My presentation about Haskell on the NL Hacker News Meetup

1. Welcome
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2. Haskell
Chris Eidhof
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3. Three things
• Strong types
• Purity
• Larger example
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4. Strong Types
• As programmers, we do a lot of testing:
• * Unit Testing
• * Debugging
• * Specification
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5. Strong Types
• Can we automate testing?
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6. List
Type-parameter
Constructor
• data [a] = [] | a : [a]
Type
Constructor
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7. List Example
Example: List.hs
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8. Functions
Give me a list of a And I’ll return a
list of a
• reverse :: [a] -> [a]
• reverse [] = []
• reverse (x:xs) = reverse xs ++ [x]
Pattern Matching
Example: Reverse.hs
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9. Purity
• State = Evil
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10. Purity
• Same Input
• ntial
Refere cy
Transp aren
=
• Same output
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11. Purity
• No side effects:
• * Variables
• * I/O
• * launchMissiles()
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12. Example
• sort :: [Int] -> [Int]
How do we know sort doesn’t launch missiles?
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13. Doing I/O
• putStr :: String -> IO ()
The IO type shows us it’s not pure
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14. Laziness
• if (x < 10 && x > 5)
• Example: Lazy.hs
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15. Quickcheck
• Automatic testing of pure code.
Example: Reverse.hs
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16. Software
Transactional
Memory
• Composable transactions
• No deadlocks!
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17. Fusion
• myFunction = map square . map toInt
• = map (square . toInt)
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18. Parallel code
• map :: (a -> b) -> [a] -> [b]
• parMap :: (a -> b) -> [a] -> [b]
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19. Effects
Useful
Most Nirvana
languages
Haskell
Useless
Dangerous Safe
Simon Peyton-Jones, Caging The Effects Monster
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20. 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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21. Arc Challenge
• Solution in Arc:
• (defop said req
(aform [onlink "click here"
(pr "you said: " (arg _ "foo"))]
(input "foo")
(submit)))
•
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22. Arc Challenge
arc = do name <- input
link "click here"
display $ "you said:" ++ name
See gist: http://gist.github.com/260052
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23. Arc Challenge (2)
input uses type-inference!
arc2 = do name <- input
(x,y) <- input
link "click here"
display (add x y)
display ("you said:" ++ name)
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24. 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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25. 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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26. Keep in touch
• http://github.com/chriseidhof
• @chriseidhof
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27. One more thing...
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28. Have fun
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