Erlang: Questioning Programming Language Groupthink

plop

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Failure is an not option

Joe Armstrong

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What is?

A programming language

  • Functional
  • Concurrency Oriented
  • Compiled
  • Fault-tolerant
  • Distributed

named after: Agner Krarup Erlang, a Danish mathematician.

Agner Erlang

Why?

  • The world is parallel
  • Write programs that run for ever.
  • "Let it crash" over Hope
  • Who understands threads?
  • Why care about multiprocessing?
  • Threads are definitely better than processes.
  • Mutlicore programming is voodoo.
  • Modify code when the app is still running

Erlang makes some very hard things easy.

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Peeking into the Erlangian Ant Colony

  • Let them have processes!
  • Let processes do one(or few things) very well.
  • Let them talk to each other.

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OO Sucks

-- Joe Armstrong

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Anyway ...

Message Passing is Encapsulation

Entities passing messages to each other do not know each other's innards.

The message is the contract.

"Either I understand what you are saying or I ignore you". (fail gracefully).

See also: Actor Model

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Shell

Play.

1 >erl
2 Erlang R15B (erts-5.9) [source] [64-bit] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]
3 Eshell V5.9  (abort with ^G)
4 1>

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Socket Programming, you say?

Holy beej.us!

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Message Passing

Send

1 Pid ! message
2
3 Pid ! {do_this, make_me_a_sandwich}

Receive

 1 receive
 2   PatternA [when Guard1] ->
 3     foo();
 4   PatternB [when Guard2] ->
 5     bar_bar();
 6
 7   _ ->
 8   meh
 9
10 end.

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Functional programming is weird

now that's out of the way...

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Functional Programming

Avoid state

Avoid mutable data

Eliminate Side Effects

Recursion

Small(er) functions

Profit(?)

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Punctuation!

  • Commas (,) seperate arguments, data cons, patterns.
  • Periods (.) followed by whitespace seperate functions and expressions.
  • Semicolons (;) seperate clauses. eg: in if, try, catch receive expressions.

!erlang %% Special Indiana Edition

area({square, Side}) -> Side * Side;

area({circle,Radius}) -> Pi = 3, Pi * Radius * Radius;

area(_Heh) -> ok.

Bonus

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Algebra was a lie!

Variables are single assignment in Erlang.

= is a pattern match *

Variables must start with uppercase letter.

No side effect $=>$ Parallelize programs

No hanky panky with shared memory, locks.

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Data types

  • Integer
  • Float
  • Atom
  • Tuple
  • List

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Strings? Nobody wants Strings!

1 1> [66,69,78,68,69,82].
2 "BENDER"

Bit Syntax

Writing wire protocols?

that's a lost dark art!

1 > Red = 2.
2 > Green = 61.
3 > Blue = 20.
4 > Mem = <<Red:5, Green:6, Blue:5>>.
5 <<23,180>>

Very compact and like, all binary and stuff.

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Pattern Matching

Looks like assignment. It's not.

1 X = 4.

Bound for life!

1 {X,Y} = {its, out_there}

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Pattern Matching

Interesting

 1 function greet(Gender,Name)
 2   if Gender == male then
 3     print("Hello, Mr. %s!", Name)
 4   else if Gender == female then
 5     print("Hello, Mrs. %s!", Name)
 6   else
 7     print("Hello, %s!", Name)
 8 end
 9
10 !erlang
11 greet(male, Name) ->
12   io:format("Hello, Mr. ~s!", [Name]);
13 greet(female, Name) ->
14   io:format("Hello, Mrs. ~s!", [Name]);
15 greet(_, Name) ->
16   io:format("Hello, ~s!", [Name]).

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Guards

pattern matching cannot express things like range of values etc.,

1 driving_age(X) when X < 16; X > 99 ->
2 true;
3 driving_age(_) ->

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Other Control Structures

1 if
2     Age > 16 ->
3         drive();
4     Age > 18 ->
5         vote();
6 end.

Case

1 case X off
2     1 ->
3         foo();
4     2 ->
5         bar();
6     3 ->
7         baz() %% <-
8 end.

Data Strctures

  • Tuples = {apples, 10}
  • Lists - ShoppingList = [{apples, 10}, {oranges, 12}]

List Processing: 

1 [Head|Tail] = [66,69,78,68,69,82].
2
3 !erlang
4 sum([H|T]) -> H + sum(T);
5 sum([]) -> 0.

List comprehensions:

1 [cost(A)*B || {A,B} <- ShoppingList]

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Functions

Functions:

1 Z = fun(X) -> 2*X end. %% Anonymous too!

A function by any other name

  • Functions with same name
  • Same name and different arity
  • Function order!

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Modules

An Erlang module

 1 -module(geometry).
 2 -export([area/1]).
 3
 4 area({rectangle, Base, Height}) -> Base * Height;
 5 area({circle, Radius}) -> 3.14 * Radius * Radius;
 6 area({triangle, Base, Height}) -> area({rectangle, Base, Height})/2.
 7
 8 %use
 9
10 geometry:area({rectangle, 4, 5}).
11 geometry:area({circle, 4}).
12 geometry:area({triangle, 4, 5}).

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ok.

Pradeep Gowda

@btbytes

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