CIS 194 / Winter, 2016 / Outline

outline.md

CIS 194: Introduction to Haskell

Agenda

• Why do I like Haskell?
• Why do I dislike Haskell?
• What's CIS 194?
• What's the Plan?
• Walkthrough of course materials
• Tracking your progress
• Participation
• Development environment setup

Why Do I Like Haskell?

Excellent Type System

Type Inference

main = scotty 3000 $ do
  get "/:word" $ do
    word <- param "word"
    html ("<h1>Hello, " ++ word ++ "</h1>")

Typed Side-Effects

sum :: (Int, Int) -> Int
sum (x,y) = x + y

sum2 :: (Int, Int) -> IO Int
sum2 (x,y) = do
  contents <- readFile "/secret"
  putStr contents
  return (x + y)

Concurrency Story

• "Lightweight threads" a la Go (Haskell runtime scheduler)
• Good low-level stuff (forkIO, MVar, etc.)
• Cool high-level stuff
  • channels
  • software transactional memory
  • thread-safe mutable variables

Laziness (streams for free)

fibonacci :: [Int]
fibonacci = 0 : (next 1 1)
  where next n m = n : (next m (n + m))

*Main> take 10 $ fibonacci
[0,1,1,2,3,5,8,13,21,34]

"Ad Hoc Polymorphism" w/Type Classes

A grouping of methods that operate over some type:

class ToJSON a where
  toJSON :: a -> Value

A type can instantiate these classes by providing an implementation of each method:

data Person { firstName :: Text
            , lastName :: Text }

instance ToJSON Person where
  toJSON (Person f l) = object [ "firstName" .= f
                               , "lastName" .= l ]

Then, we program against the type class instead of an implementation.

createJSONResponse :: ToJSON o => o -> WebServiceResponse ByteString

Instantiate type classes using types you didn't create.

Lots of interesting new things to learn

• Separating deterministic and nondeterministic code
• Interesting new abstractions
• How do we write programs where the state is mostly encapsulated in the call stack?

It's the Latin of FP

Influenced a lot of stuff:

• Rx (Erik Meijer)
• Java (generics - Philip Wadler et al.)
• Swift (protocols)
• JavaScript (promises chaining)
• Scala (scalaz, cats)
• C# (LINQ - Erik Meijer)

Why I Don't Like Haskell

• Lots of interesting new things to learn
• Learning curve very steep
• Poor web-dev library support (relative to say, JavaScript)
• Poor IDE support (relative to say, Java)
• Lots of language pragmas
• Records/fields are stupid (fixed in GHC 8)

Why CIS 194?

• Original course taught by Brett Yorgey @ U-Penn in Spring 2013
• According to me and most people on /r/haskell, it's the best option out there
• Coursework is challenging, illustrative

Things I've Changed

• Replaced the original IO and Monoids chapters with a single chapter from the Fall '14 version
• Added a chapter on Quickcheck

What's the Plan?

• Get together at lunch to talk about the week's lecture and ask questions about homework
• Presentations?

Walkthrough of Course Materials

• Go here
• Lectures
• Assignments

Tracking Your Progress

• Check out: http://coursework-progress.herokuapp.com
• Builds branches w/Travis CI

Participation

• Clone the Git repo
• Create a branch from master using your GitHub username
• Periodically rebase onto master (let's share tests!)
• Progress app will build your codes and run tests
• I'll buy sandwiches each week for individuals who say they've done the homework by the time I wake up on Thursdays

What's Expected of You for Next Week?

• Finish Chapter 1 homework
• Read Chapter 2 lecture