Erlang bootstrap course

The Erlang Bootstrap Class The Erlang Basics Based on the 4 day Erlang course

What are we dealing with? Data Types Pattern Matching (makes functions fun) Modules Processes

Shell (learn it, love it) As critical to Erlang dev as ﬁngers Your best tool for learning and experimenting Important facts Start with erl Stop with ctrl c, ctrl c q(). to kill nicely expressions end in a period

Shell Commands h() - history . Print the last 20 commands. b() - bindings. See all variable bindings. f() - forget. Forget all variable bindings. f(Var) - forget. Forget the binding of a variable Var. * This can ONLY be used as a command to * the shell - NOT in the body of a function! e(n) - evaluate. Evaluate the n:th command in history. e(-1) - Evaluate the previous command. * Edit the command line as in Emacs * See the User Guide for more details and examples of use of the shell.

Calculations in the Shell You can add! 1> 1 + 1. 2 You can assign values to variables! 2> X = 2. 2 3> X + 1. 3

Integers as you would expect But, whah??? 4> Y = 1111111111111111111111111111111111111111111111111111111111111111111111111111111 1111111111111111111111111111111111111111111111111111111111111111111111111111111 11111. On no he di’int! 5> Y + 1. 1111111111111111111111111111111111111111111111111111111111111111111111111111111 1111111111111111111111111111111111111111111111111111111111111111111111111111111 111112

Floats Alas, not freaky (plain ol’ 64 bit double‐precision) 6> 1.1234567890123456789012345678901234567890. 1.1234567890123457 An unusual conversion from ﬂoat to int 1> X = 1.12345. 1.12345 2> is_float(X). true 3> Y = erlang:trunc(X). 1 4> is_integer(Y). true

Lists Most common data structure. Used for storing a variable number of elements. [123, xyz] [ {person, 'Joe', 'Armstrong'}, {person, 'Robert', 'Virding'}, {person, 'Mike', 'Williams'} ]

Strings (haven’t we seen these before?) PreGy much what you’d expect 1> Slogan = "Mongo DB is Web Scale". "Mongo DB is Web Scale" But, wait, OMG, noooo! 2> io_lib:format("Mongo DB is ~s", ["Web Scale"]). [77,111,110,103,111,32,68,66,32,105,115,32,"Web Scale"] Yep, strings are just lists! 3> is_string(Slogan). ** exception error: undefined shell command is_string/1 4> is_list(Slogan). true

Binaries SomeMmes used to represent strings <<"This is a binary">> is_binary diﬀerenMates from is_list More typically used to work with actual binary data         (e.g. bitstring syntax – not covered in this presentaMon) Have protocol, will use binaries/bitstrings!

Variables (not very variable) More of a deﬁnition than a variable. They dont actually ever vary Abc A_long_non_standard_variable_name AProperErlangVariableName * Start with an Upper Case Letter. * No "funny characters". * Variables are used to store values of data structures.

Tuples pretty much the “struct” thingy for erlang {123, bcd} {abc, {def, 123}, jkl} {} {atom() = Tag, term() = Value} % very erlangy {error, {“value was bad”, 9}}

A = 10 Succeeds - binds A to 10 {B, C, D} = {10, foo, bar} Succeeds - binds B to 10, C to foo and D to bar {A, A, B} = {abc, abc, foo} Succeeds - binds A to abc, B to foo {A, A, B} = {abc, def, 123} Fails [A,B,C] = [1,2,3] Succeeds - binds A to 1, B to 2, C to 3 [A,B,C,D] = [1,2,3] Fails

[A,B|C] = [1,2,3,4,5,6,7] Succeeds - binds A = 1, B = 2, C = [3,4,5,6,7] [H|T] = [1,2,3,4] Succeeds - binds H = 1, T = [2,3,4] [H|T] = [abc] Succeeds - binds H = abc, T = [] [H|T] = [] Fails {A,_, [B|_Tail],{B}} = {abc,23,[22,x],{22}} Succeeds - binds A = abc, B = 22

ToDo List Let’s make a list! 1> ToDo = ["Shard /dev/null", "Learn Ruby", "Remove Bing from Phone"]. Write to disk (easy to encode Erlang!) 2> file:write_file("todo.bin", term_to_binary(ToDo)). ok Read from disk (easy to decode Erlang!) 3> {ok, Bin} = file:read_file("todo.bin"). {ok,<<131,108,0,0,0,3,107,0,15,83,104,97,114,100,32,47, 100,101,118,47,110,117,108,108,107,0,10,...>>} 4> binary_to_term(Bin). ["Shard /dev/null","Learn Ruby","Remove Bing from Phone"]

Standard Libraries Second most important Erlang developer tool: a decent module reference hGp://erlang.org/doc/man_index.html hGp://erldocs.com/  The file module provides an interface to the ﬁle system We will keep using them – keep an eye out

Module System -module(demo). -export([double/1]). double(X) -> times(X, 2). times(X, N) -> X * N.

Using lists Comma separated Erlang terms surrounded by brackets [this, is, "a", {List, [of, stuff]}] Used ehhhverywhere An important paGern for iteraMve operaMons lists:map/2 list comprehension The basis for “associaMve array” structure in Erlang (proplist) [{foo, "Foo" 123}] 1> L1 = [2, 3]. [2,3] 2> L2 = [1|L1]. [1,2,3] 3> [H|T] = L2. [1,2,3] 4> H. 1 5> T. [1,2]

Built In Functions (BIFs) date() time() length([1,2,3,4,5]) size({a,b,c}) atom_to_list(an_atom) list_to_tuple([1,2,3,4]) integer_to_list(2234) tuple_to_list({})

Lets do something more with lists A basic sort 5> lists:sort(ToDo). ["Learn Ruby","Remove Bing from Phone","Shard /dev/null"] Wait, that’s not what I want! 6> ToDo2 = [{2, "Shard /dev/null"}, {3, "Learn Ruby"}, {1, "Remove Bing from Phone"}]. 7> lists:sort(ToDo2). [{1,"Remove Bing from Phone"}, {2,"Shard /dev/null"}, {3,"Learn Ruby"}] Default sort comparison uses “natural order” of Erlang term We don’t need no sMnking natural order! 8> lists:sort( fun({P1, N1}, {P1, N2}) -> N1 < N2 end, ToDo2). [{3,"Learn Ruby"}, {1,"Remove Bing from Phone"}, {2,"Shard /dev/null"}

Lambda Functions Use fun to create a funcMon from within a funcMon Use a fun to pass funcMonality around like any other data structure Anonymous funcMons have access to variables visible within  their deﬁning block, even when executed elsewhere 1> X = 1. 1 2> F = fun(Y) -> X + Y end. #Fun<erl_eval.6.13229925> 3> F(10). 11

Function Heads Is deﬁned as a collection of clauses. func(Pattern1, Pattern2, ...) -> ... ; func(Pattern1, Pattern2, ...) -> ... ; ... func(Pattern1, Pattern2, ...) -> ... .

Guards is_number(X) - X is a number is_integer(X) - X is an integer is_ﬂoat(X) - X is a ﬂoat is_atom(X) - X is an atom is_tuple(X) - X is a tuple is_list(X) - X is a list length(X) == 3 - X is a list of length 3 size(X) == 2 - X is a tuple of size 2. X > Y + Z - X is > Y + Z

The State No Objects No Globals Logic Variables No Pass by Reference

Side Effects - Side effects are not “evil” - any IO is a side effect - anything not “referentially transparent” - Make function substitution difﬁcult - Side effects can catch you off guard. Be prepared.

Creating a New Process Code in Pid1 Pid2 = spawn(Mod, Func, Args) After Pid2 is process identiﬁer of the new process - this is known only to process Pid1.

Message Passing {PidA, foo} A B receive PidB ! {self(), foo} {From, Msg} -> Actions end

Echo Client -module(echo). -export([go/0]). go() -> Pid2 = spawn(fun() -> loop() end), Pid2 ! {self(), hello}, receive {Pid2, Msg} -> io:format("P1 ~w~n", [Msg]) end, Pid2 ! stop.

Echo Server loop() -> receive {From, Msg} -> From ! {self(), Msg}, loop(); stop -> true end.

Selective Message Reception ﬁrst foo then bar A receive PidC ! foo foo -> ... C end, receive bar -> ... end B PidC ! bar

sleep(T) -> receive after T -> true flush() -> end. receive {echo, Msg} -> flush() suspend() -> after receive 0 -> true after end. infinity -> true end.

Beefy Echo - Not just echo anymore - Fun with Funs - Call a process by a name

Now it gets interesting Fault Tolerance

Exit Signals EXIT EXIT EXIT Exit Signals are Sent when Processes Crash When a process crashes (e.g. failure of a BIF or a pattern match) Exit Signals are sent to all processes to which the failing process is currently linked.

A EXIT B EXIT C F EXIT EXIT D EXIT E

Trap Exit = True Processes can trap exit signals In the following diagram P1 is linked to P2 and P2 is linked to P3. An error occurs in P1 - the error propagates to P2. P2 traps the error and the error is not propagated to P3. EXIT trap link exits

Put up with Sh!t result parsed input data stream

Single VM Single machine communication VM A on Machine A Y ! msg X Y VM A on Machine A VM B on Machine B Y ! msg X Y Multiple VM network communication

Registered process local communication VM A on Machine A reg ! msg X reg VM A on Machine A VM B on Machine B {reg, B} ! msg X reg Registered process network communication

Topology with Cookies erl -name <name> -setcookie foo erl -name <name> -setcookie bar

Erlang bootstrap course

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