Are thread pools needed for pure Haskell code?

The core issue, I imagine, is the network side. If you have 10,000 links and forkIO for each link, then you potentially have 10,000 sockets you're attempting to open at once, which, depending on how your OS is configured, probably won't even be possible, much less efficient.

However, the fact that we have green threads that get "virtually" scheduled across multiple os threads (which ideally are stuck to individual cores) doesn't mean that we can just distribute work randomly without regards to cpu usage either. The issue here isn't so much that the scheduling of the CPU itself won't be handled for us, but rather that context-switches (even green ones) cost cycles. Each thread, if its working on different data, will need to pull that data into the cpu. If there's enough data, that means pulling things in and out of the cpu cache. Even absent that, it means pulling things from the cache to registers, etc.

Even if a problem is trivially parallel, it is virtually never the right idea to just break it up as small as possible and attempt to do it "all at once".