The document discusses various aspects of Erlang terms presented at a Seattle Elixir meetup in December 2017, focusing on the implementation of Erlang's type system, data structures, and memory alignment. It details the representation of different data types such as integers, floats, atoms, tuples, lists, maps, and binaries within Erlang, including examples of their binary formats and alignment properties. Additionally, it explains the tagging scheme for Erlang terms and limits related to processes, ports, and atoms.

1. UNDERSTANDING
ERLANG TERMS
SEATTLE ELIXIR MEETUP
DEC 2017

2. UNDERSTANDING ERLANG TERMS
MYTH
▸ Tail-recursive functions are much faster than body-
recursive functions

3. UNDERSTANDING ERLANG TERMS
HOW TO IMPLEMENT ERLANG’S TYPE SYSTEM?
▸ Dynamic typing

4. UNDERSTANDING ERLANG TERMS
INFORMATION = BITS + CONTEXT
+-----+-----+-----+-----+-----+-----+-----+-----+
|0x68 |0x69 |0x6c |0x6c |0x6f |0x00 |0x00 |0x00 |
+-----+-----+-----+-----+-----+-----+-----+-----+
|0x100|0x101|0x102|0x103|0x104|0x105|0x106|0x107|
+-----+-----+-----+-----+-----+-----+-----+-----+
+-----+-----+-----+-----+-----+-----+-----+-----+
| h | e | l | l | o | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
|0x100|0x101|0x102|0x103|0x104|0x105|0x106|0x107|
+-----+-----+-----+-----+-----+-----+-----+-----+
+-----+-----+-----+-----+-----+-----+-----+-----+
| 26984 | 27756 | 111 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+
|0x100|0x101|0x102|0x103|0x104|0x105|0x106|0x107|
+-----+-----+-----+-----+-----+-----+-----+-----+
+-----+-----+-----+-----+-----+-----+-----+-----+
| 1819044200 | 111 |
+-----+-----+-----+-----+-----+-----+-----+-----+
|0x100|0x101|0x102|0x103|0x104|0x105|0x106|0x107|
+-----+-----+-----+-----+-----+-----+-----+-----+

5. UNDERSTANDING ERLANG TERMS
ERLANG’S BUILDING BLOCK(PARTIALLY)
▸ Numbers
▸ 1, 2, 100
▸ 1.0, 3.14
▸ Atoms
▸ ok, true, false
▸ Tuples
▸ {}, {1, 2, 3}, {ok, 100}
▸ Lists
▸ [], [1, 2, 3]
▸ Maps
▸ #{name => “Jon Snow”, job => “King in the North”}
▸ Binaries
▸ <<“hello”>>, <<1, 2, 3>>

6. UNDERSTANDING ERLANG TERMS
A NAIVE IMPLEMENTATION
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx000 integer
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx001 float
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx010 atom
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx011 tuple
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx100 list
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx101 map
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx110 binary
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx111 *not in use*

7. UNDERSTANDING ERLANG TERMS
ERLANG TERM
/* erts/emulator/beam/sys.h */
typedef unsigned long Eterm erts_align_attribute(sizeof(long));
typedef unsigned long Uint erts_align_attribute(sizeof(long));
typedef Uint UWord;
/* erts/emulator/beam/erl_term.h */
typedef UWord Wterm;
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx 32-bits
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx 64-bits

8. int i = 100;
printf(“%p”, &i);
UNDERSTANDING ERLANG TERMS
DATA ALIGNMENT PROPERTY
+-----+-----+-----+-----+
| 100 |
+-----+-----+-----+-----+
|0x100|0x101|0x102|0x103|
+-----+-----+-----+-----+
+-----+-----+-----+-----+
| 100 |
+-----+-----+-----+-----+
|0x204|0x205|0x206|0x207|
+-----+-----+-----+-----+
+-----+-----+-----+-----+
| 100 |
+-----+-----+-----+-----+
|0x40c|0x40d|0x40e|0x40f|
+-----+-----+-----+-----+
0001 0000 0000
0010 0000 0100
0011 0000 1000
0100 0000 1100
+-----+-----+-----+-----+
| 100 |
+-----+-----+-----+-----+
|0x308|0x309|0x30a|0x30b|
+-----+-----+-----+-----+

9. UNDERSTANDING ERLANG TERMS
A STAGED TAG SCHEME FOR ERLANG
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx01 list
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx10 boxed
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx11 immediate 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0011 pid
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0111 port
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111 small int
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1011 immediate 2
xxxxxxxx xxxxxxxx xxxxxxxx xx001011 atom
xxxxxxxx xxxxxxxx xxxxxxxx xx011011 catch
11111111 11111111 11111111 11111011 nil

10. UNDERSTANDING ERLANG TERMS
BOXED HEADER
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header
xxxxxxxx xxxxxxxx xxxxxxxx xx000000 tuple
xxxxxxxx xxxxxxxx xxxxxxxx xx000100 binary match context
xxxxxxxx xxxxxxxx xxxxxxxx xx001000 pos big int
xxxxxxxx xxxxxxxx xxxxxxxx xx001100 neg big int
xxxxxxxx xxxxxxxx xxxxxxxx xx010000 reference
xxxxxxxx xxxxxxxx xxxxxxxx xx010100 function
xxxxxxxx xxxxxxxx xxxxxxxx xx011000 float
xxxxxxxx xxxxxxxx xxxxxxxx xx011100 export
xxxxxxxx xxxxxxxx xxxxxxxx xx100000 refc binary
xxxxxxxx xxxxxxxx xxxxxxxx xx100100 heap binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101000 sub binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101100 *not used
xxxxxxxx xxxxxxxx xxxxxxxx xx110000 external pid
xxxxxxxx xxxxxxxx xxxxxxxx xx110100 external port
xxxxxxxx xxxxxxxx xxxxxxxx xx111000 external reference
xxxxxxxx xxxxxxxx xxxxxxxx xx111100 map

11. UNDERSTANDING ERLANG TERMS
IMMEDIATE 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx11 immediate 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111 small int
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0011 pid
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0111 port

12. UNDERSTANDING ERLANG TERMS
IMMEDIATE 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx11 immediate 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111 small int
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0011 pid
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0111 port

13. UNDERSTANDING ERLANG TERMS
SMALL INT xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111 32-bits
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111 64-bits
01111111 11111111 11111111 11111111 TMAX 32
10000000 00000000 00000000 00001111 TMIN 32
-134,217,728 - 134,217,727
01111111 11111111 11111111 11111111 11111111 11111111 11111111 11111111 TMAX 64
10000000 00000000 00000000 00000000 00000000 00000000 00000000 00001111 TMIN 64
-576,460,752,303,423,488 - 576,460,752,303,423,487

14. UNDERSTANDING ERLANG TERMS
IMMEDIATE 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx11 immediate 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111 small int
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0011 pid
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0111 port

15. UNDERSTANDING ERLANG TERMS
PID
▸ Default 2^18 processes(262,144)
▸ Min 1024
▸ Max 2^27 - 1 = 134,217,727, same as TMAX 32
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0011
<0.157.0>
00000000 00000000 00000100 11101000 00000000 00000000 00001001 11010011
<0.4520.2>
00000000 00000000 00000101 01000001 00000000 00010001 00011010 10000011
% iex --erl "+P 1024" -S mix
Interactive Elixir (1.5.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> :erlang.system_info(:process_limit)
1024

16. UNDERSTANDING ERLANG TERMS
IMMEDIATE 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx11 immediate 1
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1111 small int
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0011 pid
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0111 port

17. UNDERSTANDING ERLANG TERMS
PORT
▸ Default 2^16 ports (65536)
▸ Max 2^27 - 1 = 134,217,727, same as TMAX 32
xxxxxxxx xxxxxxxx xxxxxxxx xxxx0111

18. UNDERSTANDING ERLANG TERMS
IMMEDIATE 2
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1011 immediate 2
xxxxxxxx xxxxxxxx xxxxxxxx xx001011 atom
xxxxxxxx xxxxxxxx xxxxxxxx xx011011 catch
11111111 11111111 11111111 11111011 nil

19. UNDERSTANDING ERLANG TERMS
IMMEDIATE 2
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1011 immediate 2
xxxxxxxx xxxxxxxx xxxxxxxx xx001011 atom
xxxxxxxx xxxxxxxx xxxxxxxx xx011011 catch
11111111 11111111 11111111 11111011 nil

20. UNDERSTANDING ERLANG TERMS
ATOM
▸ Default 2^20 = 1,048,576
▸ Min 8192
▸ Max 67108864 on 32-bits
▸ Max 2147483647 on 64-bits
xxxxxxxx xxxxxxxx xxxxxxxx xx001011
xxxxxxxx xxxxxxxx xxxxxxxx xx001011 32-bits
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xx001011 64-bits
11111111 11111111 11111111 11001011 MAX 32
67,108,863
64MB
00000000 00000000 00000000 00011111 11111111 11111111 11111111 11001011 MAX 64
2,147,483,647
2GB

21. UNDERSTANDING ERLANG TERMS
IMMEDIATE 2
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1011 immediate 2
xxxxxxxx xxxxxxxx xxxxxxxx xx001011 atom
xxxxxxxx xxxxxxxx xxxxxxxx xx011011 catch
11111111 11111111 11111111 11111011 nil

22. UNDERSTANDING ERLANG TERMS
CATCH
▸ Stores the address of catch block
xxxxxxxx xxxxxxxx xxxxxxxx xx011011

23. UNDERSTANDING ERLANG TERMS
IMMEDIATE 2
xxxxxxxx xxxxxxxx xxxxxxxx xxxx1011 immediate 2
xxxxxxxx xxxxxxxx xxxxxxxx xx001011 atom
xxxxxxxx xxxxxxxx xxxxxxxx xx011011 catch
11111111 11111111 11111111 11111011 nil

24. UNDERSTANDING ERLANG TERMS
NIL
▸ []
▸ Not nil in Elixir
11111111 11111111 11111111 11011011
11111111 11111111 11111111 11001011 32-bits
11111111 11111111 11111111 11111111 11111111 11111111 11111111 11001011 64-bits

25. UNDERSTANDING ERLANG TERMS
PRIMARY TAG
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx01 list
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx10 boxed
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header

26. UNDERSTANDING ERLANG TERMS
PRIMARY TAG
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx01 list
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx10 boxed
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header

27. UNDERSTANDING ERLANG TERMS
LIST(CONS)
▸ car
▸ cdr
▸ [car | cdr]
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx01
[1]
0x100 0x000000000000001f car: 1
0x108 0xfffffffffffffffb cdr: []
0x100 00000000 00000000 00000001 00000000
0x101 00000000 00000000 00000001 00000001
[1, 2]
0x200 0x000000000000002f car: 2
0x208 0xfffffffffffffffb cdr: []
0x210 0x000000000000001f car: 1
0x218 0x0000000000000201 cdr: [2]
0x210 00000000 00000000 00000011 00010000
0x211 00000000 00000000 00000011 00010001
[1 | 2]
0x300 0x000000000000001f car: 1
0x308 0x000000000000002f cdr: 2
0x300 00000000 00000000 00000011 00000000
0x301 00000000 00000000 00000011 00000001

28. UNDERSTANDING ERLANG TERMS
PRIMARY TAG
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx01 list
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx10 boxed
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header

29. UNDERSTANDING ERLANG TERMS
BOXED xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx10
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx10

30. UNDERSTANDING ERLANG TERMS
BOXED HEADER
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header
xxxxxxxx xxxxxxxx xxxxxxxx xx000000 tuple
xxxxxxxx xxxxxxxx xxxxxxxx xx000100 binary match context
xxxxxxxx xxxxxxxx xxxxxxxx xx001000 pos big int
xxxxxxxx xxxxxxxx xxxxxxxx xx001100 neg big int
xxxxxxxx xxxxxxxx xxxxxxxx xx010000 reference
xxxxxxxx xxxxxxxx xxxxxxxx xx010100 function
xxxxxxxx xxxxxxxx xxxxxxxx xx011000 float
xxxxxxxx xxxxxxxx xxxxxxxx xx011100 export
xxxxxxxx xxxxxxxx xxxxxxxx xx100000 refc binary
xxxxxxxx xxxxxxxx xxxxxxxx xx100100 heap binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101000 sub binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101100 *not used
xxxxxxxx xxxxxxxx xxxxxxxx xx110000 external pid
xxxxxxxx xxxxxxxx xxxxxxxx xx110100 external port
xxxxxxxx xxxxxxxx xxxxxxxx xx111000 external reference
xxxxxxxx xxxxxxxx xxxxxxxx xx111100 map

31. UNDERSTANDING ERLANG TERMS
BOXED HEADER
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header
xxxxxxxx xxxxxxxx xxxxxxxx xx000000 tuple
xxxxxxxx xxxxxxxx xxxxxxxx xx000100 binary match context
xxxxxxxx xxxxxxxx xxxxxxxx xx001000 pos big int
xxxxxxxx xxxxxxxx xxxxxxxx xx001100 neg big int
xxxxxxxx xxxxxxxx xxxxxxxx xx010000 reference
xxxxxxxx xxxxxxxx xxxxxxxx xx010100 function
xxxxxxxx xxxxxxxx xxxxxxxx xx011000 float
xxxxxxxx xxxxxxxx xxxxxxxx xx011100 export
xxxxxxxx xxxxxxxx xxxxxxxx xx100000 refc binary
xxxxxxxx xxxxxxxx xxxxxxxx xx100100 heap binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101000 sub binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101100 *not used
xxxxxxxx xxxxxxxx xxxxxxxx xx110000 external pid
xxxxxxxx xxxxxxxx xxxxxxxx xx110100 external port
xxxxxxxx xxxxxxxx xxxxxxxx xx111000 external reference
xxxxxxxx xxxxxxxx xxxxxxxx xx111100 map

32. UNDERSTANDING ERLANG TERMS
TUPLE xxxxxxxx xxxxxxxx xxxxxxxx xx000000
{}
0x100 0x0000000000000000 Arity(0) … 0000 0000
0x100 00000000 00000000 00000001 00000000
0x102 00000000 00000000 00000001 00000010
{1}
0x200 0x0000000000000040 Arity(1) … 0100 0000
0x208 0x000000000000001f 1
0x200 00000000 00000000 00000010 00000000
0x202 00000000 00000000 00000010 00000010
{1, 2}
0x300 0x0000000000000080 Arity(2) … 1000 0000
0x308 0x000000000000001f 1
0x310 0x000000000000002f 2
0x300 00000000 00000000 00000011 00000000
0x302 00000000 00000000 00000011 00000010

33. UNDERSTANDING ERLANG TERMS
BOXED HEADER
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header
xxxxxxxx xxxxxxxx xxxxxxxx xx000000 tuple
xxxxxxxx xxxxxxxx xxxxxxxx xx000100 binary match context
xxxxxxxx xxxxxxxx xxxxxxxx xx001000 pos big int
xxxxxxxx xxxxxxxx xxxxxxxx xx001100 neg big int
xxxxxxxx xxxxxxxx xxxxxxxx xx010000 reference
xxxxxxxx xxxxxxxx xxxxxxxx xx010100 function
xxxxxxxx xxxxxxxx xxxxxxxx xx011000 float
xxxxxxxx xxxxxxxx xxxxxxxx xx011100 export
xxxxxxxx xxxxxxxx xxxxxxxx xx100000 refc binary
xxxxxxxx xxxxxxxx xxxxxxxx xx100100 heap binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101000 sub binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101100 *not used
xxxxxxxx xxxxxxxx xxxxxxxx xx110000 external pid
xxxxxxxx xxxxxxxx xxxxxxxx xx110100 external port
xxxxxxxx xxxxxxxx xxxxxxxx xx111000 external reference
xxxxxxxx xxxxxxxx xxxxxxxx xx111100 map

34. UNDERSTANDING ERLANG TERMS
BIG INT xxxxxxxx xxxxxxxx xxxxxxxx xx001x00
(1 <<< 59) - 1
01111111_11111111_11111111_11111111_11111111_11111111_11111111_11111111 MAX small int
(1 <<< 59)
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_01001000 pos bignum
00001000_00000000_00000000_00000000_00000000_00000000_00000000_00000000
(1 <<< 59) + 1
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_01001000 pos bignum
00001000_00000000_00000000_00000000_00000000_00000000_00000000_00000001
(-1 <<< 59) - 1
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_01001100 neg bignum
00001000_00000000_00000000_00000000_00000000_00000000_00000000_00000001
(-1 <<< 59)
10000000_00000000_00000000_00000000_00000000_00000000_00000000_00001111 MIN small int

35. UNDERSTANDING ERLANG TERMS
BOXED HEADER
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header
xxxxxxxx xxxxxxxx xxxxxxxx xx000000 tuple
xxxxxxxx xxxxxxxx xxxxxxxx xx000100 binary match context
xxxxxxxx xxxxxxxx xxxxxxxx xx001000 pos big int
xxxxxxxx xxxxxxxx xxxxxxxx xx001100 neg big int
xxxxxxxx xxxxxxxx xxxxxxxx xx010000 reference
xxxxxxxx xxxxxxxx xxxxxxxx xx010100 function
xxxxxxxx xxxxxxxx xxxxxxxx xx011000 float
xxxxxxxx xxxxxxxx xxxxxxxx xx011100 export
xxxxxxxx xxxxxxxx xxxxxxxx xx100000 refc binary
xxxxxxxx xxxxxxxx xxxxxxxx xx100100 heap binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101000 sub binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101100 *not used
xxxxxxxx xxxxxxxx xxxxxxxx xx110000 external pid
xxxxxxxx xxxxxxxx xxxxxxxx xx110100 external port
xxxxxxxx xxxxxxxx xxxxxxxx xx111000 external reference
xxxxxxxx xxxxxxxx xxxxxxxx xx111100 map

36. UNDERSTANDING ERLANG TERMS
FLOAT
▸ IEEE754
xxxxxxxx xxxxxxxx xxxxxxxx xx011000
0.0
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_01011000 float
00000000_00000000_00000000_00000000_00000000_00000000_00000000_00000000
1.0
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_01011000 float
00111111_11110000_00000000_00000000_00000000_00000000_00000000_00000000
-1.0
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_01011000 float
10111111_11110000_00000000_00000000_00000000_00000000_00000000_00000000

37. UNDERSTANDING ERLANG TERMS
BOXED HEADER
xxxxxxxx xxxxxxxx xxxxxxxx xxxxxx00 header
xxxxxxxx xxxxxxxx xxxxxxxx xx000000 tuple
xxxxxxxx xxxxxxxx xxxxxxxx xx000100 binary match context
xxxxxxxx xxxxxxxx xxxxxxxx xx001000 pos big int
xxxxxxxx xxxxxxxx xxxxxxxx xx001100 neg big int
xxxxxxxx xxxxxxxx xxxxxxxx xx010000 reference
xxxxxxxx xxxxxxxx xxxxxxxx xx010100 function
xxxxxxxx xxxxxxxx xxxxxxxx xx011000 float
xxxxxxxx xxxxxxxx xxxxxxxx xx011100 export
xxxxxxxx xxxxxxxx xxxxxxxx xx100000 refc binary
xxxxxxxx xxxxxxxx xxxxxxxx xx100100 heap binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101000 sub binary
xxxxxxxx xxxxxxxx xxxxxxxx xx101100 *not used
xxxxxxxx xxxxxxxx xxxxxxxx xx110000 external pid
xxxxxxxx xxxxxxxx xxxxxxxx xx110100 external port
xxxxxxxx xxxxxxxx xxxxxxxx xx111000 external reference
xxxxxxxx xxxxxxxx xxxxxxxx xx111100 map

38. UNDERSTANDING ERLANG TERMS
HEAP BINARY xxxxxxxx xxxxxxxx xxxxxxxx xx100100
<<>>
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_01100100 heap binary
00000000_00000000_00000000_00000000_00000000_00000000_00000000_00000000
“hello”
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_10100100 heap binary
00000000_00000000_00000000_00000000_00000000_00000000_00000000_00000101 size(5)
00000000_00000000_00000000_01101111_01101100_01101100_01100101_01101000
o l l e h
“hello world!”
00000000_00000000_00000000_00000000_10000110_10010011_10001000_00101010 boxed
00000000_00000000_00000000_00000000_00000000_00000000_00000000_11100100 heap binary
00000000_00000000_00000000_00000000_00000000_00000000_00000000_00001100 size(12)
01101111_01110111_00100000_01101111_01101100_01101100_01100101_01101000
o w s o l l e h
00000000_00000000_00000000_00000000_00100001_01100100_01101100_01110010
! d l r

39. UNDERSTANDING ERLANG TERMS
MYTH WALKTHROUGH
▸ Tail-recursive functions are much faster than body-
recursive functions

40. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE VS BODY-RECURSIVE
defmodule Foo do
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

41. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x0000000042fcae49 | [1,2,3,4,5]
| 0x0000000015ac1af0 | 0x0000000019ede830 | BEAM PC 'Elixir.Foo':run1/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

42. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x0000000042fcae59 | [2,3,4,5]
| 0x0000000015ac1af0 | 0xfffffffffffffffb | []
| 0x0000000015ac1ae8 | 0x000000000000001f | 1
| 0x0000000015ac1ae0 | 0x0000000019ede830 | BEAM PC 'Elixir.Foo':run1/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

43. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x0000000042fcae69 | [3,4,5]
| 0x0000000015ac1af0 | 0x0000000015ac13c9 | [2]
| 0x0000000015ac1ae8 | 0x000000000000002f | 2
| 0x0000000015ac1ae0 | 0x0000000019ede830 | BEAM PC 'Elixir.Foo':run1/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
| 0x0000000015ac13d0 | 0xfffffffffffffffb | []
| 0x0000000015ac13c8 | 0x000000000000002f | 2
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

44. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x0000000042fcae79 | [4,5]
| 0x0000000015ac1af0 | 0x0000000015ac13d9 | [3,2]
| 0x0000000015ac1ae8 | 0x000000000000003f | 3
| 0x0000000015ac1ae0 | 0x0000000019ede830 | BEAM PC 'Elixir.Foo':run1/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
| 0x0000000015ac13e0 | 0x0000000015ac13c9 | [2]
| 0x0000000015ac13d8 | 0x000000000000003f | 3
| 0x0000000015ac13d0 | 0xfffffffffffffffb | []
| 0x0000000015ac13c8 | 0x000000000000002f | 2
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

45. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x0000000042fcae89 | [5]
| 0x0000000015ac1af0 | 0x0000000015ac13e9 | [4,3,2]
| 0x0000000015ac1ae8 | 0x000000000000004f | 4
| 0x0000000015ac1ae0 | 0x0000000019ede830 | BEAM PC 'Elixir.Foo':run1/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
| 0x0000000015ac13f0 | 0x0000000015ac13d9 | [3,2]
| 0x0000000015ac13e8 | 0x000000000000004f | 4
| 0x0000000015ac13e0 | 0x0000000015ac13c9 | [2]
| 0x0000000015ac13d8 | 0x000000000000003f | 3
| 0x0000000015ac13d0 | 0xfffffffffffffffb | []
| 0x0000000015ac13c8 | 0x000000000000002f | 2
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

46. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0xfffffffffffffffb | []
| 0x0000000015ac1af0 | 0x0000000015ac13f9 | [5,4,3,2]
| 0x0000000015ac1ae8 | 0x000000000000005f | 5
| 0x0000000015ac1ae0 | 0x0000000019ede830 | BEAM PC 'Elixir.Foo':run1/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
| 0x0000000015ac1400 | 0x0000000015ac13e9 | [4,3,2]
| 0x0000000015ac13f8 | 0x000000000000005f | 5
| 0x0000000015ac13f0 | 0x0000000015ac13d9 | [3,2]
| 0x0000000015ac13e8 | 0x000000000000004f | 4
| 0x0000000015ac13e0 | 0x0000000015ac13c9 | [2]
| 0x0000000015ac13d8 | 0x000000000000003f | 3
| 0x0000000015ac13d0 | 0xfffffffffffffffb | []
| 0x0000000015ac13c8 | 0x000000000000002f | 2
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

47. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x0000000015ac1409 | [6,5,4,3,2]
| 0x0000000015ac1af0 | 0x0000000019ede830 | BEAM PC 'Elixir.Foo':run1/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
| 0x0000000015ac1410 | 0x0000000015ac13f9 | [5,4,3,2]
| 0x0000000015ac1408 | 0x000000000000006f | 6
| 0x0000000015ac1400 | 0x0000000015ac13e9 | [4,3,2]
| 0x0000000015ac13f8 | 0x000000000000005f | 5
| 0x0000000015ac13f0 | 0x0000000015ac13d9 | [3,2]
| 0x0000000015ac13e8 | 0x000000000000004f | 4
| 0x0000000015ac13e0 | 0x0000000015ac13c9 | [2]
| 0x0000000015ac13d8 | 0x000000000000003f | 3
| 0x0000000015ac13d0 | 0xfffffffffffffffb | []
| 0x0000000015ac13c8 | 0x000000000000002f | 2
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

48. UNDERSTANDING ERLANG TERMS
TAIL-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1b08 | 0x0000000015ac1459 | [2,3,4,5,6]
| 0x0000000015ac1b00 | 0x000000001525a148 | BEAM PC normal-process-exit
|--------------------|--------------------|
|--------------------|--------------------|
| 0x0000000015ac1460 | 0x0000000015ac1449 | [3,4,5,6]
| 0x0000000015ac1458 | 0x000000000000002f | 2
| 0x0000000015ac1450 | 0x0000000015ac1439 | [4,5,6]
| 0x0000000015ac1448 | 0x000000000000003f | 3
| 0x0000000015ac1440 | 0x0000000015ac1429 | [5,6]
| 0x0000000015ac1438 | 0x000000000000004f | 4
| 0x0000000015ac1430 | 0x0000000015ac1419 | [6]
| 0x0000000015ac1428 | 0x000000000000005f | 5
| 0x0000000015ac1420 | 0xfffffffffffffffb | []
| 0x0000000015ac1418 | 0x000000000000006f | 6
| 0x0000000015ac1410 | 0x0000000015ac13f9 | [5,4,3,2]
| 0x0000000015ac1408 | 0x000000000000006f | 6
| 0x0000000015ac1400 | 0x0000000015ac13e9 | [4,3,2]
| 0x0000000015ac13f8 | 0x000000000000005f | 5
| 0x0000000015ac13f0 | 0x0000000015ac13d9 | [3,2]
| 0x0000000015ac13e8 | 0x000000000000004f | 4
| 0x0000000015ac13e0 | 0x0000000015ac13c9 | [2]
| 0x0000000015ac13d8 | 0x000000000000003f | 3
| 0x0000000015ac13d0 | 0xfffffffffffffffb | []
| 0x0000000015ac13c8 | 0x000000000000002f | 2
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run1 do
add1_tail([1, 2, 3, 4, 5])
end
def add1_tail(list) do
do_add1_tail(list, [])
end
defp do_add1_tail([], acc) do
Enum.reverse(acc)
end
defp do_add1_tail([h | t], acc) do
do_add1_tail(t, [h + 1 | acc])
end
end

49. UNDERSTANDING ERLANG TERMS
BODY-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x0000000042fcae59 | [2,3,4,5]
| 0x0000000015ac1af0 | 0x000000000000001f | 1
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ae8 | 0x0000000019ede8b0 | BEAM PC 'Elixir.Foo':run2/0 + 0x5
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run2 do
add1_body([1, 2, 3, 4, 5])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

50. UNDERSTANDING ERLANG TERMS
BODY-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x000000000000002f | 2
| 0x0000000015ac1af0 | 0x0000000019ede8b0 | BEAM PC 'Elixir.Foo':run2/0 + 0x5
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ae8 | 0x0000000042fcae69 | [3,4,5]
| 0x0000000015ac1ae0 | 0x000000000000002f | 2
| 0x0000000015ac1ad8 | 0x0000000019ede000 | BEAM PC ‘Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run2 do
add1_body([1, 2, 3, 4, 5])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

51. UNDERSTANDING ERLANG TERMS
BODY-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x000000000000002f | 2
| 0x0000000015ac1af0 | 0x0000000019ede8b0 | BEAM PC 'Elixir.Foo':run2/0 + 0x5
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ae8 | 0x000000000000003f | 3
| 0x0000000015ac1ae0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ad8 | 0x0000000042fcae79 | [4,5]
| 0x0000000015ac1ad0 | 0x000000000000003f | 3
| 0x0000000015ac1ac8 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run2 do
add1_body([1, 2, 3, 4, 5])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

52. UNDERSTANDING ERLANG TERMS
BODY-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x000000000000002f | 2
| 0x0000000015ac1af0 | 0x0000000019ede8b0 | BEAM PC 'Elixir.Foo':run2/0 + 0x5
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ae8 | 0x000000000000003f | 3
| 0x0000000015ac1ae0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ad8 | 0x000000000000004f | 4
| 0x0000000015ac1ad0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ac8 | 0x0000000042fcae89 | [5]
| 0x0000000015ac1ac0 | 0x000000000000004f | 4
| 0x0000000015ac1ab8 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run2 do
add1_body([1, 2, 3, 4, 5])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

53. UNDERSTANDING ERLANG TERMS
BODY-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x000000000000002f | 2
| 0x0000000015ac1af0 | 0x0000000019ede8b0 | BEAM PC 'Elixir.Foo':run2/0 + 0x5
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ae8 | 0x000000000000003f | 3
| 0x0000000015ac1ae0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ad8 | 0x000000000000004f | 4
| 0x0000000015ac1ad0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ac8 | 0x000000000000005f | 5
| 0x0000000015ac1ac0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ab8 | 0xfffffffffffffffb | []
| 0x0000000015ac1ab0 | 0x000000000000005f | 5
| 0x0000000015ac1aa8 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run2 do
add1_body([1, 2, 3, 4, 5])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

54. UNDERSTANDING ERLANG TERMS
BODY-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1af8 | 0x000000000000002f | 2
| 0x0000000015ac1af0 | 0x0000000019ede8b0 | BEAM PC 'Elixir.Foo':run2/0 + 0x5
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ae8 | 0x000000000000003f | 3
| 0x0000000015ac1ae0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ad8 | 0x000000000000004f | 4
| 0x0000000015ac1ad0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ac8 | 0x000000000000005f | 5
| 0x0000000015ac1ac0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1ab8 | 0x000000000000006f | 6
| 0x0000000015ac1ab0 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1aa8 | 0x0000000019ede000 | BEAM PC 'Elixir.Foo':add1_body/1 + 0x12
|--------------------|--------------------|
|--------------------|--------------------|
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run2 do
add1_body([1, 2, 3, 4, 5])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

55. UNDERSTANDING ERLANG TERMS
BODY-RECURSIVE
| BEAM STACK |
| Address | Contents |
|--------------------|--------------------| BEAM ACTIVATION RECORD
| 0x0000000015ac1b08 | 0x0000000015ac1409 | [2,3,4,5,6]
| 0x0000000015ac1b00 | 0x000000001525a148 | BEAM PC normal-process-exit
|--------------------|--------------------|
|--------------------|--------------------|
| 0x0000000015ac1410 | 0x0000000015ac13f9 | [3,4,5,6]
| 0x0000000015ac1408 | 0x000000000000002f | 2
| 0x0000000015ac1400 | 0x0000000015ac13e9 | [4,5,6]
| 0x0000000015ac13f8 | 0x000000000000003f | 3
| 0x0000000015ac13f0 | 0x0000000015ac13d9 | [5,6]
| 0x0000000015ac13e8 | 0x000000000000004f | 4
| 0x0000000015ac13e0 | 0x0000000015ac13c9 | [6]
| 0x0000000015ac13d8 | 0x000000000000005f | 5
| 0x0000000015ac13d0 | 0xfffffffffffffffb | []
| 0x0000000015ac13c8 | 0x000000000000006f | 6
|--------------------|--------------------|
| Address | Contents |
| H E A P |
defmodule Foo do
def run2 do
add1_body([1, 2, 3, 4, 5])
end
def add1_body([]) do
[]
end
def add1_body([h | t]) do
[h + 1 | add1_body(t)]
end
end

56. UNDERSTANDING ERLANG TERMS
MYTH DEMYSTIFY
▸ Use the version that makes your code cleaner (hint: it is
usually the body-recursive version)
▸ A tail-recursive function that does not need to reverse the
list at the end is faster than a body-recursive function

57. UNDERSTANDING ERLANG TERMS
RESOURCES
▸ The Seven Myths of Erlang Performance
▸ http://erlang.org/doc/efficiency_guide/myths.html
▸ A Staged Tag Scheme for Erlang(2000)
▸ http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.142.2827
▸ The Erlang Type System and Tags(The BEAM Book)
▸ https://happi.github.io/theBeamBook/#CH-TypeSystem
▸ Data Types Memory Layout(BEAM-WISDOM)
▸ http://beam-wisdoms.clau.se/en/latest/indepth-memory-layout.html
▸ Erlang/OTP source code
▸ https://github.com/erlang/otp/blob/master/erts/emulator/beam/erl_term.h
▸ ETerm library
▸ https://github.com/sunboshan/eterm