 @@ impl ::std::fmt::Display for UserErrorWithCode {
 impl UserError for UserErrorWithCode {}
 
 struct TestState {
-	pub memory: Rc<MemoryInstance>,
+	pub memory: MemoryRef,
 	pub values: RefCell<Vec<i32>>,
 }