Exercises are great and all, but once I try to go past exercises, I start getting lost in layers of monad transformers, and I struggle to figure out the "right" way to do anything. It just feels like there are all these layers of abstraction and complexity that are required for even simple applications, and I can't seem to reach the point where it all clicks for me and becomes manageable.
Reading the TOC and chapter notes for that book, I'm not convinced it'll help me. Even the "final project" at the end of the book is described as "a little weird, but small". I don't need more weird, small examples, I need a full-fledged application! I've tried picking out random Haskell OSS apps and studying them, but without newbie-friendly documentation to walk me through the code, I find it very hard to follow what's happening....
You will immediately spot some places where a monad will help you. Where that happens, use the monad, elsewhere, keep it simple. With time you will regret some of that simple code, when that happens, change it (Haskell's best use case is on refactoring). With even more time you'll get a taste for where it's good to go complex from the start.
I agree with you but the book has chapters where you write a URL shortener and the final project is an implementation of the `finger` Unix command. For me, the book was a stepping stone towards materials like Write You a Scheme (Scheme interpreter in Haskell).
I will admit though, I read Haskell Book for the ideas and learning typed functional programming. I wasn't planning on using Haskell everyday for projects.
As an example, the first "real" project I wrote in Haskell was one or two weeks into learning it: a web testing framework/DSL (a la Capybara[0]) that spoke a (custom) protocol over socket to a headless webkit instance[1].
Looking back, I abused stuff like type-classes and such, but then again, it was all a learning experience. I should probably be a little embarrassed about how I left the project, but in the interest of providing some inspiration, here it is in all its gory -- I mean, glory: https://github.com/cstrahan/happybara
[0]: http://teamcapybara.github.io/capybara/
[1]: https://github.com/thoughtbot/capybara-webkit/tree/master/sr...
I tried that. When I got stuck (fairly early on), I ended up switching my project from Haskell to Clojure :/
[0]: https://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_...
http://haskell-servant.readthedocs.io/en/stable/index.html
Servant allows you to write a type that describes a server/API, and automatically derives a specification for it. You can then write handlers for the different API endpoints, and it will only compile if every endpoint is correctly handled. That includes content types, status codes, and so on, everything is verified, and things are often derived for you. All (de)serialization to/from JSON is automatic (although you can write your own instances if you want your JSON to look a certain way).
For example, from the Servant docs:
type API = "position" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] Position
:<|> "hello" :> QueryParam "name" String :> Get '[JSON] HelloMessage
:<|> "marketing" :> ReqBody '[JSON] ClientInfo :> Post '[JSON] Email
GET /position/:x/:y/ returns a Position object as JSON
GET /hello?name=whatever returns a HelloMessage object as JSON
POST /marketing given a JSON request that represents a ClientInfo object,
returns an Email object as JSON
You can then write functions of the following types
handlePosition :: Int -> Int -> Handler Position
handleHello :: Maybe String -> Handler HelloMessage
handleMarketing :: ClientInfo -> Handler Email
If you add a new API endpoint, and forget to write a Handler for it, you'll get to know. If you change the Position type to work with x, y, and z coordinates but forget to update your handlers, you'll get to know. If you'd like to also allow clients to request HTML instead for some endpoint, just change the '[JSON] to '[JSON,HTML]. The Haskell typesystem will make sure that someone requesting a text/html Content-Type doesn't get hit with a 500 or something.
Note the Maybe String there: if you wrote a handler for the /hello endpoint that had the type String -> Handler HelloMessage, Servant would complain: you're expecting a query parameter, which the client doesn't have to provide. This is in stark contrast to, say, the "NoneType has no attribute whatever" problems that one risks having to face with Django: if it compiles, it meets the spec.
Of course, static typing won't prevent you from responding to /hello with Lovecraft quotes.
--
> I start getting lost in layers of monad transformers
Handler is actually a monad transformer, and it's a great first one: it's essentially[0]
type Handler a = ExceptT ServantErr (ReaderT Config IO) a
* throw a ServantErr
* read values from a Config object
* perform IO actions (read files, make other network requests, log things, "fire missiles")
After a couple of months, you realise it's not too wrong to say you understand how to use them, and you slowly begin to be able to rely on the type system for support. Libraries like Servant are a great example of how this can really, really help. I stopped to think last week how similar it is to pair-programming with a really intelligent but sometimes obtuse friend who likes pointing out how "this doesn't follow from your assumptions", all the way from
> "Wait, but you can't add two books together."
to
> "What if someone PUTs to /login?"
which is what we had above, to
> "And what happens if I try to withdraw all my money exactly when the payment I've made to you is getting executed?"
(in this case, you discover the wonders of software transactional memory[1]). A better typesystem allows you to let the compiler handle more of the busywork that you'd originally have written tests or comments for.
It is sometimes confusing starting out (when one discovers that 3 isn't an Int, but a "Num a => a", for instance), but it gets better. Once you get past the Project Euler/"look ma, infinite lists!" stuff, "real" Haskell does have a tendency to make people underestimate how much they really know, but as I've discovered, taking the plunge reveals that one has progressed much farther than one thinks.
Also, #haskell on Freenode is one of the friendliest places I've encountered on the internet[0].
[0]: http://haskell-servant.readthedocs.io/en/stable/tutorial/Ser...
[1]: https://www.schoolofhaskell.com/school/advanced-haskell/beau...
[2]: Here's a hilarious example (NSFW language warning): https://gist.github.com/quchen/5280339
--
Other stuff:
Corrode is a C-to-Rust converter written in literate Haskell (as in, it's like a blog post that you can compile and run, to give a slightly strained analogy).
https://github.com/jameysharp/corrode
webshit weekly actually "quoted" a previous HN comment of mine about it, in which I was falsely slandered: I'm not a Rust evangelist!
I wrote a small "script" to make tmux status bars a while back that was a good example of how fun "scripting"-style thing can be in Haskell.
https://github.com/mrkgnao/remux
And there's always XMonad!
> This is equivalent to an API that defines these three endpoints:
It would be cool to use the text that follows this as the spec; i.e. write a converter from this description to Haskell.
> GET /position/:x/:y/ returns a Position object as JSON
Or, you know, a quasiquoter (which is sort of like a Haskell macro thing, although Template Haskell is closer to that).
Depends on how complex you want to do things. You can just use yesod [0] to create a web app and compare it to an existing one you'd written.
An interesting use of Haskell is with concurrency [1], so anything along those lines should give you something pretty awesome.
[1]: http://chimera.labs.oreilly.com/books/1230000000929/index.ht...
About what extensions people consider kosher, Stephen Diehl's "What I Wish I Knew When Learning Haskell" has a great list that sorts them into "common", "uhh", and "you obviously know what you're doing".
That's incredibly easy, of your list of "problems to attack and programs to write some time soon" pick any one and get busy. It'll feel real-world very soon and there's no better way to grasp "real-world Haskell". The first iteration will be ugly, but on the plus side you'll notice that yourself and that's quite some progress already. Keep at it a bit more and perceived "decent post-newb proficiency" is within reach. Well-rounded mature libraries for "real-world periphery" (DBs, json/yaml/html/xml/all that jazz) abound, too.
What I find much more annoying is the lack of freelance projects. It's literally the language of academics, hobbyists, weekenders, late-nighters, githubbers. (Certainly also suitable for bootstrappers imho in a "Viaweb-Lisp-advantage" kinda way) If you want to avoid full-time employ, you're gonna have to keep doing .net/java/golang/node and such projects. Haskell code-bases I feel are private well-guarded silos within academia and a number of corps, no wider "projects ecosystem" to speak of.
http://www.thaiopensource.com/relaxng/derivative.html
This document describes an algorithm for validating an XML document against a RELAX NG schema. This algorithm is based on the idea of what's called a derivative (sometimes called a residual). It is not the only possible algorithm for RELAX NG validation. This document does not describe any algorithms for transforming a RELAX NG schema into simplified form, nor for determining whether a RELAX NG schema is correct.
We use Haskell to describe the algorithm. Do not worry if you don't know Haskell; we use only a tiny subset which should be easily understandable.
You don't have to use the most esoteric constructs the language offers, or get into the weeds of how lenses work (just a random example of a seemingly 'esoteric' library) or language extensions to write code that does something useful.
That said, a project centred Haskell book would be nice to have.