I disagree. Threads and green threads are not different in any way that is related to purity. Haskell makes threads much more pleasant due to Haskell code gene

I disagree. Threads and green threads are not different in any way that is related to purity. Haskell makes threads much more pleasant due to Haskell code generally being much more orthogonal. But even in C, it would make more sense to create green threads rather than actual threads (at least any thread beyond the first thread-per-core) in most settings you'd actually use threads for.