I am using the library oncurrent of GHC. I'd like to know how two different threads can communicate each other. To solve the problem I thought in something like a global variable. If there any method to avoid the use of that kind of variables in haskell.

You can use the various concurrent data structures in the Concurrent library. See: http://www.haskell.org/ghc/docs/latest/set/sec-concurrency-abstractions. html The simplest form of communication is through an MVar, which is a thread-safe mutable variable. Cheers, Simon