It seems to me that there is a qualitative difference between having synchronous HTTP requests in the JavaScript standard library and having access to System.IO

It seems to me that there is a qualitative difference between having synchronous HTTP requests in the JavaScript standard library and having access to System.IO.Unsafe.unsafePerformIO in Haskell. Still, I take your point that in theory JavaScript could remove all synchronous I/O side effecting functions and replace them with continuation-based ones. Anyway, we've wandered pretty far off the main thread, which wasn't mine to begin with, so I'll let lmm take up the discussion again if he wants.