<s>
Opa	B-Language
is	O
an	O
open-source	B-Application
programming	O
language	O
for	O
developing	O
scalable	B-Architecture
web	B-Application
applications	I-Application
.	O
</s>
<s>
It	O
can	O
be	O
used	O
for	O
both	O
client-side	O
and	O
server-side	B-Language
scripting	I-Language
,	O
where	O
complete	O
programs	O
are	O
written	O
in	O
Opa	B-Language
and	O
subsequently	O
compiled	B-Language
to	O
Node.js	B-Language
on	O
the	O
server	B-Application
and	O
JavaScript	B-Language
on	O
the	O
client	B-Protocol
,	O
with	O
the	O
compiler	B-Language
automating	O
all	O
communication	O
between	O
the	O
two	O
.	O
</s>
<s>
Opa	B-Language
implements	O
strong	O
,	O
static	O
typing	O
,	O
which	O
can	O
be	O
helpful	O
in	O
protecting	O
against	O
security	O
issues	O
such	O
as	O
SQL	B-Language
injections	I-Language
and	O
cross-site	O
scripting	O
attacks	O
.	O
</s>
<s>
in	O
June	O
2011	O
,	O
under	O
a	O
GNU	B-Operating_System
Affero	O
General	O
Public	O
License	O
.	O
</s>
<s>
Later	O
,	O
the	O
license	O
changed	O
to	O
the	B-License
MIT	I-License
license	I-License
for	O
the	O
framework	O
part	O
(	O
library	O
)	O
and	O
AGPL	O
for	O
the	O
compiler	B-Language
so	O
that	O
applications	O
written	O
in	O
Opa	B-Language
can	O
be	O
released	O
under	O
any	O
license	O
,	O
proprietary	O
or	O
open	O
source	O
.	O
</s>
<s>
Opa	B-Language
consists	O
of	O
a	O
web	B-Application
server	I-Application
,	O
a	O
database	O
and	O
distributed	O
execution	O
engine	O
.	O
</s>
<s>
Code	O
written	O
in	O
Opa	B-Language
is	O
compiled	B-Language
to	O
JavaScript	B-Language
using	O
Node.js	B-Language
on	O
the	O
server	B-Language
side	I-Language
and	O
to	O
JavaScript	B-Language
using	O
jQuery	B-Language
for	O
cross-browser	O
compatibility	O
on	O
the	O
client	B-Protocol
side	O
.	O
</s>
<s>
The	O
advantage	O
of	O
the	O
approach	O
compared	O
to	O
certain	O
Rich	B-Application
Internet	I-Application
Application	I-Application
(	O
RIA	O
)	O
platforms	O
is	O
that	O
users	O
are	O
not	O
required	O
to	O
install	O
a	O
plugin	B-Protocol
in	O
their	O
browser	O
.	O
</s>
<s>
Opa	B-Language
shares	O
motivations	O
with	O
web	B-Application
frameworks	I-Application
,	O
but	O
takes	O
a	O
different	O
approach	O
.	O
</s>
<s>
Its	O
designers	O
assert	O
that	O
this	O
helps	O
Opa	B-Language
to	O
avoid	O
many	O
security	O
issues	O
,	O
like	O
SQL	B-Language
injections	I-Language
or	O
cross-site	O
scripting	O
(	O
XSS	O
)	O
attacks	O
.	O
</s>
<s>
The	O
core	O
language	O
is	O
functional	B-Language
and	O
has	O
a	O
static	O
type	O
system	O
with	O
type	O
inference	O
.	O
</s>
<s>
Opa	B-Language
also	O
provides	O
sessions	O
which	O
encapsulate	O
an	O
imperative	B-Application
state	O
and	O
communicate	O
using	O
message	B-Architecture
passing	I-Architecture
,	O
similar	O
to	O
Erlang	B-Operating_System
processes	O
.	O
</s>
<s>
Opa	B-Language
provides	O
many	O
structures	O
or	O
functions	O
that	O
are	O
common	O
in	O
web	O
development	O
,	O
as	O
first-class	O
objects	O
,	O
</s>
<s>
for	O
instance	O
HTML	B-Language
and	O
parsers	B-Language
,	O
based	O
on	O
Parsing	O
Expression	O
Grammars	O
.	O
</s>
<s>
Because	O
of	O
this	O
adhesion	O
between	O
the	O
language	O
and	O
web-related	O
concepts	O
,	O
Opa	B-Language
is	O
not	O
intended	O
for	O
non-web	O
applications	O
(	O
for	O
instance	O
desktop	B-Application
applications	I-Application
)	O
.	O
</s>
<s>
The	O
0.9.0	O
release	O
in	O
February	O
2012	O
introduced	O
database	O
mapping	O
technology	O
for	O
the	O
non-relational	O
,	O
document-oriented	O
database	O
MongoDB	B-Application
,	O
similar	O
to	O
object-relational	B-General_Concept
mapping	I-General_Concept
.	O
</s>
<s>
The	O
1.1.0	O
release	O
in	O
February	O
2013	O
also	O
added	O
support	O
for	O
PostgreSQL	B-Application
,	O
paving	O
the	O
way	O
for	O
the	O
support	O
of	O
several	O
SQL	B-Language
databases	I-Language
.	O
</s>
<s>
The	O
traditional	O
Hello	O
world	O
program	O
,	O
producing	O
a	O
web	B-Application
server	I-Application
that	O
serves	O
a	O
static	O
page	O
with	O
"	O
Hello	O
,	O
web	O
!	O
"	O
</s>
<s>
as	O
its	O
content	O
,	O
can	O
be	O
written	O
in	O
Opa	B-Language
as	O
:	O
</s>
<s>
It	O
can	O
be	O
compiled	B-Language
to	O
a	O
stand-alone	O
executable	O
JS	O
file	O
with	O
:	O
</s>
<s>
Running	O
the	O
resulting	O
executable	O
JS	O
file	O
launches	O
the	O
web	B-Application
application	I-Application
:	O
</s>
