Haskell web programming (a simple tutorial)
yannesposito.com
yannesposito.com
My first dose of Haskell was attempting to write an XMonad config. That turned out to be a frustrating, several hour long ordeal for me that had me write off Haskell as undecipherable, alien hieroglyphics for quite some time after that. I'd urge anyone reading the tutorial who likes what they see to put in the hard time to get a handle on the language, understand what its good for and what it isn't and then return to web programming with it.
But it is clear in my mind, you must know Haskell to do something useful with it. And in particular, make a web application, even if using a web framework.
As you stated, Haskell is not a language you could learn in 3 hours like I learned the bases of Python. It takes a very long time to learn, particularly when you're not used to functional programming.
But an intent of this tutorial was to promote Haskell. Clearly, if someone want to do something a bit more useful, he will very soon realize he must understand Haskell.
I added a specific advice in my conclusion similar to yours and pointing some essential resources.
Also, what about Clojure? That runs on the JVM but how does it fare on the "correctness" claim of Haskell, i.e., "If it compiles it's close to what the programmer intended".
I'm learning Haskell right now and really loving it. I wonder if I should stick with it for more production projects or make a switch to Erlang/Clojure. I know Erlang has quite a few success stories behind it, CouchDB being one of them.
> Also, what about Clojure? That runs on the JVM but how
> does it fare on the "correctness" claim of Haskell, i.e.,
> "If it compiles it's close to what the programmer
> intended".
If you want JVM and reasonably advanced type systems (given that Haskell's claim to correctness essentially boils down to this), try Scala.
For example, the fastest Erlang programs on the benchmarks game are all slower than all the GHC Haskell programs,
* http://shootout.alioth.debian.org/u64q/benchmark.php?test=al...
Median slowdown relative to GHC is 7x.
GHC is aiming for the high performance computing world of late, as well. The Erlang language just doesn't have the facilities for raw number crunching that's needed.
It's not to hard for you to go get benchmark numbers in haskell, check out http://www.serpentine.com/blog/2009/09/29/criterion-a-new-be... or read about how snap does its benchmarking.
As for erlang, there's a lot of papers about reliability. I'm not saying the performance isn't there, it's just not a focus of the erlang community. You'll have a much tougher time figuring out instruction counts for a given function in erlang. Given that erlang is interpreted, without a major focus on performance, it seems unlikely erlang would be quicker.
This is just - what i perceive as - the general consensus. I'd love for you to generate some numbers to prove me wrong.
Also this performance usually isn't the best way to argument about languages for web applications. Often your bottleneck is somewhere else and i you really have resource intensive stuff why not just interface C?
No it isn't, it is compiled to bytecode and executed on a virtual machine, just like most scripting languages are.
>Since this is not about a simple CGI application a benchmark in which you compare the running time of a compiled executable with the interpretation and running time a another program it doesn't make a lot of sense.
Read the site, this concern is explicitly addressed. The tests are long running, not short tests repeated a million times. So the byte code compilation step ends up too small to notice. That is why using a PHP bytecode caching plugin doesn't increase PHP's performance on the shootout, and why java isn't dead last from the massive startup time of the JVM.
>Also this performance usually isn't the best way to argument about languages for web applications
This is a myth. Very few web applications actually have one CPU intensive portion that can be moved out into a C module. Most web apps have pretty flat profiles, where overall language speedups make the biggest difference. When 90% of your time is spent in 10% of the code, a C module is a reasonable option. When 90% of your time is spent in 90% of your code, your only option is a faster language.
Of course, the project still needs some performance work, as highlighted in this post:
http://labs.evolope.com/post/16054431816/a-comparison-of-web...
So CB is a long way from its theoretical maximum (largely because I haven't done any performance engineering on it yet).
"Performance", by the way, depends largely on the task at hand. For complex template rendering/delivery, Erlang will probably beat anything out there because of Erlang's intelligent use of immutable data and I/O lists. Most other languages do a "memcpy" for every chunk of the template as it's being built into a final string. Erlang, on the other hand, keeps around pointers to the separate pieces of memory and then does a scattered write (writev) when it's time to send the result to the client.
What templating engine are you talking about? I'd assume ErlyDTL for obvious reasons, but it is certainly not beating "anything out there". Are you just comparing it to scripting languages or what?
http://news.ycombinator.com/item?id=1448463
Anyway, benchmarks will always show GHC beating Erlang/BEAM, but here's a Simon (or one of his co-authors)saying that he's not into reinventing wheels, he's just going to copy good parts of erlang:
http://blog.romanandreg.com/post/6797527470/erlang-and-haske...
slide 41's pretty funny (by akka author):
http://www.slideshare.net/jboner/akka-scala-summit-oscon-201...
I have a hazy recollection of a F# user or maybe a Softie saying they're doing the same thing with MailboxProcessors, but i can't google it. Who/why are all these people borrowing/copying/appropriating from OTP behaviors?
"If your program compile it will be very close to what the programmer intended".
There's nothing I've seen in Haskell, or any other language, that will prevent you implementing the wrong algorithm, or the wrong business rules etc. If you're doing a phsyics simulator (to take an example I've played around with), Haskell is not going to stop you badly miscalculating the gravitational effect of multiple objects, or calculating the mass of an object incorrectly.
The former is when you want the program to do X, and X itself is wrong. Sure, defending against this is rarely possible. The latter is in my experience far more common. When you want the program to do X, but you actually told it to do Y.
Haskell catches most of the latter cases.
One of the main reasons is the functional purity. Because return values are the only result of a function, and these values are type-checked against a specification, it is much harder to do the wrong thing.
For example, an imperative program will idiomatically have methods whose sole effect is mutating their arguments or the objects. Even with a type system -- if you just omit the effects, the compiler really has no way to know that you meant the effects to happen, and you will simply have a bug. A purely functional program will idiomatically use a different style. Instead of mutating arguments and objects, it will return a new value. If you forget to return a new value, the compiler will catch that.
Additionally, the types in Haskell are generic by default. The more generic the type of a function, the more restricted is what that function can do. The restrictions narrow the space of wrong things you can do.
Another interesting property of Haskell is where it chooses to be on the "restrictiveness+guarantess vs. powerful+unguaranteed" trade-off.
It seems many don't even notice that the other side of the "power" coin is "guarantees". The more powerful a construct is -- the less you can say about what it will do. In Haskell, the purity and generic types of functions are two things that heavily restrict code. But there are many other mechanisms to restrict rather than empower code. In many contexts, restrictiveness is considered a bad thing, but in Haskell, this restrictiveness is very useful for reasoning about code.
For example, the function of type: (forall a. a -> a) in Haskell can only be the identity function. It cannot print "hello world", and it cannot mutate variables. These restrictions are useful because we now can have useful laws. For example: id . f = f . id = f. And we can derive these useful laws from the type itself, without even looking at the code!
By helping us find the most restrictive sub-language that can express the solution, we rule out even more bugs.
It is on the Pareto frontier of the two. Which verges on tautological, but it's still a useful observation. If you can trivially extend the power of a construct while maintaining the same guarantees, the construct was simply broken, and extending the guarantees of a construct without affecting its power means simply that the guarantees were excessively conservative in the first place.
(This can almost be copied and pasted any time anybody claims two properties of a program aren't in conflict, the most common probably being security and ease of programming.)
We may be wrong about where the frontier is... but I doubt we've very wrong.
So using an untyped language only to run an analysis on it later doesn't really seem all that appealing to me, even if it could somehow provide the guarantees of Haskell or dependently typed languages.
Some of them may class as "design" rather than "coding" bugs but they still aren't usually what the programmer intended, and I would say that the majority of production defects that I see in our systems these days would fall into this category.
1. You think about an algorithm
2. You (try to) code it
3. A first time to discover an error: compilation and/or syntax verification.
4. Then you test your code
With a standard language (C/C++/Java/Python/Ruby/Javascript...) The probability of 3 to fail is very small when you are used to your language. But the probability to fail at step 4 is High.
In haskell this is exactly the opposite, with many advantages:
Compiler error in Haskell gives you a meaningful and informative message. Most of the time, it even gives you a hint on how to solve the problem.
In the end, most of the time, once the compilation phase passed, my program worked as I expected.
Furthermore, with Haskell you can use quickcheck, which most of the time will make better test than yours. I know there is some quickcheck equivalent in other languages but I believe this is not standard.
In code where the types start to to tell you less, e.g. when doing a bunch of number crunching, and you have a signature that looks like:
func :: Int -> Int -> Int -> Int
newtype Velocity = Velocity Int
newtype Force = Force Int
newtype Iteration = Iteration Int
The problems you might run into are more (for example) using the wrong algorithm for calculating gravity, adjusting the position of one planet before using it to evaluate its impacts on other planets, failing in collision detection etc.
I don't see anything in Haskell that would prevent me from doing any of those.
That doesn't help at all, are you serious? Cause you genuinely sound like you are trolling. Especially when you say things like "if a language can't prevent every possible bug then there's no poing having it prevent any bugs".
http://necrobious.blogspot.com/2009/03/fun-example-of-haskel...
http://flygdynamikern.blogspot.com/2011/04/announce-dimensio...
http://book.realworldhaskell.org/read/data-structures.html#x...
"point of vue" -> point of view
"shell command are executed" -> "shell commands are executed"
"benchmark are here" -> "the benchmarks are here"
Also, heroku works just fine with haskell, as long as you compile a static binary on an ubuntu machine. I compile mine on virtual box (via way of vagrant), and have 3 haskell apps (none of them really production ready though) running on heroku right now.
Lastly, I don't really think haskell is the right language for a lot of web programming. In haskell, you really want your logic to be as purely functional as possible, but that gets pretty hard to do for web apps that rely on external apis.
When I said there is no heroku for Haskell it because it is not official. Furthermore, do you have to use a 64bit architecture when compiling (just curious)? I might try it myself.
And I even think that the argument against the number of Haskellers is in fact a force instead of a drawback. It filter passionate people.
By external APIs, you mean that it would be difficult to write a simple "connect to twitter/facebook" application? Could you tell us about any bad experience you had?
I never said anything about number of people...
External apis: nearly all the logic in my app leads to either talking to the db, or calling an http api (and haskell can't use https apis that well, you have to use its bindings to libcurl). In general, it feels extremely difficult to pull my logic out of IO (but maybe I'm just too dumb to).
It's not that its particularly difficult, it's just that it's not a problem I think haskell is well suited to solve. I've used haskell to teach my self automated logic proofs (just resolution refutation), compression algorithms (huffman encoding and lz77), typechecking and breaking encryption (enigma). Every time university exams roll around, I learning them on paper. It's by far and away the best language I've used for stuff like that. It feels like web-apps with lots of IO aren't really a problem haskell is well suited to solve.
Would be interested in known your experiences and opinion compared to something like django or rails.
How difficult is it to build something with a complex domain model?
Yes.
>Would be interested in known your experiences and opinion compared to something like django or rails.
Rails is an abomination, so pretty much anything would be better than that. We rolled our own framework in scala. Despite initial resistance from some dynamic language proponents here, it is now the only thing anyone here will use for web development. Even the python fan won't touch django now.
>How difficult is it to build something with a complex domain model?
Easier than in dynamic languages. The problem with complex models is that humans can't hold all that information in their heads. Static typing lets us offload a bunch of important information to the compiler and have it error check for us as we go. Our biggest issue has been that scala isn't strong enough, I'd much prefer to be using haskell.
Or is it more than you just want to use something statically typed that isn't Java?
We use scala in a pretty functional way, but most of the gains there come in the processing/displaying side rather than the model. Just because that portion lends itself to a functional style of applying chains of functions on data to transform it.
I hear that. I learned Haskell because I thought it would help me understand Scala better (namely the type system), but Haskell spoiled me and I don't want to use Scala anymore. Only thing keeping me on Scala atm is Lift, a truly excellent rethinking of web frameworks. If I ever find the time I want to start porting Lift to Haskell.
I don't like the way templates are forced to use snippets to load data, in particular as it results in snippets being dependent on the database, and thus not suitable for unit testing. The framework should be pulling the data into the view, which then passes it as an argument to the snippet to do whatever transformation on it. That way snippets are entirely self-contained and easy to unit-test. This is the sort of thing that I think would have ended up the way I want had it been written in haskell instead of scala just because writing small, self-contained functions is the typical haskell way of doing things.
I hate the very opinionated nature of the form handling, and the even more opinionated responses from the developers to questions like "how can I make a form that works normally and doesn't require a session". People are not bad for wanting forms to behave normally, and the "security" show the lift devs put on to justify the form handling in lift is insulting.
While I find templates acceptable, they are seriously missing out on type safety. I should get a compiler error if I create a template with invalid html. Ocsigen got this part very right (an ocaml framework/appserver). Not only do I not get this benefit for my markup, but lift itself generates invalid html on me when using virtually any of the included form generation methods. Very annoying.
Ideally what I want is a framework that gives me the type safety of yesod, but with the structure of lift. I hate faux-mvc web frameworks and find they make things more difficult rather than making things easier, and unfortunately yesod copies the typical rails clone structure. Lift doesn't make the mistake of trying to hide http from me, and gives me the simple and correct mapping of template to url, leaves the domain model in the model instead of cramming it into "controllers" that shouldn't be involved.
As for Yesod, I was also thinking it would be nice to have the type safety and parallel/concurrent performance of Yesod with the templating and structure of Lift. Don't know Yesod well enough yet though. Too many side projects.
The one caveat I'd suggest from experience: good web frameworks exist now, so use them, and don't write your own like we did. I'd recommend Yesod.
You shouldn’t need to know Haskell.
Don’t pay attention to all the syntax.
If you are curious you can take a look at Applicative Functor.
The opening is not really an attack. This is the way I myself started to be interested in Haskell web programming.
You could also say, for anybody that's looked at lisps, *ML, F#, scala, erlang, the syntax isn't too bad, and for anybody that knows rails, they'll recognize the same REST'y MVC layers/abstractions. I gather the target audience is folks using dynamically typed languages.
Haskell web frameworks handle parallel tasks perfectly. For example even better than node.js
Very nice blog post. After working on a Python stack during my part time job (check out Apache libcloud), I can safely say that using a statically typed language is a necessity for any non-trivial project. Without a type system you offload the entire framework onto the programmer. To actually grok the project you have to hold the complexity all in your head instead of using the types as a guide and that makes getting started incredibly challenging.
The source code of my website is here:
https://github.com/yogsototh/Scratch/
And even if not clearly stated, everyone is free to use it and steal from it. Of course, I'd appreciate to be mentioned.
If you want to have the full behavior you should copy not only the css but a bit of js.
More precisely:
output/Scratch/css
output/Scratch/assets/css
output/Scratch/js/init.js
+ jquery
You certainly also need to get the layouts:
layout/article.html
layout/default.html
If you want any more information, simply mail me, I'll be happy to help.