A ThreadId is an abstract type representing a handle to a thread. ThreadId is an instance of Eq, Ord and Show, total ordering over ThreadIds. The Show instance lets you convert an arbitrary-valued ThreadId to string form; showing a ThreadId value is occasionally useful when debugging or diagnosing the behaviour of a concurrent program.
Note: in GHC, if you have a ThreadId, you essentially have a pointer to the thread itself. This means the thread itself can't be garbage collected until you drop the ThreadId. This misfeature will hopefully be corrected at a later date.
Note: Hugs does not provide any operations on other threads; it defines ThreadId as a synonym for ().
Returns the ThreadId of the calling thread (GHC only).