This document summarizes Nicolas Hery's experience building a website in Haskell after coming from a Node.js background. It discusses choosing a web framework in Haskell, using types to document data, handling optional values, refactoring code, and deploying to Docker and Heroku. It also notes both benefits of Haskell like compiler-checked refactoring but also challenges like syntax and documentation.

1. Building a website
in Haskell
coming
from Node.js
@nicolas_hery

2. I'm a web developer
I work at Busbud
on our website
we are a JavaScript
shop (Node.js, React)
interested in
Functional
Programming

3. make API/DB calls, put data in HTML, respond
to HTTP requests
work on a team, modify other people's code
constantly adapt to business change
a web developer in a startup needs to...

4. Was it hard challenging? Yes
Was it possible? Yes
Would I do it again? Yes!
my experience
building something in Haskell

5. The app

6. developer.marvel.com

8. port existing JavaScript app to Haskell

9. Jumping in

10. Hands-on warmup
seanhess.github.io/2015/08/04/practical-haskell-getting-started.html
Course and exercises
www.seas.upenn.edu/~cis194/spring13
Book
haskellbook.com
Libraries
haskelliseasy.com
Best practices
dev.stephendiehl.com/hask
Getting started

11. choosing a web framework
Yesod
battle-tested
great docs
big framework
Scotty
lightweight
familiar
not a lot of docs
Servant
leverages types
most different
fairly new

12. Code examples

13. types
document your
domain data

14. function renderCharacterDetails(character) {
// what is the shape of `character`?
};

15. data Character = Character
{ id :: CharacterId
, name :: Text
, description :: Maybe Text
, urls :: [Url]
, thumbnail :: Image
, comics :: ComicList
}
renderCharacterDetails :: Character -> Html
renderCharacterDetails character = -- ...

16. var t = require('tcomb');
var Character = t.struct({
id: t.Number,
name: t.String,
description: t.maybe(t.String),
urls: t.list(Url),
thumbnail: Image,
comics: ComicList
}, 'Character');
function renderCharacterDetails(character) {
// ...
};

17. Maybe/Either
force you to
handle possible cases
(missing data, errors)

18. Character.getMarvelUrl = function(character) {
// will this always return a string?
return character.urls.find(function(url) {
return url.type === 'detail';
}).url;
};

19. getMarvelUrl :: Character -> Text
getMarvelUrl character =
let defaultUrl = "http://marvel.com"
maybeUrl = find isDetailUrl (urls character)
in case maybeUrl of
Nothing -> defaultUrl
Just detailUrl -> U._url detailUrl

20. some things that
are trivial in JavaScript
are hard take longer in Haskell
(ex: IO)

21. function getCharactersController(req, res, next) {
var offset = req.query.offset;
// ...
}
GET /characters?offset=20
(offset is optional)
getCharactersController :: HandlerM ()
getCharactersController = do
offset :: Int <- param "offset"
`rescue` (_ -> return 0)
-- ...

22. explicit JSON decoding
challenge when working with more
complex data structures
artyom.me/aeson

23. {
"code": 200,
"status": "Ok",
"data": {
"total": 1,
"count": 1,
"results": [
{
"id": 1009610,
"name": "Spider-Man",
// ...
}
]
}
}
GET /api/characters/1009610

24. var body = JSON.parse(response);
var character = Character(body.data.results[0]);
return {
character: character
};

25. data CharacterResponse = CharacterResponse
{ character :: Character }
instance FromJSON CharacterResponse where
parseJSON (Object o) = do
_data <- o .: "data"
_results :: [Character] <- _data .: "results"
case _results of
[] -> fail "data.results should be non-empty"
(x:_) -> return CharacterResponse { character=x }
parseJSON _ = mzero

26. refactoring
is easy

27. data Comic = Comic
{ id :: ComicId
- , title :: Text
+ , name :: Text
, description :: Maybe Text
, urls :: [Url]
$ stack build
Views/Components/ComicsList.hs:23:57:
Not in scope: ‘C.title’
Views/Components/ComicDetails.hs:29:31:
Not in scope: ‘C.title’
Views/Components/ComicDetails.hs:30:33:
Not in scope: ‘C.title’
Views/Pages/Comic.hs:27:50:
Not in scope: ‘C.title’
Controllers/Comic.hs:71:23:
Not in scope: ‘C.title’

28. Shipping

29. Docker & Docker Compose
docs.docker.com/compose/install
Heroku Toolbelt & Heroku Docker Plugin
devcenter.heroku.com/articles/docker
Dockerfile
hub.docker.com/r/thoughtbot/heroku-haskell-stack
Heroku Docker & Stack
$ heroku docker:release

30. observations
the Good

31. if it compiles
it works

32. optimize
for change

33. type-safe URLs
type-safe i18n
type-safe JS hooks

34. focus on logical errors
versus malformed code

35. influence the way you
write JavaScript

36. observations
the Bad & the Ugly

37. much syntax
haskellforall.com/2015/09/how-to-make-your-haskell-code-more.html
. $ !! @ <$> <*> <- >> >>= & ^. ^.. %~ .~ =~ ¯_(ツ)_/¯

38. implicit versus
explicit and qualified imports
import Data.Text.Lazy
import Network.HTTP.Types
import Text.Blaze.Html.Renderer.Text
import Web.Scotty.Trans
import qualified Data.Text.Lazy as TL
import Network.HTTP.Types (status404, status500)
import Text.Blaze.Html.Renderer.Text (renderHtml)
import Web.Scotty.Trans (ActionT, html, param, status)

39. record labels can cause
naming conflicts
import qualified Models.Character as C
import qualified Models.ComicSummary as CS
characterDetailsView :: Character -> Html
characterDetailsView character =
-- ...
H.img ! A.alt (toValue (C.name character))
-- ...
H.li $ toHtml (CS.name comicSummary)

40. danger of too much abstraction
www.yesodweb.com/blog/2015/10/beginner-friendly-code-and-apis
someFunc =
maybe getDefaultValue otherFunc . flip lookup someMap
someFunc key =
case lookup key someMap of
Nothing -> getDefaultValue
Just value -> otherFunc value

41. documentation
often lack examples
(type signatures not enough for beginners)
exceptions:
www.yesodweb.com/book
github.com/Gabriel439/Haskell-Pipes-Library
github.com/Gabriel439/Haskell-Turtle-Library
...

42. Wrap up

43. google "startup blog haskell"
www.wagonhq.com
www.frontrowed.com
...
are people actually doing this?

44. github.com/pbrisbin/tee-io
github.com/thoughtbot/carnival
github.com/liqd/thentos
open-source
example apps are really helpful
pinboard.in/u:nicolashery/t:haskell/t:example

45. some
closing thoughts
learn the basics and then build something
don't try to learn everything at once
don't be afraid to ask for help

46. Thank you.
github.com/nicolashery/example-marvel-haskell