[Haskell-cafe] Investing in languages (Was: What isyourfavouriteHaskell "aha" moment?)

Sergiu Ivanov sivanov at colimite.fr
Mon Jul 16 08:54:47 UTC 2018

Hi Paul,

Thus quoth  PY  on Mon Jul 16 2018 at 09:44 (+0200):
> So, motivation of monads introduction (for me, sure, I'm very
> subjective) is to workaround Haskell model,

Sometimes (e.g., when you want to be able to prove correctness) you
actually want to express everything in a small basis of
concepts/operations/tools.  To me, monads are cool precisely because
they allow _explicit_ sequencing of actions in a non-sequential model.

By the way, the lift function you mentioned in a previous E-mail does
have a value for the programmer: it shows at which level of the monad
transformer stack the action takes place (for example).

Thus quoth  PY  on Mon Jul 16 2018 at 09:44 (+0200):
> 1. State monad allows you to mark change of THIS state, so you can easy
> find where THIS state is changing (tracking changes)
> 2. Singleton with FSM allows you to *control* change and to isolate all
> change logic in one place
> 1st allows spaghetti, 2nd - does not. 2nd force you to another model:

You seem to like it when your paradigm forces you to do something (just
like I do).  Now, monads force you to program in a certain way.
Furthermore, you may say that FSM are a workaround of the way in which
conventional operative languages manipulate state.

My point is: whether monads are a workaround or a solution depends on
the angle at which you look at the situation.  (I think you say
something similar too.)


Thus quoth  PY  on Mon Jul 16 2018 at 09:44 (+0200):
>> So I think if you don't see anybody explicitly mentioning spaghetti
>> issues with State that's for some people it's just hiding in plain
>> sight and they either aren't consciously aware of it, or find that
>> area so self-explaining that they do not think they really need to
>> explain that.
> IMHO State monad solution is orthogonal to my point. It does not force
> you to isolate state change in one place with explicit control, it only
> marks place where it happens. This info is needed to compiler, not to
> me. For me - no benefits. Benefit to me - to isolate changing, but with
> State I can (and all of us do it!) smear change points throughout the
> code. So, my question is: what exact problem does solve State monad?
> Which problem? Mine or compiler? Haskell pure lambda-only-abstraction
> limitation? OK, if we imagine another Haskell, similar to F#, will I
> need State monad yet? IMHO - no. My point is: State monad is super, in
> Haskell, and absolutely waste in other languages. I will isolate
> mutability in another manner: more safe, robust and controllable. Recap:
> 1. State monad allows you to mark change of THIS state, so you can easy
> find where THIS state is changing (tracking changes)
> 2. Singleton with FSM allows you to *control* change and to isolate all
> change logic in one place
> 1st allows spaghetti, 2nd - does not. 2nd force you to another model:
> not changes, but change requests, which can return: "not possible". With
> Haskell way the check "possible/not possible" will happen in locations
> where you change state in State monad: anywhere. So, my initial point
> is: State monad is about Haskell abstraction problems, not about
> developer problems.
>> Sorry, but that's not what OO is about.
>> Also, I do not think that you're using general FSMs, else you'd be
>> having transition spaghetti.
> To be precise, then yes, you are right. But such model forces me more,
> then monadic model. When you create singleton "PlayerBehavior", and have
> all setters/getters in this singleton and already check (in one place!)
> changes - next step is to switch from checks to explicit FSM - in the
> same place. Haskell nothing offers for this. You *can* do it, but monads
> don't force you and they are about Haskell problems, not mine.
> Motivation of State monad is not to solve problem but to introduce state
> mutability in Haskell, this is my point. OK, State monad has helpful
> side-effect: allows to track change of concrete THIS state, but I can do
> it with my editor, it's more valuable to Haskell itself, then to me,
> because no problem to mutate state: Haskell allows it, Haskell also does
> not guard you to mutate this state anywhere in the application.
> I'm agree with you 100%. My point is related to accents only, my thesis
> is: monads have value, but it's small, it's justified in Haskell with
> its limitation to one abstraction, but I don't need monads in other
> languages, their value in other languages is super-small (if even
> exists). So, motivation of monads introduction (for me, sure, I'm very
> subjective) is to workaround Haskell model, not to make code more safe,
> I'm absolutely sure: monads nothing to do with safety. It's like to use
> aspirin with serious medical problem :)
>> Let me repeat: What you call a "message" is just a standard
>> synchronous function call. The one difference is that the caller
>> allows the target type to influence what function gets actually
>> called, and while that's powerful it's quite far from what people
>> assume if you throw that "message" terminology around.
> I mentioned Erlang early: the same - you send message to FSM which will
> be lightweight process. Idea of agents and messages is the same in
> Smalltalk, in QNX, in Erlang, etc, etc... So, "message" does not always
> mean "synchronous call". For example, QNX "optimizes" local messages, so
> they are more lightweight in comparison with remotely messages (which
> are naturally asynchronous). But "message" abstraction is the same and
> is more high-level then synchronous/asynchronous dichotomy. It allows
> you to isolate logic - this is the point. Haskell nothing to do with it:
> you smear logic anywhere. But now you mark it explicitly. And you have
> illusion that your code is more safe.
>> But that's not the point. The point is that Haskell makes it easy to
>> write non-spaghetti.
> How? In Haskell I propagate data to a lot of functions (as argument or
> as hidden argument - in some monad), but with singleton+FSM - you can
> not do it - data is hidden for you, you can only *call logic*, not
> *access data*. Logic in Haskell is forced to be smeared between a lot of
> functions. You *CAN* avoid it, but Haskell does not force you.
>> BTW you have similar claims about FSMs. Ordinarily they are spaghetti
>> incarnate, but you say they work quite beautifully if done right.
>> (I'm staying sceptical because your arguments in that direction didn't
>> make sense to me, but that might be because I'm lacking background
>> information, and filling in these gaps is really too far off-topic to
>> be of interest.)
> I respect your position. Everybody has different experience, and this is
> basically very good!
>> We often repeat this: “side-effects”, “tracks”, “safe”. But what does
>> it actually mean? Can I have side-effects in Haskell? Yes. Can I mix
>> side-effects? Yes. But in more difficult way than in ML or F#, for
>> example. What is the benefit?
>> That it is difficult to accidentally introduce side effects.
>> Or, rather, the problems of side effects. Formally, no Haskell program
>> can have a side effect (unless using UnsafeIO or FFI, but that's not
>> what we're talking about here).
> Actually if we look to this from high-level, as to "black box" - we see
> that it's truth. Haskell allows to have them, to mix them but in
> different manner.
>> Yes they will. Some tests will fail if they expect specific output. If
>> the program has a text-based user interface, it will become unusable.
> And wise-versa: if I will remove "print" from such tests and add "pure"
> - they can fail too. IMHO purity/impurity in your example is related to
> expected behavior and it violation, not to point that "more pure - less
> bugs". Pure function can violate its contract as well as impure.
>> Yes they will become buggy. You'll get aliasing issues. And these are
>> the nastiest thing to debug because they will hit you if and only if
>> the program is so large that you don't know all the data flows
>> anymore, and your assumptions about what might be an alias start to
>> fall down. Or not you but maybe the new coworker who doesn't yet know
>> all the parts of the program.
>> That's exactly why data flow is being pushed to being explicit.
> So, to avoid this I should not mix read/write monads, to avoid RWST. In
> this case they should be removed from the language. And monad
> transformers too. My point is: there is some misunderstanding - I often
> listen "side-effects are related to errors", "we should avoid them",
> "they leads to errors", etc, etc, but IMHO pure/impure is needed to FP
> language compiler, not to me. This is the real motto. Adding of
> side-effects does not lead to bugs automatically. Mostly it does not.
> More correct is to say: distinguish of pure/impure code is better to
> analyze the code, to manipulate with it, to transform it (as programmer
> I can transform F# code *easy because no monads*, in Haskell *compiler*
> can transform code easy *because monads*). More important argument for
> me is example with Free monads. They allows to simulate behavior, to
> check logic without to involve real external actions (side-effects).
> Yes, OK, this is argument. It's not explicitly related to buggy code,
> but it's useful. It remember me homoiconic Lisp code where code can be
> processed as data, as AST.
> Actually, I had a big interesting discussion in my company with people
> which does not like FP (the root why I become to ask such questions to
> himself). And I got their arguments. I tried to find solid base of mine.
> But currently I see that I like Haskell solutions itself, and I can not
> show concrete examples where they are needed in real world, without
> Haskell specific limitations. I know that those limitations lead to slow
> compilation, to big and complex compiler, I can not prove that
> side-effects means "lead to error", or (more interesting) that it's bad
> to separate side-effects from each other. F#, ML, Lisps have "do-" block
> and no problem with it. They don't need transformers to mix 2 different
> effects in one do-block. If you can prove that this decision leads to
> bugs and Haskell solution does not: it will be bomb :) I think, will be
> a lot of people in CS which will not agree with you ever.
> ---
> Best regards, Paul
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