<s>
In	O
computing	O
,	O
reactive	B-Architecture
programming	I-Architecture
is	O
a	O
declarative	B-Language
programming	I-Language
paradigm	O
concerned	O
with	O
data	B-Architecture
streams	I-Architecture
and	O
the	O
propagation	O
of	O
change	O
.	O
</s>
<s>
With	O
this	O
paradigm	O
,	O
it	O
's	O
possible	O
to	O
express	O
static	O
(	O
e.g.	O
,	O
arrays	O
)	O
or	O
dynamic	O
(	O
e.g.	O
,	O
event	O
emitters	O
)	O
data	B-Architecture
streams	I-Architecture
with	O
ease	O
,	O
and	O
also	O
communicate	O
that	O
an	O
inferred	O
dependency	O
within	O
the	O
associated	O
execution	O
model	O
exists	O
,	O
which	O
facilitates	O
the	O
automatic	O
propagation	O
of	O
the	O
changed	O
data	O
flow	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
an	O
imperative	B-Application
programming	I-Application
setting	O
,	O
a	O
:=	O
b	O
+	O
c	O
would	O
mean	O
that	O
a	O
is	O
being	O
assigned	O
the	O
result	O
of	O
b	O
+	O
c	O
in	O
the	O
instant	O
the	O
expression	O
is	O
evaluated	O
,	O
and	O
later	O
,	O
the	O
values	O
of	O
b	O
and	O
c	O
can	O
be	O
changed	O
with	O
no	O
effect	O
on	O
the	O
value	O
of	O
a	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
in	O
reactive	B-Architecture
programming	I-Architecture
,	O
the	O
value	O
of	O
a	O
is	O
automatically	O
updated	O
whenever	O
the	O
values	O
of	O
b	O
or	O
c	O
change	O
,	O
without	O
the	O
program	O
having	O
to	O
explicitly	O
re-execute	O
the	O
statement	O
a	O
:=	O
b	O
+	O
c	O
to	O
determine	O
the	O
presently	O
assigned	O
value	O
of	O
a	O
.	O
</s>
<s>
Another	O
example	O
is	O
a	O
hardware	O
description	O
language	O
such	O
as	O
Verilog	B-Language
,	O
where	O
reactive	B-Architecture
programming	I-Architecture
enables	O
changes	O
to	O
be	O
modeled	O
as	O
they	O
propagate	O
through	O
circuits	O
.	O
</s>
<s>
Reactive	B-Architecture
programming	I-Architecture
has	O
been	O
proposed	O
as	O
a	O
way	O
to	O
simplify	O
the	O
creation	O
of	O
interactive	O
user	O
interfaces	O
and	O
near-real-time	O
system	O
animation	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
a	O
model	O
–	O
view	O
–	O
controller	O
(	O
MVC	O
)	O
architecture	O
,	O
reactive	B-Architecture
programming	I-Architecture
can	O
facilitate	O
changes	O
in	O
an	O
underlying	O
model	O
that	O
are	O
reflected	O
automatically	O
in	O
an	O
associated	O
view	O
.	O
</s>
<s>
Several	O
popular	O
approaches	O
are	O
employed	O
in	O
the	O
creation	O
of	O
reactive	B-Architecture
programming	I-Architecture
languages	O
.	O
</s>
<s>
Other	O
approaches	O
are	O
articulated	O
in	O
the	O
definition	O
,	O
and	O
use	O
of	O
programming	O
libraries	O
,	O
or	O
embedded	O
domain-specific	B-Language
languages	I-Language
,	O
that	O
enable	O
reactivity	O
alongside	O
or	O
on	O
top	O
of	O
the	O
programming	O
language	O
.	O
</s>
<s>
A	O
variety	O
of	O
models	O
and	O
semantics	O
govern	O
reactive	B-Architecture
programming	I-Architecture
.	O
</s>
<s>
Reactive	B-Architecture
programming	I-Architecture
language	O
runtimes	O
are	O
represented	O
by	O
a	O
graph	O
that	O
identifies	O
the	O
dependencies	O
among	O
the	O
involved	O
reactive	O
values	O
.	O
</s>
<s>
The	O
graph	O
of	O
dependencies	O
are	O
maintained	O
implicitly	O
within	O
an	O
event	B-General_Concept
loop	I-General_Concept
.	O
</s>
<s>
This	O
facilitates	O
an	O
addressing	O
of	O
the	O
callback	O
's	O
control	O
inversion	O
in	O
two	O
ways	O
:	O
either	O
a	O
graph	O
is	O
specified	O
explicitly	O
(	O
typically	O
using	O
a	O
domain-specific	B-Language
language	I-Language
(	O
DSL	O
)	O
,	O
which	O
may	O
be	O
embedded	O
)	O
,	O
or	O
a	O
graph	O
is	O
implicitly	O
defined	O
with	O
expression	O
and	O
generation	O
using	O
an	O
effective	O
,	O
archetypal	O
language	O
.	O
</s>
<s>
This	O
is	O
usually	O
achieved	O
by	O
topologically	B-Algorithm
sorting	I-Algorithm
expressions	O
and	O
updating	O
values	O
in	O
topological	O
order	O
.	O
</s>
<s>
Topological	B-Algorithm
sorting	I-Algorithm
of	O
dependencies	O
depends	O
on	O
the	O
dependency	O
graph	O
being	O
a	O
directed	O
acyclic	O
graph	O
(	O
DAG	O
)	O
.	O
</s>
<s>
Usually	O
,	O
reactive	B-Architecture
programming	I-Architecture
languages	O
expect	O
such	O
cycles	O
to	O
be	O
"	O
broken	O
"	O
by	O
placing	O
some	O
element	O
along	O
a	O
"	O
back	O
edge	O
"	O
to	O
permit	O
reactive	O
updating	O
to	O
terminate	O
.	O
</s>
<s>
Reactive	O
languages	O
typically	O
assume	O
that	O
their	O
expressions	O
are	O
purely	B-Application
functional	I-Application
.	O
</s>
<s>
Properly	O
encapsulated	O
object-oriented	B-Language
libraries	O
offer	O
an	O
encapsulated	O
notion	O
of	O
state	O
.	O
</s>
<s>
For	O
instance	O
,	O
callbacks	O
can	O
be	O
installed	O
in	O
the	O
getters	O
of	O
the	O
object-oriented	B-Language
library	O
to	O
notify	O
the	O
reactive	O
update	O
engine	O
about	O
state	O
changes	O
,	O
and	O
changes	O
in	O
the	O
reactive	O
component	O
can	O
be	O
pushed	O
to	O
the	O
object-oriented	B-Language
library	O
through	O
getters	O
.	O
</s>
<s>
Permitting	O
dynamic	O
updating	O
of	O
dependencies	O
provides	O
significant	O
expressive	O
power	O
(	O
for	O
instance	O
,	O
dynamic	O
dependencies	O
routinely	O
occur	O
in	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
programs	O
)	O
.	O
</s>
<s>
Reactive	B-Architecture
programming	I-Architecture
languages	O
can	O
range	O
from	O
very	O
explicit	O
ones	O
where	O
data	O
flows	O
are	O
set	O
up	O
by	O
using	O
arrows	O
,	O
to	O
implicit	O
where	O
the	O
data	O
flows	O
are	O
derived	O
from	O
language	O
constructs	O
that	O
look	O
similar	O
to	O
those	O
of	O
imperative	B-Application
or	O
functional	B-Language
programming	I-Language
.	O
</s>
<s>
For	O
example	O
,	O
in	O
implicitly	O
lifted	O
functional	B-Application
reactive	I-Application
programming	I-Application
(	O
FRP	O
)	O
a	O
function	O
call	O
might	O
implicitly	O
cause	O
a	O
node	O
in	O
a	O
data	O
flow	O
graph	O
to	O
be	O
constructed	O
.	O
</s>
<s>
Reactive	B-Architecture
programming	I-Architecture
libraries	O
for	O
dynamic	O
languages	O
(	O
such	O
as	O
the	O
Lisp	O
"	O
Cells	O
"	O
and	O
Python	O
"	O
Trellis	O
"	O
libraries	O
)	O
can	O
construct	O
a	O
dependency	O
graph	O
from	O
runtime	O
analysis	O
of	O
the	O
values	O
read	O
during	O
a	O
function	O
's	O
execution	O
,	O
allowing	O
data	O
flow	O
specifications	O
to	O
be	O
both	O
implicit	O
and	O
dynamic	O
.	O
</s>
<s>
Sometimes	O
the	O
term	O
reactive	B-Architecture
programming	I-Architecture
refers	O
to	O
the	O
architectural	O
level	O
of	O
software	O
engineering	O
,	O
where	O
individual	O
nodes	O
in	O
the	O
data	O
flow	O
graph	O
are	O
ordinary	O
programs	O
that	O
communicate	O
with	O
each	O
other	O
.	O
</s>
<s>
Reactive	B-Architecture
programming	I-Architecture
can	O
be	O
purely	O
static	O
where	O
the	O
data	O
flows	O
are	O
set	O
up	O
statically	O
,	O
or	O
be	O
dynamic	O
where	O
the	O
data	O
flows	O
can	O
change	O
during	O
the	O
execution	O
of	O
a	O
program	O
.	O
</s>
<s>
True	O
dynamic	O
reactive	B-Architecture
programming	I-Architecture
however	O
could	O
use	O
imperative	B-Application
programming	I-Application
to	O
reconstruct	O
the	O
data	O
flow	O
graph	O
.	O
</s>
<s>
Reactive	B-Architecture
programming	I-Architecture
could	O
be	O
said	O
to	O
be	O
of	O
higher	O
order	O
if	O
it	O
supports	O
the	O
idea	O
that	O
data	O
flows	O
could	O
be	O
used	O
to	O
construct	O
other	O
data	O
flows	O
.	O
</s>
<s>
This	O
can	O
be	O
called	O
differentiated	O
reactive	B-Architecture
programming	I-Architecture
.	O
</s>
<s>
Here	O
differentiated	O
reactive	B-Architecture
programming	I-Architecture
could	O
potentially	O
be	O
used	O
to	O
give	O
the	O
spell	O
checker	O
lower	O
priority	O
,	O
allowing	O
it	O
to	O
be	O
delayed	O
while	O
keeping	O
other	O
data-flows	O
instantaneous	O
.	O
</s>
<s>
One	O
inherent	O
problem	O
for	O
reactive	B-Architecture
programming	I-Architecture
is	O
that	O
most	O
computations	O
that	O
would	O
be	O
evaluated	O
and	O
forgotten	O
in	O
a	O
normal	O
programming	O
language	O
,	O
needs	O
to	O
be	O
represented	O
in	O
the	O
memory	O
as	O
data-structures	O
.	O
</s>
<s>
This	O
could	O
potentially	O
make	O
reactive	B-Architecture
programming	I-Architecture
highly	O
memory	O
consuming	O
.	O
</s>
<s>
On	O
the	O
other	O
side	O
,	O
reactive	B-Architecture
programming	I-Architecture
is	O
a	O
form	O
of	O
what	O
could	O
be	O
described	O
as	O
"	O
explicit	O
parallelism	O
"	O
,	O
and	O
could	O
therefore	O
be	O
beneficial	O
for	O
utilizing	O
the	O
power	O
of	O
parallel	O
hardware	O
.	O
</s>
<s>
Reactive	B-Architecture
programming	I-Architecture
has	O
principal	O
similarities	O
with	O
the	O
observer	B-Language
pattern	I-Language
commonly	O
used	O
in	O
object-oriented	B-Language
programming	I-Language
.	O
</s>
<s>
For	O
example	O
,	O
the	O
observer	B-Language
pattern	I-Language
commonly	O
describes	O
data-flows	O
between	O
whole	O
objects/classes	O
,	O
whereas	O
object-oriented	B-Language
reactive	B-Architecture
programming	I-Architecture
could	O
target	O
the	O
members	O
of	O
objects/classes	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
fuse	O
reactive	B-Architecture
programming	I-Architecture
with	O
ordinary	O
imperative	B-Application
programming	I-Application
.	O
</s>
<s>
In	O
such	O
a	O
paradigm	O
,	O
imperative	B-Application
programs	I-Application
operate	O
upon	O
reactive	O
data	O
structures	O
.	O
</s>
<s>
Such	O
a	O
set-up	O
is	O
analogous	O
to	O
imperative	B-Application
constraint	O
programming	O
;	O
however	O
,	O
while	O
imperative	B-Application
constraint	O
programming	O
manages	O
bidirectional	O
data-flow	O
constraints	O
,	O
imperative	B-Application
reactive	B-Architecture
programming	I-Architecture
manages	O
one-way	O
data-flow	O
constraints	O
.	O
</s>
<s>
Object-oriented	B-Language
reactive	B-Architecture
programming	I-Architecture
(	O
OORP	O
)	O
is	O
a	O
combination	O
of	O
object	B-Language
oriented	I-Language
programming	I-Language
and	O
reactive	B-Architecture
programming	I-Architecture
.	O
</s>
<s>
If	O
an	O
OORP	O
language	O
maintains	O
its	O
imperative	B-Application
methods	O
,	O
it	O
would	O
also	O
fall	O
under	O
the	O
category	O
of	O
imperative	B-Application
reactive	B-Architecture
programming	I-Architecture
.	O
</s>
<s>
Functional	B-Application
reactive	I-Application
programming	I-Application
(	O
FRP	O
)	O
is	O
a	O
programming	O
paradigm	O
for	O
reactive	B-Architecture
programming	I-Architecture
on	O
functional	B-Language
programming	I-Language
.	O
</s>
<s>
Actors	O
have	O
been	O
proposed	O
to	O
design	O
reactive	O
systems	O
,	O
often	O
in	O
combination	O
with	O
Functional	B-Application
Reactive	I-Application
Programming	I-Application
(	O
FRP	O
)	O
and	O
Reactive	B-Language
Streams	I-Language
to	O
develop	O
distributed	O
reactive	O
systems	O
.	O
</s>
<s>
An	O
example	O
of	O
a	O
rule	O
based	O
reactive	B-Architecture
programming	I-Architecture
language	O
is	O
Ampersand	O
,	O
which	O
is	O
founded	O
in	O
relation	O
algebra	O
.	O
</s>
<s>
ReactiveX	B-Application
,	O
an	O
API	O
for	O
implementing	O
reactive	B-Architecture
programming	I-Architecture
with	O
streams	O
,	O
observables	O
and	O
operators	O
with	O
multiple	O
language	O
implementations	O
including	O
RxJs	O
,	O
RxJava	O
,	O
Rx.NET	O
,	O
RxPy	O
and	O
RxSwift	O
.	O
</s>
<s>
Elm	B-Language
(	O
programming	O
language	O
)	O
Reactive	O
composition	O
of	O
web	O
user	O
interfaces	O
.	O
</s>
<s>
Svelte	B-Language
,	O
brings	O
reactivity	O
in	O
the	O
form	O
of	O
a	O
variant	O
JavaScript	B-Language
syntax	I-Language
that	O
looks	O
like	O
JavaScript	B-Language
but	O
is	O
naturally	O
reactive	O
where	O
JavaScript	B-Language
normally	O
is	O
n't	O
.	O
</s>
<s>
,	O
brings	O
reactivity	O
to	O
JavaScript	B-Language
without	O
changing	O
JavaScript	B-Language
syntax	I-Language
semantics	O
,	O
along	O
with	O
reactive	O
JSX	B-Language
templating	O
.	O
</s>
