The author helped me narrow it down to some issues with how ghc by default allocates a stack space that is rarely enough, and once it starts growing the stack space the RAM per connection gets pretty ridiculous. Using higher default stack space helped remedy this some, but the per-connection RAM cost was still way higher than Golang/Python which I was comparing to.

So... separate project, I write a load-tester in haskell for a websocket server. I need to issue some HTTP requests, and I see Brian O'Sullivan made a nice library, wreq. I use it as described and quickly discover it uses ridiculous amounts of memory because it doesn't mention that you should always re-use the Session (the underlying http-client emphasized the importance of re-using the Manager): https://github.com/bos/wreq/issues/17

(I am sorry that this issue prolly came off as a bit whiny there, I was very frustrated that such a gap was omitted from the docs)

So, my program is working pretty nicely, until I discover that its not actually sending multiple HTTP requests at once (even though the underlying http-client lib has a thread-safe TCP connection pool). After browsing some code, I see the problem: https://github.com/bos/wreq/issues/57

The solution that was so far implemented seems equally weird to me.... letting different requests stomp over the Session's cookie jar... I forked it so that I could have multiple wreq Sessions use the same Manager, and now it finally works as it should.

I won't even go into how some of these libs have occasionally wanted conflicting dependencies which leads into its own 'cabal hell' (googling for that is entertaining unless its happening to you).

I've only been writing Haskell for a bit over a year now, but everytime I write code with it, despite my love of the language, the libraries and run-time end up frustrating me.