CSC302 2011S, Class 23: Haskell (2)

Overview:
* Haskell v. Clojure.
* The Second Haskell Lab.

Admin:
* Missing (unexcused): JQ
* Missing (excused): AC, DG, CF3, CF4
* Pop Tarts!
* There is a lot of illness going around campus.  
  Please try to take care of yourselves.
* Because so many of you seem so ill, and could probably use the extra 
  rest, there will be no class on Friday.  I still expect questions on 
  the last Haskell section.
  * We'll do Monads and arrows and similar junk after break
* Mid-semester exam to be distributed tomorrow.  Due Saturday after break.
* A few of you reported on Piazzza being insanely slow at times.  
  I talked to the Piazzza people at the conference, and they asked that
  you send them email when you encounter slowness.

Why learn Haskell if you already know Clojure?
* Purity - A different approach; we're used to side effects
  * We tend to compromise on purity, even when we know that it's useful
  * Purity helps keep programs provably correct and reliable
  * Force us to think differently
  * Parallel programming is much easier in a pure environment
  * We can believe that the compiler takes advantage of it
  * Side effects in a pure language is a huge stretch.  Let's make our 
    brains hurt some more with monads
* Syntactic sugar - It's good for us to see multiple syntaxes for
  the same sweet concept.
  * Currying is different (much simpler than l-s and r-s)
  * We needed another really confusing language
* Strong, Static Typing with Type Inference
  * Much better than in any language we've studied
  * Rare bugs
* Tail recursion is optimized

Why learn Clojure if you're going to learn Haskell?
* Combine different paradigms and learn how they can be productively
  mixed.
  * Clojure is on the JVM and the CLR and therefore can access
    large libraries
  * (Not in Tate, but a reason to learn Clojure.)
* Baby steps
* The data == code idea
  * Perhaps we should show this more
* Macros

Lab!

digitToInt x = (fromEnum x) - (fromEnum '0')

or

digitToInt '0' = 0
digitToInt '1' = 1
digitToInt '2' = 2
digitToInt '3' = 3
digitToInt '4' = 4
digitToInt '5' = 5
digitToInt '6' = 6
digitToInt '7' = 7
digitToInt '8' = 8
digitToInt '9' = 9

---

atoi str = foldl (\x y -> 10*x + y) 0 (map digitToInt str)

or

atoi = (foldl (\x y -> 10*x + y) 0) . (map digitToInt)