diff --git a/src/interpreter/env_native.rs b/src/interpreter/env_native.rs
index 19216af..ec86577 100644
--- a/src/interpreter/env_native.rs
+++ b/src/interpreter/env_native.rs
@@ -1,5 +1,6 @@
 use std::sync::Arc;
 use std::collections::HashMap;
+use std::borrow::Cow;
 use parking_lot::RwLock;
 use elements::{FunctionType, Internal, ValueType};
 use interpreter::Error;
@@ -22,20 +23,62 @@ pub trait UserFunctionExecutor {
 	fn execute(&mut self, name: &str, context: CallerContext) -> Result<Option<RuntimeValue>, Error>;
 }
 
+#[derive(Clone)]
+pub enum UserFunctionDescriptor {
+	Static(&'static str, &'static [ValueType]),
+	Heap(String, Vec<ValueType>),
+}
+
 /// User function type.
+#[derive(Clone)]
 pub struct UserFunction {
-	/// User function name.
-	pub name: String,
-	/// User function parameters (for signature matching).
-	pub params: Vec<ValueType>,
-	/// User function return type (for signature matching).
+	pub desc: UserFunctionDescriptor,
 	pub result: Option<ValueType>,
 }
 
+impl UserFunction {
+	/// New function with statically known params
+	pub fn statik(name: &'static str, params: &'static [ValueType], result: Option<ValueType>) -> Self {
+		UserFunction {
+			desc: UserFunctionDescriptor::Static(name, params),
+			result: result,
+		}
+	}
+
+	/// New function with statically unknown params
+	pub fn heap(name: String, params: Vec<ValueType>, result: Option<ValueType>) -> Self {
+		UserFunction {
+			desc: UserFunctionDescriptor::Heap(name, params),
+			result: result,
+		}	
+	}
+
+	/// Name of the function
+	pub fn name(&self) -> &str {
+		match self.desc {
+			UserFunctionDescriptor::Static(name, _) => name,
+			UserFunctionDescriptor::Heap(ref name, _) => name,
+		}
+	}
+
+	/// Arguments of the function
+	pub fn params(&self) -> &[ValueType] {
+		match self.desc {
+			UserFunctionDescriptor::Static(_, params) => params,
+			UserFunctionDescriptor::Heap(_, ref params) => params,
+		}		
+	}
+
+	/// Return type of the function
+	pub fn result(&self) -> Option<ValueType> {
+		self.result
+	}
+}
+
 /// Set of user-defined functions
 pub struct UserFunctions<'a> {
 	/// Functions list.
-	pub functions: Vec<UserFunction>,
+	pub functions: Cow<'static, [UserFunction]>,
 	/// Functions executor.
 	pub executor: &'a mut UserFunctionExecutor,
 }
@@ -49,7 +92,7 @@ pub struct NativeModuleInstance<'a> {
 	/// By-name functions index.
 	by_name: HashMap<String, u32>,
 	/// User functions list.
-	functions: Vec<UserFunction>,
+	functions: Cow<'static, [UserFunction]>,
 }
 
 impl<'a> NativeModuleInstance<'a> {
@@ -58,7 +101,7 @@ impl<'a> NativeModuleInstance<'a> {
 		Ok(NativeModuleInstance {
 			env: env,
 			executor: RwLock::new(functions.executor),
-			by_name: functions.functions.iter().enumerate().map(|(i, f)| (f.name.clone(), i as u32)).collect(),
+			by_name: functions.functions.iter().enumerate().map(|(i, f)| (f.name().to_owned(), i as u32)).collect(),
 			functions: functions.functions,
 		})
 	}
@@ -98,7 +141,7 @@ impl<'a> ModuleInstanceInterface for NativeModuleInstance<'a> {
 		self.functions
 			.get((index - NATIVE_INDEX_FUNC_MIN) as usize)
 			.ok_or(Error::Native(format!("missing native env function with index {}", index)))
-			.map(|f| FunctionType::new(f.params.clone(), f.result.clone()))
+			.map(|f| FunctionType::new(f.params().to_vec(), f.result().clone()))
 	}
 
 	fn table(&self, index: ItemIndex) -> Result<Arc<TableInstance>, Error> {
@@ -130,7 +173,7 @@ impl<'a> ModuleInstanceInterface for NativeModuleInstance<'a> {
 		self.functions
 			.get((index - NATIVE_INDEX_FUNC_MIN) as usize)
 			.ok_or(Error::Native(format!("trying to call native function with index {}", index)))
-			.and_then(|f| self.executor.write().execute(&f.name, outer))
+			.and_then(|f| self.executor.write().execute(&f.name(), outer))
 	}
 }
 
diff --git a/src/interpreter/tests/basics.rs b/src/interpreter/tests/basics.rs
index b13cb00..2931529 100644
--- a/src/interpreter/tests/basics.rs
+++ b/src/interpreter/tests/basics.rs
@@ -5,7 +5,7 @@ use builder::module;
 use elements::{ExportEntry, Internal, ImportEntry, External, GlobalEntry, GlobalType,
 	InitExpr, ValueType, BlockType, Opcodes, Opcode, FunctionType};
 use interpreter::Error;
-use interpreter::env_native::{env_native_module, UserFunction, UserFunctions, UserFunctionExecutor};
+use interpreter::env_native::{env_native_module, UserFunction, UserFunctions, UserFunctionExecutor, UserFunctionDescriptor};
 use interpreter::imports::ModuleImports;
 use interpreter::memory::MemoryInstance;
 use interpreter::module::{ModuleInstanceInterface, CallerContext, ItemIndex, ExecutionParams};
@@ -123,6 +123,25 @@ fn global_get_set() {
 	assert_eq!(module.execute_index(2, vec![].into()).unwrap_err(), Error::Variable("trying to update variable of type I32 with value of type Some(I64)".into()));
 }
 
+const SIGNATURE_I32: &'static [ValueType] = &[ValueType::I32];
+
+const SIGNATURES: &'static [UserFunction] = &[
+	UserFunction {
+		desc: UserFunctionDescriptor::Static(
+			"add",
+			SIGNATURE_I32,
+		),
+		result: Some(ValueType::I32),
+	},
+	UserFunction {
+		desc: UserFunctionDescriptor::Static(
+			"sub",
+			SIGNATURE_I32,
+		),
+		result: Some(ValueType::I32),
+	},
+];
+
 #[test]
 fn single_program_different_modules() {
 	// user function executor
@@ -170,15 +189,7 @@ fn single_program_different_modules() {
 	{
 		let functions: UserFunctions = UserFunctions {
 			executor: &mut executor,
-			functions: vec![UserFunction {
-				name: "add".into(),
-				params: vec![ValueType::I32],
-				result: Some(ValueType::I32),
-			}, UserFunction {
-				name: "sub".into(),
-				params: vec![ValueType::I32],
-				result: Some(ValueType::I32),
-			}],
+			functions: ::std::borrow::Cow::from(SIGNATURES),
 		};
 		let native_env_instance = Arc::new(env_native_module(env_instance, functions).unwrap());
 		let params = ExecutionParams::with_external("env".into(), native_env_instance);