Erlang bootstrap course

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

What are we dealing with?Data TypesPattern Matching (makes functions fun)ModulesProcesses

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

Shell Commandsh() - 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 = 2.23> X + 1.3

Integers as you would expect But, whah???4> Y =1111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111. On no he di’int!5> Y + 1.11111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111112

Floats Alas, not freaky (plain ol’ 64 bit double‐precision)6> 1.1234567890123456789012345678901234567890.1.1234567890123457 An unusual conversion from ﬂoat to int1> X = 1.12345.1.123452> is_float(X).true3> Y = erlang:trunc(X).14> is_integer(Y).true

ListsMost common data structure. Used for storing a variablenumber 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

BinariesSomeMmes used to represent strings <<"This is a binary">>is_binary diﬀerenMates from is_listMore 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 actuallyever varyAbcA_long_non_standard_variable_nameAProperErlangVariableName * Start with an Upper Case Letter. * No "funny characters". * Variables are used to store values of data structures.

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

Pattern Matching

A = 10Succeeds - binds A to 10{B, C, D} = {10, foo, bar}Succeeds - binds B to 10, C to foo and Dto 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

Modules & Functions

ToDo ListLet’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)). okRead 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 LibrariesSecond 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 systemWe 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 listsComma separated Erlang terms surrounded by brackets [this, is, "a", {List, [of, stuff]}]Used ehhhverywhereAn important paGern for iteraMve operaMons lists:map/2 list comprehensionThe 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.15> 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 FunctionsUse fun to create a funcMon from within a funcMonUse a fun to pass funcMonality around like any other data structureAnonymous 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 HeadsIs deﬁned as a collection of clauses.func(Pattern1, Pattern2, ...) ->... ;func(Pattern1, Pattern2, ...) ->... ;...func(Pattern1, Pattern2, ...) ->... .

Guardsis_number(X) - X is a numberis_integer(X) - X is an integeris_ﬂoat(X) - X is a ﬂoatis_atom(X) - X is an atomis_tuple(X) - X is a tupleis_list(X) - X is a listlength(X) == 3 - X is a list of length 3size(X) == 2 - X is a tuple of size 2.X > Y + Z - X is > Y + Z

The Tyrannical State

The StateNo ObjectsNo GlobalsLogic VariablesNo Pass by Reference

In the LoopYour State

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 offguard. Be prepared.

Concurrent Programming

Creating a New ProcessCode in Pid1Pid2 = spawn(Mod, Func, Args)AfterPid2 is process identiﬁer of the new process -this is known only to process Pid1.

Message Passing {PidA, foo} A B receivePidB ! {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 Serverloop() -> receive {From, Msg} -> From ! {self(), Msg}, loop(); stop -> true end.

Selective Message Reception ﬁrst foo then bar A receivePidC ! foo foo -> ... C end, receive bar -> ... end BPidC ! 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 EXITExit Signals are Sent when Processes Crash When a processcrashes (e.g. failure of a BIF or a pattern match) Exit Signalsare sent to all processes to which the failing process iscurrently linked.

A EXIT B EXIT C F EXIT EXITD EXIT E

Trap Exit = TrueProcesses can trap exit signalsIn the following diagram P1 is linked to P2 and P2 is linked toP3. An error occurs in P1 - the error propagates to P2. P2traps the error and the error is not propagated to P3. EXIT trap link exits

Put up with Sh!tresult parsed input data stream

Distribution

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 regRegistered process network communication

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

http://lanyrd.com	187
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Erlang bootstrap course

by Martin Logan on Sep 18, 2011

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