+ Monadic input-output with OCaml.
+ There are many possibilities to implement monadic IO with OCaml.
+It's very annoying to copy and paste types and implementations
+over the projects, but the real reason was the "diamond problem":
+two libraries are using almost same implementation, but different
+ There will be type signatures and implementations for monadic
+input-output using Lwt and using direct OCaml input-output
+(since OCaml type system implements "monad with errors", where
+monad that carries 'a is the 'a itself, and errors are propagated
+ Module IO_Types contains signatures:
+ - IO_Monad -- just return, bind, bind_rev
+ - IO_MonadError -- IO_Monad + error, catch
+ - IO_Common (functions' signatures that are common to Lwt,
+ Direct IO and possible future IO libraries)
+ There are no need for signatures of IO_Lwt and IO_Direct, so there are
+ Module implementations:
+- IO_Pure -- no Lwt, no -threads, no exceptions handling
+- IO_PureError -- IO_Pure + exceptions
+- IO_Direct -- IO via OCaml's Pervasives + Unix + Sys + Thread modules
+ IO_Lwt and IO_Direct must match all the signatures contained in IO_Types.