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But Haskell’s execution model has no underlying support for continuations, and yet there are perfectly efficient implementations. You actually can not implemen

• rbehrends 9y ago
  > But Haskell’s execution model has no underlying support for continuations, and yet there are perfectly efficient implementations. What you need is a way to capture and manipulate stack frames at a very basic level. Haskell cannot magically avoid that. As I recall (though it has been a while), Control.Monad.CC reifies stack frames explicitly. Monads function in this context as a very basic metaprogramming technique (or as some sort of AOP, if you want to look at it this way). > In c++ at least, such an implementation would certainly use undefined behavior This is what I mean by support in the execution model. You do need to have access to stack frames, and C++ doesn't permit that. There are workarounds [1], but they generally require non-trivial metaprogamming, due to the insane complexity of C++. You don't have that problem with, say, Smalltalk, as most Smalltalk VMs gives you the necessary functionality. For example, one of the original continuation-based web frame works, Seaside, was all Smalltalk [2]. [1] http://www.filpizlo.com/papers/baker-ccpe09-accurate.pdf; http://www.filpizlo.com/papers/baker-ccpe09-accurate.pdf; the context here is GC, but the underlying problem is similar, access to stack frames (in this context, for root scanning). [2] http://seaside.st/about/examples http://seaside.st/about/examples