The blog post is an exploration of an alternative way to structure code in Haskell.
Why is the bar such that Haskell blog posts have to either demonstrate something clearly better then the status quo or that they need to explain the fundamentals of the language?
wk_end 4 hours ago [-]
The audience is going to meet the article where they're at.
It's fine for, say, a blog post aimed at Haskellers to assume Haskell knowledge, but when posted on a board largely consisting of people without Haskell knowledge, it's natural that you're going to get at least a few people saying, "hey, I don't understand this".
But I'll be honest - I'm familiar with Haskell and the ML module system and the underlying concept (that typeclasses and modules are roughly equivalent in some sense), but I'm unfamiliar with Backpack so I still struggled to follow it a little. The target audience is an extremely narrow niche. So I think it's just somewhat poorly written; it doesn't feel like the author really had an audience in mind, other than themselves. There's probably ways of writing this - without spending too much time regurgitating the basics - that would be more palatable to more people.
solomonb 3 hours ago [-]
> The audience is going to meet the article where they're at.
I hear you on this point but anyone can post anything on this forum. The burden should not be on the author to write a post that aligns with whatever forum their blog might get posted onto.
teddyh 1 hours ago [-]
The author is free to ignore any and all complaints they consider unfounded. It’s not even like the author is recieving any complaints personally; they have to come here to see any. And if they come here, they will get to read the viewpoint visible from here.
mvdtnz 1 hours ago [-]
Maybe if the post title used the word "Haskell" it wouldn't attract the opinions of people not interested in Haskell? Pretty obvious stuff.
randomNumber7 33 minutes ago [-]
It contains "type class".
indigoabstract 10 hours ago [-]
Delightfully weird and niche article.
And I wouldn't be surprised if there were more retired left handed surgeons in their 50s living in rural Switzerland than people who understand what he's talking about.
Which highlighted the fact that typeclasses can basically be thought of as an additional vtable argument passed to every function that has typeclass constraints. (And, indeed, thinking about them this way often allows one to avoid typeclasses entirely and achieve the same kind of polymorphism in simpler and more flexible ways.)
Backpacks can achieve something similar, except by declaring an abstract "signature" for your dependency, rather than a function argument. (Backpacks can also be used to do a lot more than this, of course, but that's outside the scope of the OP article.)
throwaway81523 1 hours ago [-]
I think Oleg Kiselyov also has a post somewhere explaining how you can use Ocaml modules to do something similiar to Haskell typeclasses.
mrkeen 8 hours ago [-]
In this case that’s the >>= from Maybe.Monad. As long as you satisfy the signature, it’s happy . do has nothing to do with Monads! Who lied to you?
Could have been a stronger point by using a non-monadic >>=.
I've been doing a lot of parsing lately and I find I don't need to reach all the way for monad (applicative is usually enough). But I guess that's what ApplicativeDo[1] is for.
We’ve got to be explicit now which Functor or Monad we’re importing, and you can’t have do notation for different Monads in the same module.
This is a bit rough for usability (not to mention also undermines the point above).
But overall I like the approach of trying something radically new.
ApplicativeDo can be thought of as a compile-time transformation that turns sequential code into possibly parallel* code just by analyzing the way variables are used. Once I've had that realization I really appreciated it more.
*: By "possibly parallel" I mean the blocks of code are known to be independent from each other. They could be actually executed in parallel if the Applicative instance does so.
ambicapter 6 hours ago [-]
Does that mean it may or may not run the code in parallel once compiled? Doesn't seem very ergonomic to the developer if true.
kccqzy 5 hours ago [-]
Why isn't it ergonomic? You just read the documentation or source for the Applicative class in question to find out.
If the Applicative class chooses to decide randomly whether to run in parallel, that's the class's fault, not the language or the compiler's problem.
SkiFire13 13 hours ago [-]
I feel like this article would be much more approachable if it didn't assume readers already know Ocaml and Haskell and their module system
nine_k 12 hours ago [-]
Indeed so, but that would require a few months worth of work from the reader, unfortunately.
(I have a t-shirt with a lambda in a circle, reminiscent of the anarchist emblem, and words "no class, no state". It's definitely possible to explain to a passer-by who never studied FP what it refers to, but not in such a way that the joke remained funny. Possibly the same deal is with the bumper sticker saying "my other car is cdr".)
codebje 12 hours ago [-]
"Humor can be dissected, as a frog can, but the thing dies in the process and the innards are discouraging to any but the pure scientific mind." - E. B. White.
tmtvl 21 minutes ago [-]
Unexplained humour dies over time. Think of the classic 'a dog walks into an inn and says "I think I'll open the other one"'.
Supermancho 32 minutes ago [-]
Contemporary rephrasing:
"<InsertSubject> can be dissected, like a frog. It turns out that nobody is very interested and the frog dies of it."
DaiPlusPlus 10 hours ago [-]
> a lambda in a circle
How often do people think you're a Half-Life fan instead?
nine_k 5 hours ago [-]
Likely never. The Half-Life has the lambda with the top crooked, neatly inscribed in a circle [1]. The anarchist version has everything straight, and with the three sticks protruding out of the circle (like the A would). The visual rhyme is obvious.
The CS understanding of "No class, no state" is just about at the edge of something Aphex Twin might care about. But the stylized A really isn't that similar to a Lambda
Thanks, I scanned through all the comments/links but this is the actual resource one wants to read to get familiar with Backpack.
nesarkvechnep 10 hours ago [-]
Yeah, it’s better to not assume anything and be stuck in the basics forever, right?
4ad 12 hours ago [-]
It would also be a useless article. It's fine to write for an audience, if you're not in the target audience, move on.
Vosporos 11 hours ago [-]
It's okay not to know things
whateveracct 1 hours ago [-]
Uh so? There's a lot of value in articles that don't rehash the basics for the noob reader.
actionfromafar 12 hours ago [-]
Yeah... meanwhile, in lesser languages, type classes is all we have.
vjerancrnjak 5 hours ago [-]
How to reuse `readFile` `writeFile` program with this module trick?
Assuming `IO.readFile` and `IO.writeFile` is replaced by HTTP requests.
I can define `writeFile` and `readFile` in a type class and then implement the effect for HTTP variant, hiding the HTTP client beneath.
Is it just wiring it up in mixins, cabal file?
I think general conclusion is that there's no need for dependency injection, environment objects, or similar tricks if module system is rich enough.
For a long time I questioned why Python needs anything but `async def` or `def` (async should be achievable through module or `yield` usage) and `import` statements, to achieve maximal reuse, given dynamic nature of language and modules. We could ignore all object-oriented features, decorators, replace them with modules. Would be flatter and readable compared to bloated feature set.
yuppiemephisto 1 hours ago [-]
I enjoyed this, hadn't used backpack and this is a nice tutorial
semiinfinitely 11 hours ago [-]
even for someone moderately interested in FP, this one goes above my head and the only take-away I can get from it is "maybe use ocaml instead of haskell"
acomar 4 hours ago [-]
it's experimenting with a feature that was added to Haskell but just never caught on. it's theoretically very powerful but it's far enough from idiomatic Haskell that it sees basically zero usage in the wild as the cost for using it is that very few people can understand what you've done.
seanparsons 7 hours ago [-]
I'm reasonably versed in Haskell and my response would be that it shouldn't make that much difference to you what they've written in here. I've yet to see any code in the wild using the backpack extension.
kccqzy 6 hours ago [-]
It's more of an experiment: what if we take this beloved feature from OCaml to port it to Haskell (backpack), and see if it can replace a different beloved Haskell feature (type classes).
test6554 12 hours ago [-]
I just picture Charlie from “It’s Always Sunny in Philadelphia” connecting the dots.
edem 2 hours ago [-]
As someone who has zero OCaml knowledge...can someone explain what's going on?
wk_end 1 hours ago [-]
OCaml (also SML) has a really powerful module system; it's so powerful that to call it a "module" system is maybe misleading to the average developer. You might think of a "module" system as a way of dividing your program into different files, and the ML module system certainly subsumes that case, but it goes beyond that.
The key to this is that modules in ML are actually kind of a separate programming language. Not only can you define modules, you can define "functor modules" - modules that are kind of like functions; you can pass modules as arguments to functors to produce new modules. And there's a (structural!) type system here, too: the modules a functor can accept are specified to have a specific structure, and if you try to pass an incompatible module to a functor you get a type error.
(Incidentally: the naming of functors in the module system is really unfortunate here, because it overlaps with the name of functors in category theory/Haskell.)
This sounds extremely abstract and kind of insane; it's easier to understand it in practice. A typical example might be that you want to define a hash table, which requires some kind of hash operation for a key. What you'd do idiomatically in ML is define a HashTable functor which takes, as an argument, a module X consisting of a type t and and a hash function. This would generate a HashTable(X) module specialized to that particular key type.
What's interesting here is that there's an overlap here with things like interfaces in Java or typeclasses in Haskell, where your HashTable type would demand that the key type adheres to a Hashable interface. It turns out they're kind of (kind of!) just different takes on the same thing. There's even some interest in the OCaml world in trying to "close the gap" with a feature called "modular implicits" [0].
The other thing to know is that there's an esoteric feature of Haskell called "Backpack", which is an attempt to bring the ML module system to Haskell. It's not exactly widely used or anything like that, but it's there and been in GHC for several years now.
This article is basically just demonstrating how Backpack lets you use modules in much the same way as you'd traditionally use typeclasses in Haskell.
Thank you for such a concise summary. I had no idea that types in ocaml are that powerful.
platz 7 hours ago [-]
In the 2010-2020 era, readers of Hacker News used to know how to read Haskell and had strong opinions (pro and con) of it.
That era is now over.
marcosdumay 5 hours ago [-]
The problem is that this is not only about Haskell's fundamentals. It's about them being extended with backpack, an extension that very few people use.
And honestly, I don't see the point there either. I know that backpack is meant to solve some problem everybody has, but I haven't been able to fully understand what that problem is. Specifically on this article, I don't see the gain over using a type class.
ghc 2 hours ago [-]
There are still dozens of us!
pyrale 6 hours ago [-]
Maybe in 2010, but you have an idealized view of 2020 HN.
platz 6 hours ago [-]
peak FP debate was 2015/2016-ish
leethomas 5 hours ago [-]
Orrrrr just use a lisp?
12_throw_away 11 hours ago [-]
"Functors" "Monads" "Typeclasses" <- concepts dreamed up by the utterly deranged
> Have you ever seen a Number grazing in the fields? Or a Functor chirping in the trees? No? That’s because they’re LIES. LIES told by the bourgeoisie to keep common folk down.
jerf 6 hours ago [-]
Grumbling about how typeclasses could just be normal datatypes has been an undercurrent in the Haskell world for a long time, particularly as a way to solve the orphan instances problem. However, the syntactic grease provided by typeclasses is pretty appealing, and in the end I think this may have died down because people developed engineering solutions to the orphan instances problem (mostly "don't") that mitigated the theoretical problems enough that even most Haskellers don't care anymore... but I guess a few do still somewhat, because such things never truly go away.
(And yes, I recognize the quoted sentence as being humorous and labeling the entire article as not entirely serious... my point is that it is not entirely unserious either. The idea has been kicking around seriously for a while.)
steve_gh 10 hours ago [-]
There is a really interesting interview with Simon Peyton-Jones referenced on HN yesterday.He talks a lot about why Haskell came about, and some of the thinking behind the design choices that were made.
The blog post is an exploration of an alternative way to structure code in Haskell.
Why is the bar such that Haskell blog posts have to either demonstrate something clearly better then the status quo or that they need to explain the fundamentals of the language?
It's fine for, say, a blog post aimed at Haskellers to assume Haskell knowledge, but when posted on a board largely consisting of people without Haskell knowledge, it's natural that you're going to get at least a few people saying, "hey, I don't understand this".
But I'll be honest - I'm familiar with Haskell and the ML module system and the underlying concept (that typeclasses and modules are roughly equivalent in some sense), but I'm unfamiliar with Backpack so I still struggled to follow it a little. The target audience is an extremely narrow niche. So I think it's just somewhat poorly written; it doesn't feel like the author really had an audience in mind, other than themselves. There's probably ways of writing this - without spending too much time regurgitating the basics - that would be more palatable to more people.
I hear you on this point but anyone can post anything on this forum. The burden should not be on the author to write a post that aligns with whatever forum their blog might get posted onto.
And I wouldn't be surprised if there were more retired left handed surgeons in their 50s living in rural Switzerland than people who understand what he's talking about.
The author then uses Backpacks to achieve ad-hoc polymorphism without typeclasses.
There is a well-known article from a long time ago which was conceptually similar: https://www.haskellforall.com/2012/05/scrap-your-type-classe...
Which highlighted the fact that typeclasses can basically be thought of as an additional vtable argument passed to every function that has typeclass constraints. (And, indeed, thinking about them this way often allows one to avoid typeclasses entirely and achieve the same kind of polymorphism in simpler and more flexible ways.)
Backpacks can achieve something similar, except by declaring an abstract "signature" for your dependency, rather than a function argument. (Backpacks can also be used to do a lot more than this, of course, but that's outside the scope of the OP article.)
I've been doing a lot of parsing lately and I find I don't need to reach all the way for monad (applicative is usually enough). But I guess that's what ApplicativeDo[1] is for.
This is a bit rough for usability (not to mention also undermines the point above).But overall I like the approach of trying something radically new.
[1] https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/appl...
*: By "possibly parallel" I mean the blocks of code are known to be independent from each other. They could be actually executed in parallel if the Applicative instance does so.
If the Applicative class chooses to decide randomly whether to run in parallel, that's the class's fault, not the language or the compiler's problem.
(I have a t-shirt with a lambda in a circle, reminiscent of the anarchist emblem, and words "no class, no state". It's definitely possible to explain to a passer-by who never studied FP what it refers to, but not in such a way that the joke remained funny. Possibly the same deal is with the bumper sticker saying "my other car is cdr".)
How often do people think you're a Half-Life fan instead?
[1]: https://en.wikipedia.org/wiki/Gordon_Freeman#/media/File:Gor...
[2]: https://en.wikipedia.org/wiki/Anarchist_symbolism#/media/Fil...
Assuming `IO.readFile` and `IO.writeFile` is replaced by HTTP requests. I can define `writeFile` and `readFile` in a type class and then implement the effect for HTTP variant, hiding the HTTP client beneath.
Is it just wiring it up in mixins, cabal file?
I think general conclusion is that there's no need for dependency injection, environment objects, or similar tricks if module system is rich enough.
For a long time I questioned why Python needs anything but `async def` or `def` (async should be achievable through module or `yield` usage) and `import` statements, to achieve maximal reuse, given dynamic nature of language and modules. We could ignore all object-oriented features, decorators, replace them with modules. Would be flatter and readable compared to bloated feature set.
The key to this is that modules in ML are actually kind of a separate programming language. Not only can you define modules, you can define "functor modules" - modules that are kind of like functions; you can pass modules as arguments to functors to produce new modules. And there's a (structural!) type system here, too: the modules a functor can accept are specified to have a specific structure, and if you try to pass an incompatible module to a functor you get a type error.
(Incidentally: the naming of functors in the module system is really unfortunate here, because it overlaps with the name of functors in category theory/Haskell.)
This sounds extremely abstract and kind of insane; it's easier to understand it in practice. A typical example might be that you want to define a hash table, which requires some kind of hash operation for a key. What you'd do idiomatically in ML is define a HashTable functor which takes, as an argument, a module X consisting of a type t and and a hash function. This would generate a HashTable(X) module specialized to that particular key type.
What's interesting here is that there's an overlap here with things like interfaces in Java or typeclasses in Haskell, where your HashTable type would demand that the key type adheres to a Hashable interface. It turns out they're kind of (kind of!) just different takes on the same thing. There's even some interest in the OCaml world in trying to "close the gap" with a feature called "modular implicits" [0].
The other thing to know is that there's an esoteric feature of Haskell called "Backpack", which is an attempt to bring the ML module system to Haskell. It's not exactly widely used or anything like that, but it's there and been in GHC for several years now.
This article is basically just demonstrating how Backpack lets you use modules in much the same way as you'd traditionally use typeclasses in Haskell.
[0] https://www.cl.cam.ac.uk/~jdy22/papers/modular-implicits.pdf
That era is now over.
And honestly, I don't see the point there either. I know that backpack is meant to solve some problem everybody has, but I haven't been able to fully understand what that problem is. Specifically on this article, I don't see the gain over using a type class.
> Have you ever seen a Number grazing in the fields? Or a Functor chirping in the trees? No? That’s because they’re LIES. LIES told by the bourgeoisie to keep common folk down.
(And yes, I recognize the quoted sentence as being humorous and labeling the entire article as not entirely serious... my point is that it is not entirely unserious either. The idea has been kicking around seriously for a while.)
https://news.ycombinator.com/item?id=45242530
https://williamcotton.com/articles/a-burrito-is-a-monad