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Introduction to Erlang

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Suitable for an audience with little exposure to functional programming. Concurrency and handling errors in Erlang are skipped due to time

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Introduction to Erlang

  1. 1. Erlang – Quick Intro<br />Brought to you by Raymond Tay<br />
  2. 2. Erlang?<br />Small language designed more than 20 years ago by Joe Armstrong, Robert Virding and Mike Williams of Ericsson’s CS Lab. <br />Functional Language.<br />Lightweight concurrency model (aka Actors). Yes, Scalahas a Actors model where they share similar syntax and machinery.<br />Great for soft real-time systems. E.g. Second Life’s chat service<br />Not good for number crunching. There are better tools for that e.g. SciPyNumPy<br />
  3. 3. Yah yah yah…WHO actually uses Erlang?<br />Amazon uses Erlang to implement SimpleDB as part of its EC2 services<br />Facebook uses Erlang to power its backend chat service for its 100 million active users<br />Ericsson uses Erlang to support its GPRS and 3G mobile networks<br />T-Mobile uses Erlang in its SMS and authentication systems<br />
  4. 4. So, what are the benefits of Erlang?<br />Easy to learn, easy to use. Expressing distributing computing is easier<br />Garbage collection is implemented on a per process basis<br />An process in Erlang is light-weight i.e. not a OS process. Each pair of processes communicate by passing messages<br />Designed to operate behind firewalls. Think private compute cloud<br />
  5. 5. Let’s learn how Erlang does it<br />Numbers<br />Variables (they’re not variable)<br />Atoms<br />Boolean Algebra and Comparison Operators<br />Tuples<br />Lists (and comprehensions)<br />Modules in Erlang<br />
  6. 6. There’s more…<br />High-order functions and Lambdas<br />Errors and catching them (We don’t have time for this)<br />Concurrency in Erlang and Actors etc (We don’t have time for this too)<br />Finally we’ll end with a demonstration<br />
  7. 7. Working with Erlang<br />In FP, statements do not exist only expressions. Expressions always return a result and more often than not, have no side effects at all; statements (depending on the lang) may / may not return a result but always have side effects.<br />Expressions, in Erlang, have to be terminated with a period followed by whitespace (n, )<br />E.g expression involving numbers<br />2 + 5.<br />5 / 2. (same as 5 div 2.)<br />5 rem 2. <br />-50 * (100 – 499). (Erlang understands the typical math operator precedences)<br />2#101 (same as the decimal number 5 only expressed in binary i.e. base 2)<br />16#cafebabe (do you know what this magic number is expressed in base 16?)<br />
  8. 8. Variables in Erlang (seriously???)<br />No such thing as a variable variable<br />In Erlang, creating a variable and associating with a value is termed bounding. You cannot change the value once bounded.<br />One = 1.<br />One = 2. (Will not succeed)<br />Two = 2.<br />Two = One + One. (Will succeed)<br />_ = 999999999 (Will always succeed. ‘_’ is the i-don’t-care-variable)<br />The ‘=‘ acts as an assignment and pattern matching operator. Pattern matching occurs with/without the ‘=‘. More later.<br />
  9. 9. Atoms (Like the physics term but not like it)<br />In physics of the 1920s, atoms were the smallest things known to man. Today, there are units smaller than the atom like hadrons, quarks. In Erlang, an atom is a literal.<br />hello. (Typing Hello. means to extract the value of Hello)<br />‘Hello there’. (anything in quotes is an atom)<br />‘_this_is_a_#really_!long_@string’.<br />
  10. 10. Boolean Algebra, Operators<br />In Erlang, the atom true is Truth whereas false is Falsehood.<br />You can use =:= and =/= for comparing equality and inquality<br />You can use == and /= for comparing equality and inequality if you don’t care about exactness / precisions<br />5 =:= 5. (will return true)<br />5 == 5. (will return true)<br />5 =:= 5.0. (will return false)<br />5 == 5.0. (will return true)<br />
  11. 11. Tuples<br />They allow you to aggregate values to form some sort of data structure.<br />X = 10, Y = 4.<br />Point = {X, Y}. (Tuple is created)<br />{MyX, _} = Point. (Pattern matching at play. MyX will get the value 10)<br />{_, _, _} = Point. (will fail and erlang will complain RHS don’t match LHS)<br />
  12. 12. Lists<br />In FP, lists are completely indispensable. Same here<br />You can create lists, obviously. You can concat, intersect them. You can retrieve the head or tail of a list via ‘hd’ and ‘tl’ commands in the ‘erlang’ module<br />MyList = [1,2,3,4,5].<br />MyList ++ [7,8] - - [1,2,3,4,5,6,78]. (equivalent to (MyList ++ ([7,8] - - [1,2,3,4,5,6,7,8])) )<br />hd( [1,2] ). (will return 1 and take note its not a list)<br />tl([1,2]). (will return [2] which is a list)<br />
  13. 13. List Comprehension<br />In other languages, its called a for-comprehension. Let’s look at code examples<br />[2 * N | | N <- [1,2,3]]. (returns [2,4,6])<br />[X | | X <- [1,2,3,4,5,6], X rem 2 =:= 0]. (returns [2,4,6] )<br />Let’s look at a more complex example (taken from sg.finance.yahoo.com)<br />[ {Quote,Price} | | {Quote, Price} <- [ {e3s, 0.4}, {g13.si, 1.66}, {'5im.si',0.22}, {e5h.si, 0.64} ], Price > 0.5]. (returns [{'g13.si',1.66},{'e5h.si',0.64}])<br />Nested-for-loops (Ugly word in Erlang, but u get the meaning)<br />[ {X, Y} | | X <- [1,2], Y <- [3,4]]. (returns [{1,3}, {1,4}, {2,3}, {2,4}]) <br />
  14. 14. How to avoid clustering all code in a single file?<br />You need modules.<br />You need to declare your module’s name, attributes, what functions are exported<br />
  15. 15. Compiling and using your modules<br />There are a couple of methods to do this but for the sake of brevity, here’s the simplest<br />
  16. 16. Functions and Lambdas<br />Functions are easy to write in Erlang. You don’t need to say ‘def’, ‘function’ or give any access specifier like ‘public’, ‘protected’ any of that nonsense.<br />Here’s an example of a Mapper function.<br />map(_, []) -> [];<br />map(F, [H|T]) -> [F(H) | map(F, T)].<br />Another example of a Folder function<br />fold(F, Start, []) -> Start;<br />fold(F, Start, [H|T]) -> fold(F, F(H, Start), T).<br />A lambda function is simply a nameless function and sometimes called an anonymous function. They can do pretty much anything except recursion.<br />The form is like this:<br />fun (Args1) -> Expression1, … , Expression N ;<br /> (Args2) -> Expression1, … , Expression N;<br />end.<br />
  17. 17. Lambdas<br />
  18. 18. More Lambdas…<br />
  19. 19. Demo<br />This is a demo of a sorting algorithm I wrote in 2007 as it illustrates most of the concepts we talked about.<br />

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