Haskell: A Great Procedural Language
entropicthoughts.com
entropicthoughts.com
Reading up to this point, I had to chuckle a bit. I have struggled with the Haskell type system more than I care to admit; It's almost never "we just don't have to care" when comparing to most other popular languages.
That being said, this article does a nice job of gently introducing some of the basics that will trip up somebody who is casually looking through code wondering what *> and <*> and <* do. As usual, there is a steep learning curve because of stuff like this all over the codebase. If I walk away for a month, I need to revisit >>= vs >> and other common operators before I can be productive. It probably doesn't help that I never actually speak to a human about these concepts so in my head it's always ">>=" and not "bind."
But I like <|> , it feels very clear that it has a sort of "or" meaning.
thing <- getThing
case thing of
writing this:
getThing >>= \case
Not so much because it is less code, but fewer variables to name.
The difference here seems to be that \case is a lambda?
Haskell’s >>= is doing something slightly different. ‘getThing >>= \case…’ means roughly ‘perform the action getThing, then match the result and perform the matched branch’
Whereas ‘getThing |> \case…’ means ‘pattern match on the action getThing, and return the matched action (without performing it).
The >>= operator can also be used for anything satisfying the Monad laws, e.g. your actions can be non-deterministic.
Interesting! I thought Elixir was mostly macros all the way down, like this article shows:
https://evuez.net/posts/cursed-elixir.html
Being able to pipe into def, defp, defmodule, if, etc. is fantastic! But apparently you say at least in the case of case, it's not a macro, and it's just a function which returns a monad—I guess it's the same effect afterall? Is that why people say Haskell can get away with lack of Lisp-style macros because of its type system and laziness?
Interesting. Probably it's just me, but I first learned monads using >>= (in OCaml), so at the beginning I found the Haskell do notation more confusing (and indentation rules didn't help). >>= is just a function and I understand its signature well. On the other hand, "do" is a syntactic sugar that I sometimes had to "mentally desugar" in some cases.
Struggling with Haskell type system is not an experience of somebody who has developed an intuition about Haskell type system. Granted, it is not a binary thing, you can have good intuition about some parts of it and struggle with others.
I think they way you put it is, while technically true, not fair. Those "most other" languages are very similar to one another. It is not C# achievement, that you don't struggle with its type system coming from Java.
This is like people struggling with Rust because of burrow checker, well, they have probably never programmed with burrow checker before.
FYI it's "borrow" (as in when someone lends something to you) not "burrow" (which is a tunnel/hole)
Given how many important projects maintained by smart people have dealt with bugs that safe Rust makes impossible, I'm inclined to doubt that.
But it shifts the need to memorize undefined behavior with the need to memorize the borrow checker rules.
If you are dealing with common system level needs like double linked lists, rust adds back in the need for that super human level memory of undefined behavior, because the borrow checker is limited to what static analysis can do.
IMHO, the best thing Rust could do right now is more clearly communicate those core limitations, and help build tools that help mitigate those problems.
Probably just my opinion, and I am not suggesting it is superior, but zig style length as part of the type is what would mitigate most of what is problematic with C/C++
Basically a char myArray[10];, really being *myArray is the main problem.
Obviously the borrow checker removes that problem, but not once you need deques treeps etc...
If I could use Rust as my only or primary language, memorizing the borrow checker rules wouldn't be that bad.
But it becomes problematic for people who need to be polyglots, in a way that even Haskell doesn't.
I really think there's ways for Rust to grow into a larger role.
But at this point it seems that even mentioning the limits is forbidden and people end up trying to invert interface contracts, and leak implementation details when they're existing systems are incompatible with the projects dogma.
It is great that they suggest limiting the size of unsafe code blocks etc....but the entire world cannot bend to decisions that ignores the real world nuances of real systems.
Rust needs to grow into a language that can adjust to very real needs without and many real needs will never fit into what static analysis can do.
Heck C would be safer if that was the real world.
I really do hope that the project grows into the role, but the amount of 'unsafe' blocks points to them not being there today, despite the spin.
With one massive difference: the borrow checker will tell you when you're wrong at compile time. Undefined behaviour will make your program compile and it will look like your program is fine until it's not.
I'd take that trade.
rust-analyzer is the tool that does this. Having it flag errors in my editor as I write code keeps me from getting too far down the wrong road. The moment I do something Rust thinks is goofy, I get a red underline and I can pause to consider my options at that moment.
You have a choice: Fight the borrow checker on the front end until your code compiles, or chase down bugs on the back end from undefined behavior and other safety problems. No single answer there, it depends on what you're doing.
Cognitive load is a big issue. In Rust I find this comes in a couple of phases, the first being figuring out what the borrow checker does and how to appease it. The second is figuring out how to structure your data to avoid overusing clone() and unsafe blocks. It's a bit like learning a functional language, where there's a short-term adaptation to the language, then a longer-term adaptation to how you think about solving problems.
You mention doubly-linked lists, which are a good example of a "non-star" data structure that doesn't nicely fit Rust's model. But: for how many problems is this the ideal data structure, really? They're horrible for memory locality. Those of us who learned from K&R cut our teeth on linked lists but for the last 20 years almost every problem I've seen has a better option.
I think use makes master: the more you use it, the more mastery you will have and the better intuition will follow.
To give an example, you use `>>=` after `readFile` to do something with the contents. You use `>>` after `putStrLn` since `putStrLn` doesn't return a meaningful value.
In a nutshell, first class effects and built in set of patterns for composing them get rid of boilerplate code. Combine that with type safety and you can churn out relatively bug free code very fast.
So why hasn't it happened? Some possibilities:
1. People are just ignorant/unenlightened.
2. Haskell is too hard to use for most people. I think that different programmers think in different ways, and therefore find different languages to be "natural". To those whom Haskell fits, it really fits, and they have a hard time understanding why it isn't that way for everyone, so they wind up at 1. But for those who it doesn't fit, it's this brick wall that never makes sense. (Yes, this is about the same as 1, just seen from the other side. It says the problem is the language, not the people - the language really doesn't fit most people very well, and we can change languages easier than we can change people.)
3. Haskell isn't a good fit for many kinds of programming. The kind of programs where it fits, it's like a superpower. The kinds where it doesn't, though, it's like picking your nose with boxing gloves on. (Shout out to Michael Pavlinch, from whom I stole that phrase.)
What kinds of programs fit? "If you can think of your program like a pipe" is the best explanation I've seen - if data flows in, gets transformed, flows out. What kind of program doesn't fit? One with lots of persistent mutable state. Especially, one where the persistent mutable state is due to the problem, not just to the implementation.
Myself, I lean toward a combination of 2 and 3.
4. History. In those types of discussions, there are always "rational" arguments presented, but this one is missing.
> One with lots of persistent mutable state.
You mean like a database? I don't see a problem here. In fact, there is a group of programs large enough, that Haskell fits nicely, that it cannot be 3; REST/HTTP APIs. This is pretty much your data goes in, data goes out.
> You mean like a database?
No, I mean like a routing switcher for a TV station. You have a set of inputs and a set of outputs, and you have various sources of control, and you have commands to switch outputs to different inputs. And when one source of control makes a change, you have to update all the other sources of control about the change, so that they have a current view of the world. The state of the current connections is the fundamental thing in the program - more even than controlling the hardware is.
But in your example, PHP is also a bad choice, and alas, it dwarfs Haskell in popularity. I can't really think of where PHP is a great fit, but Haskell isn't.
Later, Python took its place, since people realized the Java ecosystem was way too complicated and was turning off would-be CS students. Python directly targeted the academic use case by having similarities to C, Java, and Bash——it was not a better language, it just made existing imperative and object-oriented assignments easier for classroom environments. Believe it or not, a lot of programmers and even academics sort of give up on exploring significantly unfamiliar directions after graduating.
[1] https://www.cs.utexas.edu/~EWD/OtherDocs/To%20the%20Budget%2...
Imperative languages do offer many advantages over Haskell, in that most coursework and industry jobs use them and that, consequently, their ecosystems are much further developed. These advantages are a consequence of university programs' alignment with the imperative and object-oriented programming paradigms, to Oracle's benefit.
Your colleagues having never looked back at lisp is hardly evidence that Haskell would have been too difficult for them or that Oracle didn’t have a hand in this.
Functional programming just seems harder for people to get into. Perhaps it's bad for everyone that people don't make that effort, but it doesn't seem like a conspiracy
The promotional campaign that Dijkstra mentions was perhaps orchestrated by Sun Microsystems, though perhaps not since Oracle was indirectly strategically aligned with Java, as the eventual acquisition shows.
Yes, it is more difficult to get into FP. However, asking the question why it became more difficult, when historically the opposite was true, is certainly worthwhile. Surely there was some cause.
He's asking for something that is really inappropriate. He lost in the CS department, and he wants the Budget Council to decide on what languages should be taught? Like they know anything about it!
He lost a political battle, and he's appealing it to the only place he can, and he's buttering them up to do it, but the people that actually know something about the topic decided against him already.
And, you're quoting only one side of the battle. One vocal and eloquent side, but only one side. Maybe look into why the UT CS department made that change? (And not why Dijkstra says they did.)
Political campaigns are often bought.
He mentions the expensive promotional campaign that was paid towards Java.
> the people that actually know something about the topic decided against him already
Matters of pedagogy often involve value judgements and trade-offs, with knowledge alone being unable to provide definitive answers.
However, Sun/Oracle did know that more money would flow their way if undergraduates were to learn Java. The letter suggests that one or both of them decided to act accordingly.
It’s questionable to assert that every knowledgeable faculty member thought that pivoting to Java was the best option. (Dijkstra himself is a counter-example?) From the looks of it, just a single department chair——not the full CS department——had the decision-making authority on this curriculum change.
> inappropriate
Would inappropriateness make his arguments any less true?
Why in an academic context would it be inappropriate to request input from additional stakeholders? If the letter was unconventional, remember that a purpose of the tenure system is to protect unconventionality.
I mean, look, if you had a letter signed by the majority of the department, complaining about the chair's decision, then the Budget Council might consider reversing the chair, on the authority of the expertise of the majority of the department. But overrule the chair on the basis of disagreement by one professor? No way. You can't run a university that way, because there's always at least one professor who disagrees with a decision.
Admittedly, I don’t know anything about who was on the budget committee to which Dijkstra wrote this letter. But it is just ordinary for academics to write proposals outlining their priorities in hopes that the grant/budget/financing committee will bite.
I think you probably mean Sun? Or maybe that Oracle continued what Sun started.
before the first class I get an email from one student asking if they must purchase the book for the class since it $275 (this is years ago) and I was taken aback, what kind of book costs $275 - even for a college textbook that was nuts. I told him to not purchase it until we meet for the first class. I go to the office and see my copy of the book, it is programming the web with oracle forms from oracle press!!!! I talked to Dean and he was like “yea, that is what we need to teach!” needless to say that, none of the kids bought the book, and I did NOT teach oracle forms, and I was never given that class again :)
Sun and Oracle were strategically aligned at the time, so it could have been either or both.
Off the top of my head, memory safety challenges for junior Haskellers (laziness footguns), State monad being fundamentally flawed: there is an inability to get at and log your application state just before a crash, bloated tooling, GHC frequently breaking existing code. Laziness and monadic code makes debugging painfully difficult.
I like Haskell for Advent of Code only.
Could you explain a bit about what you mean here? In what way is it fundamentally flawed?
I'm not saying you're wrong; I just want to understand.
This explains in more detail:
https://github.com/haskell-effectful/effectful/blob/master/t...
I agree loosely with what haskman above says about creating relatively bug free applications, the guard rails are so robust. But those same guard rails mean you can paint yourself into a corner that it is harder to get out of without imperative state, case in point above.
- Space leaks due to laziness are a solved problem. I explain the technique to solve it at: https://h2.jaguarpaw.co.uk/posts/make-invalid-laziness-unrep... This technique has not completely percolated throughout the community, but I am confident that it does actually resolve the "laziness causes space leaks issue"
- Flawed state monad: well, you point out the analysis of its flaws from the effectful documentation. That's correct. The solution is: just use effectful (or another similar effect system. I recommend my own: Bluefin)
- GHC breakage: I've been keeping an inventory of breakage caused by new GHC versions, since GHC 9.8: https://github.com/tomjaguarpaw/tilapia/ There has been very little! The Haskell Foundation Stability Working Group has had a massive effect in removing breakage from the ecosystem.
- Laziness and monadic code makes debugging painfully difficult: I mean, sort of, but if you're using monadic code in the style of a decent effect system like effectful or Bluefin this is a non-problem. It's hardly different from programming in, say, Python from the point of view of introducing debugging printfs or logging statements.
>well, you point out the analysis of its flaws from the effectful documentation. That's correct.
Thinking deeper about this, that there is essentially no way to fix this issue with StateT, because of the type choice, the monad, the composability requirement, all conspiring together to not be undone, does that signal something deeper that is wrong with the flexibility of Haskell, that we can progressively paint ourselves into a corner like this. Could it not happen again, but with another late breaking requirement, for effectful, or bluefin?
Ah, that's great to know, thanks. It's rarely clear to me whether people read or are interested in what I say!
Well, yes, in principle even a design that is perfect according to some spec could be completely wrong if the spec needs to change. and impossible to tweak to match the new spec. This is true of any language or any system. This raises a few important questions:
1. How easy does a language make it to "unpaint" yourself from a corner?
In Haskell it's easier than in any other language I've experienced, due to its legendary refactoring experience. For example, if you "incorrectly" used the State monad and got stuck, you can wrap it up in an abstract type, change all the use sites, check that it still compiles and passes the tests, then change the definition to use new "uncornered" implementation, again check it compiles and passes the tests, then unwrap the abstract type (if you like, this stage is probably less important), then add the new feature supported by the new implementation.
2. How likely is it to paint yourself into a corner in the first place?
In Haskell, again, less likely than any other language I've experienced, because the constructs are so general. There is far more opportunity to tweak a design when you have general constructs to work with. (That said, I've met many Haskell behemoths that couldn't be easily tweaked so, particularly contorted type class hierarchies. I recommend not designing those.)
3. Why won't effectful or Bluefin lead to "corners"?
Because they're just Haskell's IO, wrapped up in a type system that gives fine-grained control over effect tracking. Anything you can do in Bluefin and effectful you can do in IO, and vice versa. So to really paint yourself into a corner with IO-based effect systems it would have to be something that you can't do in IO either, and at that point we're talking about something that can't be done in Haskell at all. So there's no real downside to using IO-based effect systems in that regard.
1. Tooling has historically been a mess, though it's rapidly getting better.
2. Error messages are opaque. They make sense to someone familiar with Haskell, but others cannot make the leap from an error message to the fix easily.
3. It's a jack of all trades. The resulting binaries are not small. Performance can be very good but can be unpredictable. It doesn't compile nicely to the web. Doesn't embed well. There is basically no compelling reason to get into it.
4. The ecosystem is aging. You can find a library for almost any obscure usecase, but it would be many years old, and possibly require tweaking before it even compiles.
Haskell gives you a wide range of tools, from simulated state like the State monad, to real ones like the ST monad and IORef inside the IO monad. For synchronization between threads you have atomic IORef, MVar, and TVar.
If you problem requires you to have persistent mutable state, Haskell helps you manage it so that you can truly separate those persistent mutable state that's due to the problem from those that's due to the implementation.
Maybe now that I'm older and wiser (debatable) a revisit is in order, but lately I prefer dynamically typed languages (like Clojure and Elixir) to statically typed ones. I'll probably add it to my TODO list, but the list is long and time is short.
I don't have a math background. I failed high school maths.
I've been working as a professional Haskell programmer for several years.
What you are saying is just demonstrably not true. So why are you saying it?
functional and logical languages are indeed very simple, small core, very general laws.. (logic, recursion, some types) but grokking this requires unplugging from a certain kind of reality.
Most people live in the land of tools, syntax and features .. they look paradoxically both simpler than sml/haskell so people are seduced by them, yet more complex at the same time (class systems are often large and full of exceptions) but that also makes it like they're learning something advanced, (and familiar, unlike greek single variables and categ-oids :).
Understanding the map signature in Haskell is more difficult than any C construct. Now do IO monad.
This is obviously false. The map type signature is significantly easier to understand than pointers, referencing and dereferencing.
I am an educator in computer science - the former takes about 30-60 seconds to grok (even in Haskell, though it translates to most languages, and even the fully generalised fmap), but it is a rare student that fully understands the latter within a full term of teaching.
I wouldn't say one is more difficult than the other.
IMO `map` is a really bad example for the point that OP is trying to make, since it's almost everywhere these days.
FlatMap might be a better example, but people call `.then` on Promises all the time.
I think it might just be familiarity at this point. Generally, programming has sort of become more `small f` functional. I'd call purely functional languages like Haskell Capital F Functional, which are still quite obscure.
I didn’t say “using map” I said understanding the type signature. For example, after introducing map can you write its type signature? That’s abstract reasoning.
Pointers are a problem in Haskell too. They exist in any random access memory system.
Sure, IORef is a thing, but it's hardly comparable to the prevalence of pointers in C. I use pointers constantly. I don't think I've ever used an IORef.
map :: (a -> b) -> [a] -> [b]
> Given a function from `a` to `b`, return a function from a `list of as` to a `list of bs`.
or:
> Given a function from `a` to `b` and a `list of as`, return a `list of bs`.
I find the first to be the more intuitive one: it turns a normal function into a function that acts on lists.
Anecdotally, I've actually found `map` to be one of the most intuitive concepts in all of programming. It was only weird until I'd played around with it for about 10m, and since then I've yet to be surprised by it's behavior in any circumstance. (Although I suppose I haven't tried using it over tricky stuff like `Set`.)
`fmap` is admittedly a bit worse...
fmap :: Functor f => (a -> b) -> f a -> f b
I’m saying that thing you did where you start representing concepts by letters which can be populated by concrete objects is not a skill most people have.
Eagerly? Yes. Imperatively? Not as much as SW devs tend to think.
When the teacher tells you to sort the papers alphabetically, he's communicating functionally, not imperatively.
When the teacher tells you to separate the list of papers by section, he's communicating functionally, not imperatively.
When he tells you to sum up the scores on all the exams, and partition by thresholds (90% and above is an A, 80% above and above is a B, etc), he's communicating functionally, not imperatively.
No one expects to be told to do it in a "for loop" style:
"Take a paper, add up the scores, and if it is more than 90%, put it in this pile. If it is between 80-90%, put it in this pile, ... Then go and do the same to the next paper."
People usually don't talk that way.
When you’re explaining someone how to sort physical objects, they will think in terms of “okay I’ll do x [a physical mutable state change] and then I’ll have achieved physical state y, and then I’ll do z (etc.)”.
Anyway: He's telling you to do something, and you do it. It doesn't get more imperative than that.
I would imagine that probably the majority of Haskell devs would understand it, which is all that matters.
99% of the population doesn't understand anything in Perl, either. That's not a mark against Perl.
There's not really first class effects though, ultimately just IO.
Could you please clarify what you mean by 'stored'?
‘traverse :: Applicative f => (a -> f b) -> t a -> f (t b)’
And you can derive it for free for your own datatypes!
The amount of code I’ve manually written in other languages to get a similar effect is painfully large.
traverse :: (Traversable t, Applicative f) => (a -> f b) -> t a -> f (t b)
data MyType ... = ... deriving (Traversable)I understand how Applicative works, but I don’t know how to apply (pun intended) to my data types.
For one client, we needed to accumulate some extra statistics for each. For another, we needed to call their web API (so async I/O) to get some of the data used in the report. By making the generic superclass use a generic Applicative type, we could keep the report business logic clear and allow the client-specific subclasses to do these client-specific things and have them compose the right way.
Wanting custom applicative types is rarer than using a standard one, but it can be a good way to represent any kind of "secondary effect" or "secondary requirement" that your functions might have. E.g. "requires this kind of authorisation" or "must happen in a database transaction". But a lot of the time you can implement custom things using reader/writer/state, or free, rather than having to write a completely from-scratch applicative.
Besides Maybe/Either `t` could represent anything, like container types Lists/Trees/Hashmaps etc, but also more complicated structures like syntax trees for programming languages or custom DSLs. Or (my favorite use case!) recursive command types for robot control. I'm doing this mostly in Rust, but borrow all the ideas from Haskell.
I tried it on couple of one liners and it found a couple of potential problematic points, one for each one liner.
There are plenty of interesting Haskell programs (e.g. Xmonad, Darcs), but a lot of people explicitly use them because they're Haskell.
Pandoc, on the other hand, is useful to pretty much anyone who has ever needed to convert documents. It's one of the first things I install on most computers.
Private messenger for desktop and mobile platforms. It's mostly written in Haskell except for UI.
> cpp-for-mobile > Template for cross-platform mobile app with native UIs and C++ core
Their flake.nix indicates they use Haskell to generate cross compiled shared library for Android, iOS, Mac, Linux and Windows.
I am not expert in Nix but at high level I can see they are indeed using Haskell.
Goes to show that success is not determined by technology.
hledger - a plain text accounting tool
CADQuery/build123d is the other big one I'm interested in.
Co-Star, the astrology SaaS, is apparently written with a Haskell backend. I'd love to have seen the casting call for that.
I believe the Mercury bank also runs most of their backend stuff on Haskell. Functional languages in general are surprisingly common among financial investment firms.
Some of Target's stuff is written in Haskell. I think there was at least one big Facebook project that was written in Haskell, but they may have moved away from it by now. Awake Security does some Haskell stuff.
One thing which might be surprising is Haskell is apparently quite strong for general backend web dev.
Yep. Mostly because of the https://www.servant.dev/ framework (but see also IHP, Yesod, and other frameworks). Servant lets you declare your HTTP API at the type level, and then it will infer the correct types for your endpoint handlers. You can also extract OpenAPI specs from it, generate clients for Haskell or other languages, etc.
My current employer, Bellroy, uses Haskell for pretty much all new code and Servant for all new HTTP APIs. https://exploring-better-ways.bellroy.com/our-technology-sta... is an older post discussing the shift to Haskell. We've found Haskell code to be much more compact than the equivalent Ruby, and significantly more robust.
I think they were using Agda or something too for a while, but it appears I can't find what I'm thinking of on their site anymore. Really interesting guys if you're located in the Baltic states.
They have a page about it: https://www.costarastrology.com/why-haskell/
It does one of the things I find most annoying about Haskell programmers, which is that they think the language is magic just because it has ADTs.
> HASKELL MAKES ILLEGAL STATES UNREPRESENTABLE.
That's not true!
It's especially not true for numeric programming, Haskell really doesn't provide good support for that. Basically only Ada and dependent-type languages do.
https://hackage.haskell.org/package/clash-prelude-1.8.2/docs...
How it compares to Ada though, I could not say.
Lispers hailing macros and metaprogramming cannot understand that power is also the very thing that makes jumping from project to project difficult, I have no intention of relearning your cleverly designed DSL or new extensions.
There's a reason why Java or PHP have plenty of widely used killer software, while monocle-wielding lispers and haskellers have very little to show after many decades.
It's not FP being the issue, it's just that their power attracts crowds interested in code more than the features/business/product.
I don't blame them, I love Haskell and Racket but I think very few teams can scale and make the compromise worth it.
also http://detexify.kirelabs.org/classify.html was surprisingly useful in university
In one such system I built had the main (REST API exposing) backend implemented in Kotlin with a separate application in Haskell doing a complex set of maths driven business rules against GIS data to calculate area specific prices.
The amount of IO on the Haskell side was fairly minimum and abstracted away quite nicely.
Haskell allowed expressing all complexity in a way that was easy to audit and translate from business/data analyst requirements.
Would do again :-) But only with the correct amount isolation so you can lean into Haskell's strong sides.
https://wiki.haskell.org/Haskell_in_industry
Imagine a "Go in industry" page. Even a "Rust in industry" would be laughable at this point.
Ok I'm maybe being a little unfair, but you get the point.
- map only works on Lists (one needs fmap for functors)
- head throwing exceptions instead of returning Maybe
- dependent types bolted on later: they're much nicer in Idris
- I don't fully understand this, but do you mean that every `IO a` function would be better off being `IO (Maybe a)`?
- AFAIK, there are no dependent types in Haskell yet, but say that the type level programming you can do today is what you mean then you are already in quite advanced territory. Yeah, I guess it could be more polished.
The choice is keep it is it is, or change it. Changing it would break vast amounts of existing code. Which option do you prefer?
{-# LANGUAGE GHC2024 #-}As I write, its links to ghc.gitlab.haskell.org are all broken.
If you want a language that is very Haskell-like without the historical baggage or the laziness, PureScript is very good. It’s main compile target is JavaScript, so it’s built for different use cases.
I once had a hard to track down bug in some code making use of conduit[0], which is introduced using examples like `main = runConduit $ (yield 1 >> yield 2) .| mapM_C print`.
Dutifully replacing every occurrence of (>>) with (>), because it was more modern, suddenly changed the semantics somewhere, due to the fact that (>>) is defined with fixity `infixl 1 >>` and (>) as `infixl 4 >` - i.e. both are left-associated operators, but (*>) binds tighter than (>>) and some of the myriad of other operators you may encounter.
-- [0] - https://github.com/snoyberg/conduit
If I were to start a new side-project using Haskell today (I probably won't), I would just stick to do-notation and even concrete types (instead of mtl-style or what have you) where possible.
"This seems rather … procedural. Even though we get all the nice guarantees of working with side effectful functions in Haskell, the code itself reads like any other procedural language would. With Haskell, we get the best of both worlds."
Working with the IO monad is much more complex, especially if you want to use other monadic types inside that code.
Yes, mixing other monadic types with IO can be a challenge. One possible solution to that is not just not use other monadic types inside the code. Just use a general-purpose effect system instead. I recommend effectful or Bluefin (the latter my own project).
It makes Haskell even more procedural, you can use intermediate variables in do blocks.
http://blog.sigfpe.com/2023/08/what-does-it-mean-for-monad-t...
Is there any parallel there, or is it an entirely wrong perception I got ?
JavaScript's `Promise` is particularly interesting because it is a Monad over "all JS values which do not contain a method called `then` in their prototype chain" or as Dominic put it 12 years ago:
> Indeed, I like the way @medikoo phrases it. There's, practically speaking, nothing wrong with being a monad on the category of non-thenables.
https://github.com/promises-aplus/promises-spec/issues/101#i...
Of course. Promise is a monad, .then is more or less equivalent to the >>= operator and await makes it look more imperative-ish just like <- in Haskell.
Note that in JS you'll need to be inside an async function to use await, just like in Haskell you'll need to be inside the do notation to use <-. Otherwise, you'll need to play with .then just like you would need to play with >>= in Haskell.
About other languages, one way to achieve asynchronous behavior without having it implemented in the language is using ReactiveX (https://reactivex.io/). It's hard to understand at first, but it if you understand the IO Monad it becomes easy, as it's basically the same.
If you think of effectful computation in Haskell to be a sort of metaprogramming, then indeed it will seem very complex, absurdly complex actually.
If you think of it as just the same as programming in any other language, except functions that do effects are tagged with a type that indicates that, then I think things will appear much simpler.
I prefer thinking of it as the latter, and it's been very effective (pun half intended) for me.
() => console.log(“abc”)
The real sell is: Haskell objects which are not Maybe are definitely there. Functions which are not IO will always give the same input for the same output.
So print(s) returns a function that when called prints s. Then there needs to be function that joins those functions, so print(a); print(b) evaluates to a function that once called prints out a and then b.
What makes Haskell special in my opinion is 1) it generalises this way of achieving "stateful" functions, 2) enforces such discipline for you and makes sure calling functions never produces side effects, and 3) some syntactic sugar (do, <-, etc) to make it easier to write this kind of code.
Also note that the above example only does output which would be the easier case. When it comes to code with input, there will suddenly be values which are unavailable until some side effects have been made, so the returned functions will also need to encode how those values are to be obtained and used, which complicates things further.
It's a starting point. You'll have to somehow chain (e.g. you want to print "def" after "abc", how would you do that?). You'll also need to put a value into your IO, e.g. if you are inside that IO, all the computations will need to produce IO. If you solve those things, you'll have a Monad.
But, wait! You're in JS, right? You have IO Monads there, perhaps only don't know yet. They are the JS Promises! Look this:
// assuming that you have a sync input function, that receives a string from stdin and returns it
let input_str = new Promise((resolve, reject) => resolve(input());
let input_int = input_str.then(s => parseInt(s));
let square = input_int.then(n => n * n);
square.then(sq => console.log(sq));
If you want to proceed with your poor man's IO, you'll need to implement your "then". Going further, the equivalent in Haskell (not the best code, though) to that is:
main = result
where input_str = getLine
input_int = input_str >>= \s -> return (read s) :: IO Int
square = input_int >>= \n -> return (n * n)
result = square >>= \sq -> putStrLn (show sq)
main = do
input_str <- getLine
let input_int = read input_str :: Int
let square = input_int * input_int
putStrLn (show square)
let input_str = await new Promise((resolve, reject) => resolve(input()));
let input_int = parseInt(input_str);
let square = input_int * input_int;
console.log(square);
In those senses, these are the same. You can emulate the IO monad in this way.
Haskell provides guarantees that make this stronger. There is no way to perform IO-like side effects except via the IO monad. And no way to “evaluate” the IO monad except to declare it as your main entry point, compile, and hit play.
(Except unsafePerformIO, of course, the highly discouraged escape hatch).
These restrictions are actually what makes the use of the IO monad powerful. As you show, it’s easy to “add” it to a language, and also almost valueless. The value is in removing everything that’s not in the IO monad.