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, ...) ->
... ;
... ;
...
... .

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

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