Welcome

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HaskellChris Eidhof

woensdag 27 januari 2010

Three things

• Strong types

• Purity

• Larger example

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Strong Types

• As programmers, we do a lot of testing:

• * Unit Testing

• * Debugging

• * Specification

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Strong Types

• Can we automate testing?

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List

• data [a] = [] | a : [a]

Type

Type-parameter

Constructor

Constructor

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List Example

Example: List.hs

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Functions

• reverse :: [a] -> [a]

• reverse [] = []

• reverse (x:xs) = reverse xs ++ [x]

Example: Reverse.hs

Give me a list of a And I’ll return alist of a

Pattern Matching

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Purity

• State = Evil

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Purity

• Same Input

• =

• Same output

Referential

Transparency

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Purity

• No side effects:

• * Variables

• * I/O

• * launchMissiles()

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Example

• sort :: [Int] -> [Int]

How do we know sort doesn’t launch missiles?

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Doing I/O

• putStr :: String -> IO ()

The IO type shows us it’s not pure

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Laziness

• if (x < 10 && x > 5)

• Example: Lazy.hs

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Quickcheck

• Automatic testing of pure code.

Example: Reverse.hs

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Software Transactional

Memory

• Composable transactions

• No deadlocks!

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Fusion

• myFunction = map square . map toInt

• = map (square . toInt)

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Parallel code

• map  :: (a -> b) -> [a] -> [b]

• parMap :: (a -> b) -> [a] -> [b]

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Effects

Dangerous SafeUseless

UsefulMost

languages

Haskell

Nirvana

Simon Peyton-Jones, Caging The Effects Monsterwoensdag 27 januari 2010

Arc Challenge

• Write a program that causes the url said (e.g. http://localhost:port/said) to produce a page with an input field and a submit button. When the submit button is pressed, that should produce a second page with a single link saying "click here." When that is clicked it should lead to a third page that says "you said: ..." where ... is whatever the user typed in the original input field. The third page must only show what the user actually typed. I.e. the value entered in the input field must not be passed in the url, or it would be possible to change the behavior of the final page by editing the url.

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Arc Challenge

• Solution in Arc:

• (defop said req

(aform [onlink "click here" (pr "you said: " (arg _ "foo"))] (input "foo") (submit)))

•

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Arc Challenge

arc = do name <- input          link "click here"          display $ "you said:" ++ name

See gist: http://gist.github.com/260052

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Arc Challenge (2)

arc2 = do  name  <- input           (x,y) <- input           link "click here"           display (add x y)           display ("you said:" ++ name)

input uses type-inference!

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Read more

• Real World Haskell - http://book.realworldhaskell.org/

• Haskell.org - http://haskell.org

• Haskell Café - http://haskell.org/haskellwiki/Mailing_lists

• Planet Haskell - http://planet.haskell.org/

• Haskell reddit - http://haskell.reddit.com

•

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Getting Started

• 1. Install the Haskell Platform

• http://hackage.haskell.org/platform/

• 2. Haskell in 10 minutes

• http://haskell.org/haskellwiki/Learn_Haskell_in_10_minutes

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Keep in touch

• http://github.com/chriseidhof

• @chriseidhof

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One more thing...

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Have fun

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