diff --git a/src/module/command.rs b/src/module/command.rs
new file mode 100644
index 00000000..e83ce232
--- /dev/null
+++ b/src/module/command.rs
@@ -0,0 +1,11 @@
+use std::result::Result;
+
+use super::ModuleError;
+use storage::backend::{StorageBackend, StorageBackendError};
+
+type CommandError = Result<ModuleError, StorageBackendError>;
+type CommandResult = Result<(), Result<ModuleError, CommandError>>;
+
+pub trait ExecutableCommand {
+    fn exec(StorageBackend) -> CommandResult;
+}
diff --git a/src/module/mod.rs b/src/module/mod.rs
index d0f711f2..2cc5aeb9 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -7,6 +7,7 @@ use std::path::Path;
 use std::result::Result;
 
 use storage::backend::{StorageBackend, StorageBackendError};
+use self::command::ExecutableCommand;
 mod command;
 
 #[derive(Debug)]
@@ -51,7 +52,7 @@ pub trait Module {
     fn execute(&self, rt : &Runtime) -> ModuleResult;
     fn shutdown(&self, rt : &Runtime) -> ModuleResult;
 
-    fn getCommandBuilder<T>() -> F
+    fn getCommandBuilder<T, F>() -> F
         where F: FnOnce(StorageBackend) -> T,
               T: ExecutableCommand;