<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
(	O
or	O
)	O
is	O
a	O
fundamental	O
function	O
in	O
most	O
dialects	O
of	O
the	O
Lisp	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
These	O
objects	O
are	O
referred	O
to	O
as	O
(	O
cons	B-Language
)	O
cells	O
,	O
conses	O
,	O
non-atomic	O
s-expressions	B-Protocol
(	O
"	O
NATSes	O
"	O
)	O
,	O
or	O
(	O
cons	B-Language
)	O
pairs	O
.	O
</s>
<s>
In	O
Lisp	B-Language
jargon	O
,	O
the	O
expression	O
"	O
to	O
cons	B-Language
x	O
onto	O
y	O
"	O
means	O
to	O
construct	O
a	O
new	O
object	O
with	O
(	O
cons	B-Language
x	O
y	O
)	O
.	O
</s>
<s>
It	O
is	O
loosely	O
related	O
to	O
the	O
object-oriented	B-Language
notion	O
of	O
a	O
constructor	O
,	O
which	O
creates	O
a	O
new	O
object	O
given	O
arguments	O
,	O
and	O
more	O
closely	O
related	O
to	O
the	O
constructor	O
function	O
of	O
an	O
algebraic	O
data	O
type	O
system	O
.	O
</s>
<s>
The	O
word	O
"	O
cons	B-Language
"	O
and	O
expressions	O
like	O
"	O
to	O
cons	B-Language
onto	O
"	O
are	O
also	O
part	O
of	O
a	O
more	O
general	O
functional	B-Language
programming	I-Language
jargon	O
.	O
</s>
<s>
Sometimes	O
operators	O
that	O
have	O
a	O
similar	O
purpose	O
,	O
especially	O
in	O
the	O
context	O
of	O
list	B-Language
processing	I-Language
,	O
are	O
pronounced	O
"	O
cons	B-Language
"	O
.	O
</s>
<s>
(	O
A	O
good	O
example	O
is	O
the	O
::	O
operator	O
in	O
ML	B-Language
,	O
Scala	B-Application
,	O
F#	B-Operating_System
,	O
Lean	B-Language
and	O
Elm	B-Language
or	O
the	O
:	O
operator	O
in	O
Haskell	B-Language
,	O
which	O
adds	O
an	O
element	O
to	O
the	O
beginning	O
of	O
a	O
list	B-General_Concept
.	O
)	O
</s>
<s>
Although	O
cons	B-Language
cells	I-Language
can	O
be	O
used	O
to	O
hold	O
ordered	O
pairs	O
of	O
data	O
,	O
they	O
are	O
more	O
commonly	O
used	O
to	O
construct	O
more	O
complex	O
compound	O
data	O
structures	O
,	O
notably	O
lists	B-Data_Structure
and	O
binary	O
trees	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Lisp	B-Language
expression	O
constructs	O
a	O
cell	O
holding	O
1	O
in	O
its	O
left	O
half	O
(	O
the	O
so-called	O
field	O
)	O
and	O
2	O
in	O
its	O
right	O
half	O
(	O
the	O
field	O
)	O
.	O
</s>
<s>
In	O
Lisp	B-Language
notation	O
,	O
the	O
value	O
looks	O
like	O
:	O
</s>
<s>
Note	O
the	O
dot	O
between	O
1	O
and	O
2	O
;	O
this	O
indicates	O
that	O
the	O
S-expression	B-Protocol
is	O
a	O
"	O
dotted	O
pair	O
"	O
(	O
a	O
so-called	O
"	O
cons	B-Language
pair	I-Language
"	O
)	O
,	O
rather	O
than	O
a	O
"	O
list.	O
"	O
</s>
<s>
In	O
Lisp	B-Language
,	O
lists	B-Data_Structure
are	O
implemented	O
on	O
top	O
of	O
cons	B-Language
pairs	I-Language
.	O
</s>
<s>
More	O
specifically	O
,	O
any	O
list	B-General_Concept
structure	O
in	O
Lisp	B-Language
is	O
either	O
:	O
</s>
<s>
An	O
empty	O
list	B-General_Concept
,	O
which	O
is	O
a	O
special	O
object	O
usually	O
called	O
.	O
</s>
<s>
A	O
cons	B-Language
cell	I-Language
whose	O
is	O
the	O
first	O
element	O
of	O
the	O
list	B-General_Concept
and	O
whose	O
is	O
a	O
list	B-General_Concept
containing	O
the	O
rest	O
of	O
the	O
elements	O
.	O
</s>
<s>
This	O
forms	O
the	O
basis	O
of	O
a	O
simple	O
,	O
singly	B-Data_Structure
linked	I-Data_Structure
list	I-Data_Structure
structure	O
whose	O
contents	O
can	O
be	O
manipulated	O
with	O
,	O
,	O
and	O
.	O
</s>
<s>
Note	O
that	O
is	O
the	O
only	O
list	B-General_Concept
that	O
is	O
not	O
also	O
a	O
cons	B-Language
pair	I-Language
.	O
</s>
<s>
As	O
an	O
example	O
,	O
consider	O
a	O
list	B-General_Concept
whose	O
elements	O
are	O
1	O
,	O
2	O
,	O
and	O
3	O
.	O
</s>
<s>
Such	O
a	O
list	B-General_Concept
can	O
be	O
created	O
in	O
three	O
steps	O
:	O
</s>
<s>
The	O
resulting	O
value	O
is	O
the	O
list	B-General_Concept
:	O
</s>
<s>
Thus	O
,	O
can	O
be	O
used	O
to	O
add	O
one	O
element	O
to	O
the	O
front	O
of	O
an	O
existing	O
linked	B-Data_Structure
list	I-Data_Structure
.	O
</s>
<s>
For	O
example	O
,	O
if	O
x	O
is	O
the	O
list	B-General_Concept
we	O
defined	O
above	O
,	O
then	O
will	O
produce	O
the	O
list	B-General_Concept
:	O
</s>
<s>
Another	O
useful	O
list	B-General_Concept
procedure	O
is	O
,	O
which	O
concatenates	O
two	O
existing	O
lists	B-Data_Structure
(	O
i.e.	O
</s>
<s>
combines	O
two	O
lists	B-Data_Structure
into	O
a	O
single	O
list	B-General_Concept
)	O
.	O
</s>
<s>
Binary	O
trees	O
that	O
only	O
store	O
data	O
in	O
their	O
leaves	B-Data_Structure
are	O
also	O
easily	O
constructed	O
with	O
.	O
</s>
<s>
Technically	O
,	O
the	O
list	B-General_Concept
(	O
1	O
2	O
3	O
)	O
in	O
the	O
previous	O
example	O
is	O
also	O
a	O
binary	O
tree	O
,	O
one	O
which	O
happens	O
to	O
be	O
particularly	O
unbalanced	O
.	O
</s>
<s>
Cons	B-Language
can	O
refer	O
to	O
the	O
general	O
process	O
of	O
memory	B-General_Concept
allocation	I-General_Concept
,	O
as	O
opposed	O
to	O
using	O
destructive	O
operations	O
of	O
the	O
kind	O
that	O
would	O
be	O
used	O
in	O
an	O
imperative	O
programming	O
language	O
.	O
</s>
<s>
I	O
sped	O
up	O
the	O
code	O
a	O
bit	O
by	O
putting	O
in	O
side	O
effects	O
instead	O
of	O
having	O
it	O
cons	B-Language
ridiculously	O
.	O
</s>
<s>
Since	O
Lisp	B-Language
has	O
first-class	B-Application
functions	I-Application
,	O
all	O
data	O
structures	O
,	O
including	O
cons	B-Language
cells	I-Language
,	O
can	O
be	O
implemented	O
using	O
functions	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
Scheme	B-Language
:	O
</s>
<s>
It	O
re-implements	O
the	O
cons	B-Language
,	O
car	O
,	O
and	O
cdr	O
operations	O
,	O
using	O
a	O
function	O
as	O
the	O
"	O
cons	B-Language
cell	I-Language
"	O
.	O
</s>
<s>
Church	O
encoding	O
is	O
a	O
usual	O
way	O
of	O
defining	O
data	O
structures	O
in	O
pure	O
lambda	B-Language
calculus	I-Language
,	O
an	O
abstract	O
,	O
theoretical	O
model	O
of	O
computation	O
that	O
is	O
closely	O
related	O
to	O
Scheme	B-Language
.	O
</s>
<s>
This	O
implementation	O
,	O
while	O
academically	O
interesting	O
,	O
is	O
impractical	O
because	O
it	O
renders	O
cons	B-Language
cells	I-Language
indistinguishable	O
from	O
any	O
other	O
Scheme	B-Language
procedure	O
,	O
as	O
well	O
as	O
introduces	O
unnecessary	O
computational	O
inefficiencies	O
.	O
</s>
<s>
This	O
encoding	O
also	O
has	O
the	O
advantage	O
of	O
being	O
implementable	O
in	O
a	O
statically	O
typed	O
language	O
that	O
does	O
n't	O
have	O
variants	O
,	O
such	O
as	O
Java	B-Language
,	O
using	O
interfaces	O
instead	O
of	O
lambdas	O
.	O
</s>
