<s>
Hack	B-Language
is	O
a	O
programming	O
language	O
for	O
the	O
HipHop	B-Language
Virtual	I-Language
Machine	I-Language
(	O
HHVM	B-Language
)	O
,	O
created	O
by	O
Facebook	B-Application
as	O
a	O
dialect	O
of	O
PHP	B-Application
.	O
</s>
<s>
The	O
language	O
implementation	O
is	O
open-source	B-Application
,	O
licensed	O
under	O
the	B-License
MIT	I-License
License	I-License
.	O
</s>
<s>
Hack	B-Language
allows	O
programmers	O
to	O
use	O
both	O
dynamic	O
typing	O
and	O
static	O
typing	O
.	O
</s>
<s>
This	O
kind	O
of	O
a	O
type	O
system	O
is	O
called	O
gradual	O
typing	O
,	O
which	O
is	O
also	O
implemented	O
in	O
other	O
programming	O
languages	O
such	O
as	O
ActionScript	B-Operating_System
.	O
</s>
<s>
Hack	B-Language
's	O
type	O
system	O
allows	O
types	O
to	O
be	O
specified	O
for	O
function	O
arguments	O
,	O
function	O
return	B-Language
values	I-Language
,	O
and	O
class	O
properties	O
;	O
however	O
,	O
types	O
of	O
local	O
variables	O
are	O
always	O
inferred	O
and	O
cannot	O
be	O
specified	O
.	O
</s>
<s>
Hack	B-Language
was	O
introduced	O
on	O
March	O
20	O
,	O
2014	O
.	O
</s>
<s>
Before	O
the	O
announcement	O
of	O
the	O
new	O
programming	O
language	O
,	O
Facebook	B-Application
had	O
already	O
implemented	O
the	O
code	O
and	O
tested	O
it	O
on	O
a	O
large	O
portion	O
of	O
its	O
web	O
site	O
.	O
</s>
<s>
Hack	B-Language
is	O
designed	O
to	O
interoperate	O
seamlessly	O
with	O
PHP	B-Application
,	O
which	O
is	O
a	O
widely	O
used	O
open-source	B-Application
scripting	B-Language
language	I-Language
that	O
has	O
a	O
focus	O
on	O
web	O
development	O
and	O
can	O
be	O
embedded	O
into	O
HTML	B-Language
.	O
</s>
<s>
A	O
majority	O
of	O
valid	O
PHP	B-Application
scripts	I-Application
are	O
also	O
valid	O
in	O
Hack	B-Language
;	O
however	O
,	O
numerous	O
less	O
frequently	O
used	O
PHP	B-Application
features	O
and	O
language	O
constructs	O
are	O
not	O
supported	O
in	O
Hack	B-Language
.	O
</s>
<s>
Hack	B-Language
extends	O
the	O
type	O
hinting	O
available	O
in	O
PHP5	O
through	O
the	O
introduction	O
of	O
static	O
typing	O
,	O
by	O
adding	O
new	O
type	O
hints	O
(	O
for	O
example	O
,	O
for	O
scalar	O
types	O
such	O
as	O
integer	O
or	O
string	O
)	O
,	O
as	O
well	O
as	O
by	O
extending	O
the	O
use	O
of	O
type	O
hints	O
(	O
for	O
example	O
,	O
for	O
class	O
properties	O
or	O
function	O
return	B-Language
values	I-Language
)	O
.	O
</s>
<s>
Since	O
Hack	B-Language
uses	O
a	O
gradual	O
typing	O
system	O
,	O
in	O
the	O
default	O
mode	O
,	O
type	O
annotations	O
are	O
not	O
mandatory	O
even	O
in	O
places	O
they	O
cannot	O
be	O
inferred	O
;	O
the	O
type	O
system	O
will	O
assume	O
the	O
author	O
is	O
correct	O
and	O
admit	O
the	O
code	O
.	O
</s>
<s>
The	O
basic	O
file	O
structure	O
of	O
a	O
Hack	B-Language
script	O
is	O
similar	O
to	O
a	O
PHP	B-Application
script	I-Application
with	O
a	O
few	O
changes	O
.	O
</s>
<s>
A	O
Hack	B-Language
file	O
does	O
not	O
include	O
the	O
<	O
?	O
php	B-Application
opening	O
markup	O
tag	O
and	O
forbids	O
using	O
top-level	O
declarations	O
.	O
</s>
<s>
Like	O
other	O
functions	O
in	O
Hack	B-Language
,	O
the	O
function	O
names	O
must	O
be	O
unique	O
within	O
a	O
project	O
–	O
i.e.	O
</s>
<s>
The	O
above	O
script	O
,	O
similar	O
to	O
PHP	B-Application
,	O
will	O
be	O
executed	O
and	O
the	O
following	O
output	O
is	O
sent	O
to	O
the	O
browser	O
:	O
</s>
<s>
Unlike	O
PHP	B-Application
,	O
Hack	B-Language
and	O
HTML	B-Language
code	I-Language
do	O
not	O
mix	O
;	O
either	O
XHP	B-Language
or	O
another	O
template	B-Application
engine	I-Application
needs	O
to	O
be	O
used	O
.	O
</s>
<s>
Like	O
PHP	B-Application
7	O
,	O
Hack	B-Language
allows	O
types	O
to	O
be	O
specified	O
for	O
function	O
arguments	O
and	O
function	O
return	B-Language
values	I-Language
.	O
</s>
<s>
Functions	O
in	O
Hack	B-Language
are	O
thus	O
annotated	O
with	O
types	O
like	O
the	O
following	O
:	O
</s>
