diff --git a/src/interpreter/program.rs b/src/interpreter/program.rs
index 20e9f5d..3e51bba 100644
--- a/src/interpreter/program.rs
+++ b/src/interpreter/program.rs
@@ -2,8 +2,9 @@
 use std::collections::HashMap;
 use elements::Module;
 use interpreter::Error;
-use interpreter::store::{Store, ModuleId};
+use interpreter::store::{ModuleId, Store, ExternVal};
 use interpreter::host::HostModule;
+use interpreter::value::RuntimeValue;
 
 /// Program instance. Program is a set of instantiated modules.
 pub struct ProgramInstance {
@@ -32,7 +33,13 @@ impl ProgramInstance {
 			let module = self.modules[import_entry.module()];
 			let extern_val = module
 				.resolve_export(&self.store, import_entry.field())
-				.ok_or_else(|| Error::Function(format!("Module {} doesn't have export {}", import_entry.module(), import_entry.field())))?;
+				.ok_or_else(|| {
+					Error::Program(format!(
+						"Module {} doesn't have export {}",
+						import_entry.module(),
+						import_entry.field()
+					))
+				})?;
 			extern_vals.push(extern_val);
 		}
 
@@ -42,14 +49,62 @@ impl ProgramInstance {
 		Ok(module_id)
 	}
 
-	pub fn add_host_module(&mut self, name: &str, host_module: HostModule) -> Result<ModuleId, Error> {
+	pub fn add_host_module(
+		&mut self,
+		name: &str,
+		host_module: HostModule,
+	) -> Result<ModuleId, Error> {
 		let module_id = host_module.allocate(&mut self.store)?;
 		self.modules.insert(name.to_owned(), module_id);
 		Ok(module_id)
 	}
 
-	/// Get one of the modules by name
-	pub fn module(&self, name: &str) -> Option<ModuleId> {
-		self.modules.get(name).cloned()
+	pub fn invoke_export<St: 'static>(
+		&mut self,
+		module_name: &str,
+		func_name: &str,
+		args: Vec<RuntimeValue>,
+		state: &mut St,
+	) -> Result<Option<RuntimeValue>, Error> {
+		let module_id = self.modules.get(module_name).ok_or_else(|| {
+			Error::Program(format!("Module {} not found", module_name))
+		})?;
+		let extern_val = module_id
+			.resolve_export(&self.store, func_name)
+			.ok_or_else(|| {
+				Error::Program(format!(
+					"Module {} doesn't have export {}",
+					module_name,
+					func_name
+				))
+			})?;
+
+		let func_id = match extern_val {
+			ExternVal::Func(func_id) => func_id,
+			unexpected => {
+				return Err(Error::Program(format!(
+					"Export {} is not a function, but {:?}",
+					func_name,
+					unexpected
+				)))
+			}
+		};
+
+		self.store.invoke(func_id, args, state)
+	}
+
+	pub fn invoke_index<St: 'static>(
+		&mut self,
+		module_name: &str,
+		func_idx: u32,
+		args: Vec<RuntimeValue>,
+		state: &mut St,
+	) -> Result<Option<RuntimeValue>, Error> {
+		let module_id = self.modules.get(module_name).ok_or_else(|| {
+			Error::Program(format!("Module {} not found", module_name))
+		})?;
+		let func_id = module_id.resolve_func(&self.store, func_idx);
+
+		self.store.invoke(func_id, args, state)
 	}
 }