CSC302 2011S, Class 15: Erlang (1)

Overview:
* Java vs. Scala.
* Your Erlang questions.
* Lab.

Admin:
* Celebration of MathCounts! teams performance
* Don't forget that Assignment 5 is due Saturday night.  
  I know that there are some questions.
* I plan to post the next assignment on Saturday night.
* No office hours today: Another CSFS meeting.
* Reminder: Summer research applications are due today.
* EC for CS table today: CUDA and GPUs.
* EC for track meet this weekend.  Watch AH run and jump.  Figure
  out when.

Why might Java be better than Scala?
* It might not.  Write a short paragraph on why not.
* No overloading in Scala.  Whoops.  Solved.
* Reflection.
* Array syntax.  Lame.  No familiarity issues allowed.
* What happens if two mixins define the same function?
  What if they're not my mixins or class?
* Resume' word
* The Perl issue: Different programmers use different language features,
  making code harder to read/maintain/whatever.

Erlang: Bits of Prolog in a Functional System
* To understand Erlang, you need to think about
  * Key features of Prolog
  * Pure functional programming (and why)
  * Parallelism

Key feature of prolog: Pattern matching
* Given two values, try to match them together
  [X,X] vs. [2,2]. => X is 2
  [X,X] vs. [2,3]. => NO.
* Matching was often implicit.
  goal is foo([X,X]).
  rule is foo([2,3]).
* Erlang makes the matching explicit.
  EXP1 = EXP2 means "try to match the two things"
* In Prolog, matching gives assignment of variables (and yes),
* In Erlang, matching gives the matched value.
* Erlang matching does not like unbound variables on the right.
* When Erlang can't match, it throws an exception.

> X1 = blue.
blue
> X1.
blue
> X2 = blue.
blue
> X1 = X2.
blue
> X3 = red.
red
> X1 = X3.
Boom!

Functional part: "Variables" don't vary.
* Three mechanisms in Scheme
  (define name val)
  (let ((name val)) ...)
  ((lambda (name) ...) val)
* They're really constants.

Why not let variables vary?  Parallelism!

--

Starting erlang

$ erl

Loading a file

1> c(prefix).

Asking it to evaluate

2> 3 + 4.

Asking it to match

3> X = [1,2].

Sequencing

4> X = 1, X + 2.