CIS 194: Intro. to Haskell

SMHUG - July 10, 2018

Agenda

  • Why do I like/dislike Haskell?
  • What's CIS 194?
  • What's the plan?
  • Development env. setup
  • Prakash intros. Week 1 Content

Why I Like Haskell

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)

MVar, Green Threads

main = do
    result <- newEmptyMVar

    forkIO (do
        sleepMs 5
        putStrLn "Calculated result!"
        putMVar result 42)

    putStrLn "Waiting..."
    value <- takeMVar result
    putStrLn ("The answer is: " ++ show value)

Channels

main = do
    messages <- newChan
    writeChan messages "unbounded"
    writeChan messages "channels"

    -- Read a message from the channel, 
    -- then output it.
    putStrLn =<< readChan messages

Laziness (Streams for Free)

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

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]

Type Classes

-- define a type class

class ToJSON a where
  toJSON :: a -> Value

Type Classes

-- define a type class

class ToJSON a where
  toJSON :: a -> Value

-- define a type

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

Type Classes

-- define a type class

class ToJSON a where
  toJSON :: a -> Value

-- define a type

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

-- the type instantiates the type class

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

Type Classes

-- define a type class

class ToJSON a where
  toJSON :: a -> Value

-- define a type

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

-- the type instantiates the type class

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

-- program against type classes

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

Lots of New Things to Learn

  • Separating deterministic from nondeterministic code
  • Interesting, new, mathy abstractions
  • How do we write programs where state is mostly on the call stack?

The Latin of FP

  • RxWhatever (Erik Meijer)
  • Java Generics (Philip Wadler et al.)
  • JavaScript (promises, Ramda)
  • Scala (for-comprehension)
  • LINQ (Erik Meijer)
  • more

Why I Don't Like Haskell

  • Lots of interesting new things to learn
  • Learning curve is very steep
  • Poor IDE support (relative to Java)

Laziness and IO

-- hGetContents :: Handle -> IO String
-- withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r

-- closes file handle before fileData is forced
wrong = do
    fileData <- withFile "test.txt" ReadMode hGetContents
    putStr fileData

-- forces fileData before lambda exits
right = withFile "test.txt" ReadMode $ \handle -> do
    fileData <- hGetContents handle
    putStr fileData

Language Extensions

{-# LANGUAGE DeriveFunctor              #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE ScopedTypeVariables        #-}

CIS 194

Why CIS 194?

  • Coursework is challenging, illustrative of concepts
  • Great homework assignments
  • According to me and many on /r/haskell, it's the best option out there

Things We've Changed

  • Added a chapter on Quickcheck
  • Added a bunch of tests
  • Replaced the original IO and Monoid chapters with something from Fall '14 version

What's the Plan?

  • Get together every-other week
  • Volunteers present concepts applicable to next week's assignment
  • Afterwards, discuss homework solutions
  • Eat pizza

What's Expected of You?

  • Show up
  • Do the homework
  • Volunteer to talk about a relevant topic (functors, monoids, lazy evaluation, Quickcheck, etc.)
  • Don't wreck the bathrooms

Tracking Progress

Let's Get Started

  • https://github.com/laser/cis-194-summer-2018
  • Write your GitHub username on the whiteboard and I'll add you as a collaborator
  • Clone repo, create branch whose name is your username, push to origin
  • Check out README.md for OSX setup steps
  • Make PRs to master adding new tests and bugfixes
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