High-Performance Web Applications in Haskell
qconlondon.com
qconlondon.com
Wrote two small services for http://www.webpop.com in Haskell, both based on Snap. One for dynamically resizing images on the fly and one for handling uploads of large files and streaming them straight to Cloudfiles.
In both cases I started by trying node.js (both cases deals with either fetching or sending files from Cloudfiles so IO operations is what takes time) since the problems seemed very suited for node's eventloop model. In both cases node.js really disappointed.
Ended up giving Snap a try and got really surprised at just how well it worked. Performance is amazing, it's not much more code than the node.js version and theres no endless nesting of callbacks. It handles high levels of concurrency incredibly well and both services ended up using around 8MB of memory each - even under load...
Main problem is the lack of libraries that you tend to take for granted in more webby languages, and obviously the learning curve is pretty steep (but also very rewarding).
http://antirez.com/post/autocomplete-with-redis.html
Sounds like a fun toy project, right? I figured I would store everything in a trie, so I could do really fast prefix searches, and then bolt on a REST interface that would send back JSON or JSONP. So I wrote some C code to handle the prefix searching in a JudySL trie (see http://judy.sf.net/ for details; it's a nice library) and bolted that onto Haskell and wrote some bridge code. What I ended up with was a really nice, easy server that took me only a few hours to write, and was every bit as memory-efficient as I'd hoped. Source code here, if anybody's curious:
https://github.com/PeterScott/acdaemon
The learning curve on Haskell, though, is really something. If you really want to see something ridiculous, try looking at the auto-generated library documentation for anything involving regular expressions. The type declarations in Text.Regex.TDFA are like something H. P. Lovecraft might write about.
On the other hand, that fact has always seemed to me a mark of why it's worthwhile to persevere: as when you are learning mathematics, things can seem entirely opaque until they 'click', but then you've really made an advance in your understanding.
Probably most people are aware of it, but the O'Reilly book _Real World Haskell_ by Bryan O'Sullivan, Don Stewart, and John Goerzen, is a great way to get started. The book is on-line at
http://book.realworldhaskell.org/
Chapter 8 discusses (one of) the Haskell regexp implementation(s), and in particular addresses one of the puzzling aspects: polymorphism in return type. The whole book is excellent.
(I should also point out that another book _Learn You A Haskell_, has just been published, and is well-regarded. I think it assumes rather less programming experience than RWH, and also covers less of the "real-world" aspects. It also can be read on-line, at
and in fact there is a coupon code on that page now for 40% off.)
Also, the Haskell community is, in my experience, extremely helpful and welcoming. People should come and join!
https://bitbucket.org/bos/attoparsec/src/tip/examples/RFC261...
It's 54 lines long, pretty trivial, and here's a really cool part: it parses incrementally on strict bytestrings, so you can just do raw socket recv calls and pass the chunks of input to the parser. Similarly, I wrote a parser for the subset of YAML that beanstalkd uses (as part of the hbeanstalk client library), and it was just 11 lines of code. It's very impressive.
(The concurrency and parallelism stuff is also notably slick, but I haven't had as much occasion to use it. Haskell's aversion to side-effects really pays off sometimes.)
By the way, you're right about the Haskell community being friendly and welcoming -- as you yourself demonstrate. :-)
I'm not familiar with Haskell, so to me the code looks like a mix of high-level declarative and low-level specialized constructs (e.g. skipWhile, takeWhile) interleaved with syntax noise. And it seems to be using quite a lot of external libraries. Also, correspondence of the code with the HTTP spec is completely non-obvious.
In contrast, here's an HTTP response parser I wrote in OCaml using just one library: https://github.com/alavrik/piqi/blob/master/piqi-tools/piqi_... (see lines 26 - 218).
I've just noticed that the code you are referring to is an example. Well, looking at the example, I can hardly come to a conclusion that Haskell shines for parsing.
By the way, that long import list is actually just importing some basic stuff from the standard library, and the attoparsec parsing library. One of the persistent minor annoyances of Haskell is writing long lists of module imports.
I was just surprised that this code was presented as a good example of Haskell's fit to the parsers domain. It would be interesting to see if my perception of this code changes once I get more familiar with the language.
Yojson (search for 'let positive_int') http://forge.ocamlcore.org/scm/browser.php?group_id=153
Mirage (using MPL for zero copy parsing) https://github.com/avsm/mirage/blob/master/lib/net/direct/mp...
In fact, I borrowed Yojson's json parser for my project (piqi.org). Also, I'm familiar with MPL. It looks very nice, but from what I heard it is fairly immature. And you definitely can't parse HTTP this way :)
http://stackoverflow.com/questions/16918/beginners-guide-to-...
http://stackoverflow.com/questions/1012573/how-to-learn-hask...
Type Systems:
Haskell and Scala are closest in that they're strongly, statically typed with Hindley-Milner type inference. Erlang is dynamically typed as is clojure. This tends to break the advantage/disadvantage scheme along the same lines as Java/Ruby wrt typing: the former languages generally give you more speed and safety, and the latter languages can do cool tricks with deciding typing at runtime, which can aid in expressiveness.
Type System Complexity/Power:
Haskell has the most sophisticated type system--by far, I think it's fair to say. This adds power but at the cost of a steeper learning curve for people unused to thinking so deeply about the contracts/promises code and data structures are making. Scala's is also fairly involved since it's basically a subset of Haskell's (well, plus traits and inheritance blah)--but not quite as consistent or clear. Clojure and Erlang have somewhat simpler type system that may be more tractable out of the gate--Erlang in particular is pretty straightforward.
"functional-ness", how sharp a break from OOP/imperative is imposed:
Scala is by far the least dogmatic about doing thing in a "functional" as opposed to an OOP or imperative way. Scala's heavy emphasis on seamless interop with Java makes it a sort of up to the programmer to place their style on the continuum between something like Java and something like ML. Haskell, with its purity-by-default, is probably the most opinionated of the group. You will be forced to approach problems very differently. I'd say clojure follows just behind, also having a strong bias toward doing functional-only styles. Erlang is also pretty interested in doing things in a particular way that could be construed as functional (not rebinding names, focus on immutability, for example), but its "big idea" is more tailored around its flagship library and virtual machine.
Syntax:
Erlang will probably feel the most foreign to a 2011 programmer. Erlang started as a modified Prolog, and the syntactic legacy of Prolog is not widespread in common contemporary languages. Haskell's syntax is rather clean and restricted--though the fondness of veteran Haskellers for custom operators can sometimes be overwhelming for a new programmer. Scala, while a hybrid OOP language that shares many idioms with Java etc, has so heavily dipped into the operator well, even for standard language constructs--the syntax can be a bit crowded. Imagine Java with twice the non-alphanumeric density. Clojure is a LISP, so it's very clean and clear syntactically, provided you are a parenthesis master or make use of SLIME, etc.
Speed (average job, single core, not scalability etc etc):
Haskell, Scala, Clojure, Erlang--descending. Though on certain workloads these players will switch places, as a general rule of thumb that's my aggregated observed performance order in "real world-ish" code scenarios. Clojure is sorta interpolated based on experiences others have relayed to me and online. From a memory standpoint, Haskell is substantially stingier than the rest. This impacts startup times and the practicality of shell tools, etc.
Compilation/VM + Concurrency:
Haskell is alone in this group in that it compiles to native code--well, at least, the most commonly-used configuration does: the compiler ghc. GHC is a beauty of a project. Very good native code generation, aggressive improvement projects (new I/O manager, LLVM backend) incredibly advanced concurrency capabilities (sparks, first class green threads, seamless async IO). Erlang uses a VM, beam, which also uses green threads (which erlang calls processes) and transparent async I/O--but, erlang goes even further to provide syntactic support for super-efficient message-passing between these processes, and facilities to transparently pass those messages between machines in a cluster. Clojure and Scala both use the JVM. It is what it is--mature, fast, but primarily designed for Java and things a lot like Java. Cool features like tail call optimization and green threads are simply not possible b/c the JVM does not (currently) support them. (I know Scala has Actors, Akka, etc, but JVM-based actors are not in the same league as what Erlang and Haskell have.)
All of these languages have a REPL, thank god.
Breadth of Libraries:
The JVM languages here obviously have a HUGE wealth of libraries available. The only concession is calling them can be somewhat awkward and "downgrade" you out of the functional idioms you'd rather be using, depending on the library. Haskell's library situation is pretty good (Hackage has libraries for most everything you need and is growing quickly every day), and Erlang is probably bringing up the rear, here. OTP is excellent, and there are many libraries related to large networked systems etc, but not as much "general" coverage.
Key Strength:
If you want to write high-uptime distributed systems, Erlang/OTP is the bees' knees. Everything about the language and runtime is tailored to that environment--supervisor trees, mnesia, good scheduling of hundreds of thousands of lightweight processes.
Haskell is great at correctness. Like its non-lazy cousin ML, if you satisfy the type system, a shockingly high percentage of the time your program is just flat out correct the first time you run it. Haskell is also pretty darn good at single-machine multicore concurrency.
Clojure is a great "modern Lisp". Many of the concerns, fairly or unfairly, leveled at Lisps in the past (no libraries, stagnant platform, closed development, etc) are non issues. Building on the JVM enables a very small developer core to make progress on the language while leveraging a mature platform that evolves independently and at scale.
Scala is a "better Java". While iterop with large Java-oriented frameworks is possible in Clojure, it's pretty much trivial in Scala. You can dip a toe incrementally into the functional world while keeping retaining your existing projects, favorite libraries, 3rd party frameworks, etc--transitioning as fast and far as appropriate.
Summary:
The good news is, all of these languages are pretty damn good, and all of them actually have healthy, thriving developer communities and razor-sharp maintainers.
If someone has never done functional programming before, I'd personally recommend trying something like clojure or Racket first to get the fundamentals down before digging into something truly mind-bending like Haskell. Scala is probably not a big enough leap to discipline yourself to grok functional just b/c it's so easy to relapse into non-functional patterns and Scala will happily comply.
http://lampsvn.epfl.ch/trac/scala/ticket/2991
http://stackoverflow.com/questions/3313929/how-do-i-disambig...
(and does Orig. Commenter have examples of clojure hiccups? Interop is very important to both (and F#). Maybe a more meaningful compare/contrast is haskell, scala and F# (which I can't do)
http://matt.might.net/articles/best-programming-languages/
---------------
http://james-iry.blogspot.com/2010/05/types-la-chart.html
(his league table of "Most Powerful Type Systems": Agda, epigram, scala, haskell
Technically Scala's inference is not Hindley-Milner since you still have to declare method argument types for JVM interop.
The speed difference between Scala and Clojure has less to do with the static/dynamic gap than the fact that Clojure relies upon reflection by default. However, once you identify your bottlenecks, adding type hints to remove reflection is pretty easy.
Good stuff.
HM is a type system first, and that means is a logic system. HM is a logic that is deliberately kept relatively weak in order to allow full inference. Scala is not HM and never has been - it's much, much more sophisticated and so Scala uses a form of type inference not based on HM. Haskell '98 is HM plus a tiny bit (and yes that tiny bit does mean that the occasional type annotation is required). GHC Haskell is HM plus a ton more and so there are a lot more GHC Haskell programs that need some type annotation. The people behind GHC Haskell have done a good job of carefully adding type system features that don't totally break the model, though, so you rarely have to annotate much unless you love playing in advanced type land.
I would hesitate before saying GHC Haskell is more sophisticated "by far". It's not at all obvious that that is so. I'd want a type theorist to spend some time with the two core logics to determine which one subsumes what parts of the other before making that call.
(Right now there's an internal server error.)