[peter-fogg]

> 2. benign effects: in Haskell "proper", you do not have effects; rather you have "codes" for effects, which get interpreted into effects by the RTS; this rules out the possibility of, e.g., using effects to implement a semantically pure interface. On the other hand, OCaml has actual effects, which can be used in an open-ended way to implement all sorts of functional interfaces.

So, to be all pedantic and stuff... You've always got unsafePerformIO, first off. This is used in Debug.Trace (https://hackage.haskell.org/package/base-4.7.0.2/docs/src/De...) to allow printf debugging in pure code. This is also used in some Haskell libraries to provide restricted effects in monads other than IO. But if you don't want to use unsafe* functions, you can always use ST to get direct access to mutable memory in a safe way. If you can runST within that interface, then you can present a pure interface to users.

[jonsterling]

1. that's why I qualified it "proper". Of course there is unsafePerformIO, but in the presence of laziness, this is really unsafe. as opposed to ML, where doing IO may harm your ability to reason about stuff, but it won't be unsafe.

Debug.Trace is also unpredictable; actually, it's perfectly predictable if you understand laziness, but it's still not what's really wanted in most cases.

2. ST is a single instance of an effect that can be interpreted into "pure" code. There are plenty of other effects that don't work like this in Haskell, not to mention the problem of composing them together.

So thanks for chiming in! But what you have said is not really that different from what I have said.

[tel]

I would love to talk to you a little about these kinds of things. I've been digging more seriously into OCaml the last few weeks and would love some signposts for making the Haskell->OCaml transition.

[jonsterling]

Hi! Please feel free to email me at any time. I can't promise that I will know everything you want to know (since I only know very little), but I would be happy to help with what I do know. jon [at] jonmsterling [dot] com.

BTW, I looked at your OCaml stuff yesterday, and it seemed pretty cool.

[tome]

Could you say something about why `unsafePerformIO` is unsafe in the presence of laziness?

I know `unsafePerformIO` can be used to violate the type system in combination with `IORef`s, for example, but I don't see what laziness has to do with it.

[spyder81]

In the presence of laziness, it can be very difficult to reason about what code executes when. When you don't know when your unsafe IO happens, things can get out of order or have other unforeseen side effects.

[glaebhoerl]

With regards to 2., could you briefly mention some other instances? I have no idea what I should be thinking of.