<s>
Lisp	B-Language
(	O
historically	O
LISP	B-Language
,	O
an	O
acronym	O
for	O
list	B-Language
processing	I-Language
)	O
is	O
a	O
family	O
of	O
programming	O
languages	O
with	O
a	O
long	O
history	O
and	O
a	O
distinctive	O
,	O
fully	O
parenthesized	O
prefix	O
notation	O
.	O
</s>
<s>
Originally	O
specified	O
in	O
1960	O
,	O
Lisp	B-Language
is	O
the	O
second-oldest	O
high-level	B-Language
programming	I-Language
language	I-Language
still	O
in	O
common	O
use	O
,	O
after	O
Fortran	B-Application
.	O
</s>
<s>
Lisp	B-Language
has	O
changed	O
since	O
its	O
early	O
days	O
,	O
and	O
many	O
dialects	O
have	O
existed	O
over	O
its	O
history	O
.	O
</s>
<s>
Today	O
,	O
the	O
best-known	O
general-purpose	O
Lisp	B-Language
dialects	O
are	O
Common	B-Language
Lisp	I-Language
,	O
Scheme	B-Language
,	O
Racket	B-Operating_System
and	O
Clojure	B-Language
.	O
</s>
<s>
Lisp	B-Language
was	O
originally	O
created	O
as	O
a	O
practical	O
mathematical	O
notation	O
for	O
computer	B-Application
programs	I-Application
,	O
influenced	O
by	O
(	O
though	O
not	O
originally	O
derived	O
from	O
)	O
the	O
notation	O
of	O
Alonzo	O
Church	O
's	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
It	O
quickly	O
became	O
a	O
favored	O
programming	O
language	O
for	O
artificial	B-Application
intelligence	I-Application
(	O
AI	B-Application
)	O
research	O
.	O
</s>
<s>
As	O
one	O
of	O
the	O
earliest	O
programming	O
languages	O
,	O
Lisp	B-Language
pioneered	O
many	O
ideas	O
in	O
computer	B-General_Concept
science	I-General_Concept
,	O
including	O
tree	B-Application
data	I-Application
structures	I-Application
,	O
automatic	B-General_Concept
storage	I-General_Concept
management	I-General_Concept
,	O
dynamic	O
typing	O
,	O
conditionals	B-Language
,	O
higher-order	B-Language
functions	I-Language
,	O
recursion	O
,	O
the	O
self-hosting	O
compiler	B-Language
,	O
and	O
the	O
read	B-Application
–	I-Application
eval	I-Application
–	I-Application
print	I-Application
loop	I-Application
.	O
</s>
<s>
The	O
name	O
LISP	B-Language
derives	O
from	O
"	O
LISt	O
Processor	O
"	O
.	O
</s>
<s>
Linked	B-Data_Structure
lists	I-Data_Structure
are	O
one	O
of	O
Lisp	B-Language
's	O
major	O
data	B-General_Concept
structures	I-General_Concept
,	O
and	O
Lisp	B-Language
source	O
code	O
is	O
made	O
of	O
lists	O
.	O
</s>
<s>
Thus	O
,	O
Lisp	B-Language
programs	O
can	O
manipulate	O
source	O
code	O
as	O
a	O
data	B-General_Concept
structure	I-General_Concept
,	O
giving	O
rise	O
to	O
the	O
macro	O
systems	O
that	O
allow	O
programmers	O
to	O
create	O
new	O
syntax	O
or	O
new	O
domain-specific	B-Language
languages	I-Language
embedded	O
in	O
Lisp	B-Language
.	O
</s>
<s>
The	O
interchangeability	O
of	O
code	O
and	O
data	O
gives	O
Lisp	B-Language
its	O
instantly	O
recognizable	O
syntax	O
.	O
</s>
<s>
All	O
program	O
code	O
is	O
written	O
as	O
s-expressions	B-Protocol
,	O
or	O
parenthesized	O
lists	O
.	O
</s>
<s>
John	O
McCarthy	O
began	O
developing	O
Lisp	B-Language
in	O
1958	O
while	O
he	O
was	O
at	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
(	O
MIT	O
)	O
.	O
</s>
<s>
He	O
showed	O
that	O
with	O
a	O
few	O
simple	O
operators	O
and	O
a	O
notation	O
for	O
anonymous	B-General_Concept
functions	I-General_Concept
borrowed	O
from	O
Church	O
,	O
one	O
can	O
build	O
a	O
Turing-complete	B-Algorithm
language	O
for	O
algorithms	O
.	O
</s>
<s>
Information	B-Language
Processing	I-Language
Language	I-Language
was	O
the	O
first	O
AI	B-Application
language	O
,	O
from	O
1955	O
or	O
1956	O
,	O
and	O
already	O
included	O
many	O
of	O
the	O
concepts	O
,	O
such	O
as	O
list-processing	O
and	O
recursion	O
,	O
which	O
came	O
to	O
be	O
used	O
in	O
Lisp	B-Language
.	O
</s>
<s>
McCarthy	O
's	O
original	O
notation	O
used	O
bracketed	O
"	O
M-expressions	B-Language
"	O
that	O
would	O
be	O
translated	O
into	O
S-expressions	B-Protocol
.	O
</s>
<s>
As	O
an	O
example	O
,	O
the	O
M-expression	B-Language
is	O
equivalent	O
to	O
the	O
S-expression	B-Protocol
.	O
</s>
<s>
Once	O
Lisp	B-Language
was	O
implemented	O
,	O
programmers	O
rapidly	O
chose	O
to	O
use	O
S-expressions	B-Protocol
,	O
and	O
M-expressions	B-Language
were	O
abandoned	O
.	O
</s>
<s>
M-expressions	B-Language
surfaced	O
again	O
with	O
short-lived	O
attempts	O
of	O
MLisp	B-Language
by	O
Horace	O
Enea	O
and	O
CGOL	B-Language
by	O
Vaughan	O
Pratt	O
.	O
</s>
<s>
Lisp	B-Language
was	O
first	O
implemented	O
by	O
Steve	O
Russell	O
on	O
an	O
IBM	B-Device
704	I-Device
computer	O
using	O
punched	B-Architecture
cards	I-Architecture
.	O
</s>
<s>
Russell	O
had	O
read	O
McCarthy	O
's	O
paper	O
and	O
realized	O
(	O
to	O
McCarthy	O
's	O
surprise	O
)	O
that	O
the	O
Lisp	B-Language
eval	O
function	O
could	O
be	O
implemented	O
in	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
The	O
result	O
was	O
a	O
working	O
Lisp	B-Language
interpreter	B-Application
which	O
could	O
be	O
used	O
to	O
run	O
Lisp	B-Language
programs	O
,	O
or	O
more	O
properly	O
,	O
"	O
evaluate	O
Lisp	B-Language
expressions	O
"	O
.	O
</s>
<s>
Two	O
assembly	O
language	O
macros	O
for	O
the	O
IBM	B-Device
704	I-Device
became	O
the	O
primitive	O
operations	O
for	O
decomposing	O
lists	O
:	O
(	O
Contents	O
of	O
the	O
Address	O
part	O
of	O
Register	O
number	O
)	O
and	O
(	O
Contents	O
of	O
the	O
Decrement	O
part	O
of	O
Register	O
number	O
)	O
,	O
where	O
"	O
register	O
"	O
refers	O
to	O
registers	B-General_Concept
of	O
the	O
computer	O
's	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
.	O
</s>
<s>
Lisp	B-Language
dialects	O
still	O
use	O
and	O
(	O
and	O
)	O
for	O
the	O
operations	O
that	O
return	O
the	O
first	O
item	O
in	O
a	O
list	O
and	O
the	O
rest	O
of	O
the	O
list	O
,	O
respectively	O
.	O
</s>
<s>
The	O
first	O
complete	O
Lisp	B-Language
compiler	B-Language
,	O
written	O
in	O
Lisp	B-Language
,	O
was	O
implemented	O
in	O
1962	O
by	O
Tim	O
Hart	O
and	O
Mike	O
Levin	O
at	O
MIT	O
,	O
and	O
could	O
be	O
compiled	B-Language
by	O
simply	O
having	O
an	O
existing	O
LISP	B-Language
interpreter	B-Application
interpret	O
the	O
compiler	B-Language
code	O
,	O
producing	O
machine	B-Language
code	I-Language
output	O
able	O
to	O
be	O
executed	O
at	O
a	O
40-fold	O
improvement	O
in	O
speed	O
over	O
that	O
of	O
the	O
interpreter	B-Application
.	O
</s>
<s>
This	O
compiler	B-Language
introduced	O
the	O
Lisp	B-Language
model	O
of	O
incremental	B-Application
compilation	I-Application
,	O
in	O
which	O
compiled	B-Language
and	O
interpreted	B-Application
functions	O
can	O
intermix	O
freely	O
.	O
</s>
<s>
The	O
language	O
used	O
in	O
Hart	O
and	O
Levin	O
's	O
memo	O
is	O
much	O
closer	O
to	O
modern	O
Lisp	B-Language
style	O
than	O
McCarthy	O
's	O
earlier	O
code	O
.	O
</s>
<s>
Garbage	B-General_Concept
collection	I-General_Concept
routines	O
were	O
developed	O
by	O
MIT	O
graduate	O
student	O
Daniel	O
Edwards	O
,	O
prior	O
to	O
1962	O
.	O
</s>
<s>
During	O
the	O
1980s	O
and	O
1990s	O
,	O
a	O
great	O
effort	O
was	O
made	O
to	O
unify	O
the	O
work	O
on	O
new	O
Lisp	B-Language
dialects	O
(	O
mostly	O
successors	O
to	O
Maclisp	B-Language
such	O
as	O
ZetaLisp	B-Language
and	O
NIL	B-Language
(	O
New	O
Implementation	O
of	O
Lisp	B-Language
)	O
into	O
a	O
single	O
language	O
.	O
</s>
<s>
The	O
new	O
language	O
,	O
Common	B-Language
Lisp	I-Language
,	O
was	O
somewhat	O
compatible	O
with	O
the	O
dialects	O
it	O
replaced	O
(	O
the	O
book	O
Common	B-Language
Lisp	I-Language
the	O
Language	O
notes	O
the	O
compatibility	B-Device
of	O
various	O
constructs	O
)	O
.	O
</s>
<s>
In	O
1994	O
,	O
ANSI	O
published	O
the	O
Common	B-Language
Lisp	I-Language
standard	O
,	O
"	O
ANSI	B-Language
X3.226-1994	I-Language
Information	O
Technology	O
Programming	O
Language	O
Common	B-Language
Lisp	I-Language
"	O
.	O
</s>
<s>
Since	O
inception	O
,	O
Lisp	B-Language
was	O
closely	O
connected	O
with	O
the	B-Application
artificial	I-Application
intelligence	I-Application
research	O
community	O
,	O
especially	O
on	O
PDP-10	B-Device
systems	O
.	O
</s>
<s>
Lisp	B-Language
was	O
used	O
as	O
the	O
implementation	O
of	O
the	O
language	O
Micro	B-Application
Planner	I-Application
,	O
which	O
was	O
used	O
in	O
the	O
famous	O
AI	B-Application
system	O
SHRDLU	O
.	O
</s>
<s>
In	O
the	O
1970s	O
,	O
as	O
AI	B-Application
research	I-Application
spawned	O
commercial	O
offshoots	O
,	O
the	O
performance	O
of	O
existing	O
Lisp	B-Language
systems	O
became	O
a	O
growing	O
issue	O
,	O
as	O
programmers	O
needed	O
to	O
be	O
familiar	O
with	O
the	O
performance	O
ramifications	O
of	O
the	O
various	O
techniques	O
and	O
choices	O
involved	O
in	O
the	O
implementation	O
of	O
Lisp	B-Language
.	O
</s>
<s>
Over	O
its	O
sixty-year	O
history	O
,	O
Lisp	B-Language
has	O
spawned	O
many	O
variations	O
on	O
the	O
core	O
theme	O
of	O
an	O
S-expression	B-Protocol
language	O
.	O
</s>
<s>
Moreover	O
,	O
each	O
given	O
dialect	O
may	O
have	O
several	O
implementations	O
—	O
for	O
instance	O
,	O
there	O
are	O
more	O
than	O
a	O
dozen	O
implementations	O
of	O
Common	B-Language
Lisp	I-Language
.	O
</s>
<s>
Differences	O
between	O
dialects	O
may	O
be	O
quite	O
visible	O
—	O
for	O
instance	O
,	O
Common	B-Language
Lisp	I-Language
uses	O
the	O
keyword	O
defun	O
to	O
name	O
a	O
function	O
,	O
but	O
Scheme	B-Language
uses	O
define	O
.	O
</s>
<s>
LISP	B-Language
1	O
–	O
First	O
implementation	O
.	O
</s>
<s>
LISP	B-Language
1.5	I-Language
–	O
First	O
widely	O
distributed	O
version	O
,	O
developed	O
by	O
McCarthy	O
and	O
others	O
at	O
MIT	O
.	O
</s>
<s>
So	O
named	O
because	O
it	O
contained	O
several	O
improvements	O
on	O
the	O
original	O
"	O
LISP	B-Language
1	O
"	O
interpreter	B-Application
,	O
but	O
was	O
not	O
a	O
major	O
restructuring	O
as	O
the	O
planned	O
LISP	B-Language
2	I-Language
would	O
be	O
.	O
</s>
<s>
Stanford	O
LISP	B-Language
1.6	O
–	O
This	O
was	O
a	O
successor	O
to	O
LISP	B-Language
1.5	I-Language
developed	O
at	O
the	O
Stanford	O
AI	B-Application
Lab	O
,	O
and	O
widely	O
distributed	O
to	O
PDP-10	B-Device
systems	O
running	O
the	O
TOPS-10	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
It	O
was	O
rendered	O
obsolete	O
by	O
Maclisp	B-Language
and	O
InterLisp	B-Language
.	O
</s>
<s>
MACLISP	B-Language
–	O
developed	O
for	O
MIT	O
's	O
Project	O
MAC	O
,	O
MACLISP	B-Language
is	O
a	O
direct	O
descendant	O
of	O
LISP	B-Language
1.5	I-Language
.	O
</s>
<s>
It	O
ran	O
on	O
the	O
PDP-10	B-Device
and	O
Multics	B-Application
systems	O
.	O
</s>
<s>
MACLISP	B-Language
would	O
later	O
come	O
to	O
be	O
called	O
Maclisp	B-Language
,	O
and	O
is	O
often	O
referred	O
to	O
as	O
MacLisp	B-Language
.	O
</s>
<s>
The	O
"	O
MAC	O
"	O
in	O
MACLISP	B-Language
is	O
related	O
neither	O
to	O
Apple	O
's	O
Macintosh	B-Device
nor	O
to	O
McCarthy	O
.	O
</s>
<s>
Interlisp	B-Language
–	O
developed	O
at	O
BBN	O
Technologies	O
for	O
PDP-10	B-Device
systems	O
running	O
the	O
TENEX	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
,	O
later	O
adopted	O
as	O
a	O
"	O
West	O
coast	O
"	O
Lisp	B-Language
for	O
the	O
Xerox	O
Lisp	B-Operating_System
machines	I-Operating_System
as	O
InterLisp-D	B-Language
.	O
A	O
small	O
version	O
called	O
"	O
InterLISP	B-Language
65	O
"	O
was	O
published	O
for	O
the	O
6502-based	O
Atari	B-Device
8-bit	I-Device
family	I-Device
computer	O
line	O
.	O
</s>
<s>
For	O
quite	O
some	O
time	O
,	O
Maclisp	B-Language
and	O
InterLisp	B-Language
were	O
strong	O
competitors	O
.	O
</s>
<s>
Franz	O
Lisp	B-Language
–	O
originally	O
a	O
University	O
of	O
California	O
,	O
Berkeley	O
project	O
;	O
later	O
developed	O
by	O
Franz	O
Inc	O
.	O
</s>
<s>
The	O
name	O
is	O
a	O
humorous	O
deformation	O
of	O
the	O
name	O
"	O
Franz	O
Liszt	O
"	O
,	O
and	O
does	O
not	O
refer	O
to	O
Allegro	B-Language
Common	I-Language
Lisp	I-Language
,	O
the	O
dialect	O
of	O
Common	B-Language
Lisp	I-Language
sold	O
by	O
Franz	O
Inc.	O
,	O
in	O
more	O
recent	O
years	O
.	O
</s>
<s>
XLISP	B-Language
,	O
which	O
AutoLISP	B-Algorithm
was	O
based	O
on	O
.	O
</s>
<s>
Standard	O
Lisp	B-Language
and	O
Portable	B-Language
Standard	I-Language
Lisp	I-Language
were	O
widely	O
used	O
and	O
ported	O
,	O
especially	O
with	O
the	O
Computer	B-Algorithm
Algebra	I-Algorithm
System	O
REDUCE	O
.	O
</s>
<s>
ZetaLisp	B-Language
,	O
also	O
termed	O
Lisp	B-Language
Machine	I-Language
Lisp	I-Language
–	O
used	O
on	O
the	O
Lisp	B-Operating_System
machines	I-Operating_System
,	O
direct	O
descendant	O
of	O
Maclisp	B-Language
.	O
</s>
<s>
ZetaLisp	B-Language
had	O
a	O
big	O
influence	O
on	O
Common	B-Language
Lisp	I-Language
.	O
</s>
<s>
LeLisp	B-Language
is	O
a	O
French	O
Lisp	B-Language
dialect	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
Interface	O
Builders	O
(	O
called	O
SOS	O
Interface	O
)	O
was	O
written	O
in	O
LeLisp	B-Language
.	O
</s>
<s>
Scheme	B-Language
(	O
1975	O
)	O
.	O
</s>
<s>
Common	B-Language
Lisp	I-Language
(	O
1984	O
)	O
,	O
as	O
described	O
by	O
Common	B-Language
Lisp	I-Language
the	O
Language	O
–	O
a	O
consolidation	O
of	O
several	O
divergent	O
attempts	O
(	O
ZetaLisp	B-Language
,	O
Spice	B-Language
Lisp	I-Language
,	O
NIL	B-Language
,	O
and	O
S-1	B-Language
Lisp	I-Language
)	O
to	O
create	O
successor	O
dialects	O
to	O
Maclisp	B-Language
,	O
with	O
substantive	O
influences	O
from	O
the	O
Scheme	B-Language
dialect	O
as	O
well	O
.	O
</s>
<s>
This	O
version	O
of	O
Common	B-Language
Lisp	I-Language
was	O
available	O
for	O
wide-ranging	O
platforms	O
and	O
was	O
accepted	O
by	O
many	O
as	O
a	O
de	O
facto	O
standard	O
until	O
the	O
publication	O
of	O
ANSI	B-Language
Common	I-Language
Lisp	I-Language
(	O
ANSI	B-Language
X3.226-1994	I-Language
)	O
.	O
</s>
<s>
Among	O
the	O
most	O
widespread	O
sub-dialects	O
of	O
Common	B-Language
Lisp	I-Language
are	O
Steel	B-Language
Bank	I-Language
Common	I-Language
Lisp	I-Language
(	O
SBCL	B-Language
)	O
,	O
CMU	O
Common	B-Language
Lisp	I-Language
(	O
CMU-CL	O
)	O
,	O
Clozure	O
OpenMCL	O
(	O
not	O
to	O
be	O
confused	O
with	O
Clojure	B-Language
!	O
</s>
<s>
)	O
,	O
GNU	O
CLisp	O
,	O
and	O
later	O
versions	O
of	O
Franz	O
Lisp	B-Language
;	O
all	O
of	O
them	O
adhere	O
to	O
the	O
later	O
ANSI	O
CL	O
standard	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
Dylan	B-Language
was	O
in	O
its	O
first	O
version	O
a	O
mix	O
of	O
Scheme	B-Language
with	O
the	O
Common	B-Application
Lisp	I-Application
Object	I-Application
System	I-Application
.	O
</s>
<s>
EuLisp	B-Language
–	O
attempt	O
to	O
develop	O
a	O
new	O
efficient	O
and	O
cleaned-up	O
Lisp	B-Language
.	O
</s>
<s>
ISLISP	B-Language
–	O
attempt	O
to	O
develop	O
a	O
new	O
efficient	O
and	O
cleaned-up	O
Lisp	B-Language
.	O
</s>
<s>
Standardized	O
as	O
ISO/IEC	O
13816:1997	O
and	O
later	O
revised	O
as	O
ISO/IEC	O
13816:2007	O
:	O
Information	O
technology	O
–	O
Programming	O
languages	O
,	O
their	O
environments	O
and	O
system	O
software	O
interfaces	O
–	O
Programming	O
language	O
ISLISP	B-Language
.	O
</s>
<s>
IEEE	O
Scheme	B-Language
–	O
IEEE	O
standard	O
,	O
1178	O
–	O
1990	O
(	O
R1995	O
)	O
.	O
</s>
<s>
ANSI	B-Language
Common	I-Language
Lisp	I-Language
–	O
an	O
American	O
National	O
Standards	O
Institute	O
(	O
ANSI	O
)	O
standard	O
for	O
Common	B-Language
Lisp	I-Language
,	O
created	O
by	O
subcommittee	O
X3J13	B-Language
,	O
chartered	O
to	O
begin	O
with	O
Common	B-Language
Lisp	I-Language
:	O
The	O
Language	O
as	O
a	O
base	O
document	O
and	O
to	O
work	O
through	O
a	O
public	O
consensus	O
process	O
to	O
find	O
solutions	O
to	O
shared	O
issues	O
of	O
portability	B-Architecture
of	O
programs	O
and	O
compatibility	B-Device
of	O
Common	B-Language
Lisp	I-Language
implementations	O
.	O
</s>
<s>
Although	O
formally	O
an	O
ANSI	O
standard	O
,	O
the	O
implementation	O
,	O
sale	O
,	O
use	O
,	O
and	O
influence	O
of	O
ANSI	B-Language
Common	I-Language
Lisp	I-Language
has	O
been	O
and	O
continues	O
to	O
be	O
seen	O
worldwide	O
.	O
</s>
<s>
ACL2	B-Application
or	O
"	O
A	O
Computational	O
Logic	O
for	O
Applicative	O
Common	B-Language
Lisp	I-Language
"	O
,	O
an	O
applicative	O
(	O
side-effect	O
free	O
)	O
variant	O
of	O
Common	B-Language
LISP	I-Language
.	O
</s>
<s>
ACL2	B-Application
is	O
both	O
a	O
programming	O
language	O
which	O
can	O
model	O
computer	O
systems	O
,	O
and	O
a	O
tool	O
to	O
help	O
proving	O
properties	O
of	O
those	O
models	O
.	O
</s>
<s>
Clojure	B-Language
,	O
a	O
recent	O
dialect	O
of	O
Lisp	B-Language
which	O
compiles	B-Language
to	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
and	O
has	O
a	O
particular	O
focus	O
on	O
concurrency	B-Operating_System
.	O
</s>
<s>
Game	B-Language
Oriented	I-Language
Assembly	I-Language
Lisp	I-Language
(	O
or	O
GOAL	B-Language
)	O
is	O
a	O
video	O
game	O
programming	O
language	O
developed	O
by	O
Andy	O
Gavin	O
at	O
Naughty	O
Dog	O
.	O
</s>
<s>
It	O
was	O
written	O
using	O
Allegro	B-Language
Common	I-Language
Lisp	I-Language
and	O
used	O
in	O
the	O
development	O
of	O
the	O
entire	O
Jak	B-Application
and	I-Application
Daxter	I-Application
series	I-Application
of	I-Application
games	I-Application
developed	O
by	O
Naughty	O
Dog	O
.	O
</s>
<s>
After	O
having	O
declined	O
somewhat	O
in	O
the	O
1990s	O
,	O
Lisp	B-Language
has	O
experienced	O
a	O
resurgence	O
of	O
interest	O
after	O
2000	O
.	O
</s>
<s>
Most	O
new	O
activity	O
has	O
been	O
focused	O
around	O
implementations	O
of	O
Common	B-Language
Lisp	I-Language
,	O
Scheme	B-Language
,	O
Emacs	B-Operating_System
Lisp	I-Operating_System
,	O
Clojure	B-Language
,	O
and	O
Racket	B-Operating_System
,	O
and	O
includes	O
development	O
of	O
new	O
portable	O
libraries	O
and	O
applications	O
.	O
</s>
<s>
Many	O
new	O
Lisp	B-Language
programmers	O
were	O
inspired	O
by	O
writers	O
such	O
as	O
Paul	O
Graham	O
and	O
Eric	O
S	O
.	O
Raymond	O
to	O
pursue	O
a	O
language	O
others	O
considered	O
antiquated	O
.	O
</s>
<s>
New	O
Lisp	B-Language
programmers	O
often	O
describe	O
the	O
language	O
as	O
an	O
eye-opening	O
experience	O
and	O
claim	O
to	O
be	O
substantially	O
more	O
productive	O
than	O
in	O
other	O
languages	O
.	O
</s>
<s>
This	O
increase	O
in	O
awareness	O
may	O
be	O
contrasted	O
to	O
the	O
"	O
AI	B-Application
winter	O
"	O
and	O
Lisp	B-Language
's	O
brief	O
gain	O
in	O
the	O
mid-1990s	O
.	O
</s>
<s>
,	O
there	O
were	O
eleven	O
actively	O
maintained	O
Common	B-Language
Lisp	I-Language
implementations	O
.	O
</s>
<s>
Scieneer	O
Common	B-Language
Lisp	I-Language
is	O
a	O
new	O
commercial	O
implementation	O
forked	O
from	O
CMUCL	O
with	O
a	O
first	O
release	O
in	O
2002	O
.	O
</s>
<s>
The	O
open	O
source	O
community	O
has	O
created	O
new	O
supporting	O
infrastructure	O
:	O
CLiki	B-Application
is	O
a	O
wiki	O
that	O
collects	O
Common	B-Language
Lisp	I-Language
related	O
information	O
,	O
the	O
Common	B-Language
Lisp	I-Language
directory	O
lists	O
resources	O
,	O
#lisp	O
is	O
a	O
popular	O
IRC	O
channel	O
and	O
allows	O
the	O
sharing	O
and	O
commenting	O
of	O
code	O
snippets	O
(	O
with	O
support	O
by	O
lisppaste	O
,	O
an	O
IRC	O
bot	O
written	O
in	O
Lisp	B-Language
)	O
,	O
Planet	O
Lisp	B-Language
collects	O
the	O
contents	O
of	O
various	O
Lisp-related	O
blogs	O
,	O
on	O
LispForum	O
users	O
discuss	O
Lisp	B-Language
topics	O
,	O
Lispjobs	O
is	O
a	O
service	O
for	O
announcing	O
job	O
offers	O
and	O
there	O
is	O
a	O
weekly	O
news	O
service	O
,	O
Weekly	O
Lisp	B-Language
News	O
.	O
</s>
<s>
Common-lisp.net	O
is	O
a	O
hosting	O
site	O
for	O
open	O
source	O
Common	B-Language
Lisp	I-Language
projects	O
.	O
</s>
<s>
Quicklisp	O
is	O
a	O
library	O
manager	O
for	O
Common	B-Language
Lisp	I-Language
.	O
</s>
<s>
Fifty	O
years	O
of	O
Lisp	B-Language
(	O
1958	O
–	O
2008	O
)	O
was	O
celebrated	O
at	O
LISP50	O
@OOPSLA	O
.	O
</s>
<s>
Other	O
events	O
include	O
the	O
European	O
Common	B-Language
Lisp	I-Language
Meeting	O
,	O
the	O
European	O
Lisp	B-Language
Symposium	O
and	O
an	O
International	O
Lisp	B-Language
Conference	O
.	O
</s>
<s>
The	O
Scheme	B-Language
community	O
actively	O
maintains	O
over	O
twenty	O
implementations	O
.	O
</s>
<s>
The	O
Revised5	O
Report	O
on	O
the	O
Algorithmic	B-Language
Language	I-Language
Scheme	B-Language
standard	O
of	O
Scheme	B-Language
was	O
widely	O
accepted	O
in	O
the	O
Scheme	B-Language
community	O
.	O
</s>
<s>
The	O
Scheme	B-Language
Requests	I-Language
for	I-Language
Implementation	I-Language
process	O
has	O
created	O
a	O
lot	O
of	O
quasi	O
standard	O
libraries	O
and	O
extensions	O
for	O
Scheme	B-Language
.	O
</s>
<s>
User	O
communities	O
of	O
individual	O
Scheme	B-Language
implementations	O
continue	O
to	O
grow	O
.	O
</s>
<s>
A	O
new	O
language	O
standardization	O
process	O
was	O
started	O
in	O
2003	O
and	O
led	O
to	O
the	O
R6RS	O
Scheme	B-Language
standard	O
in	O
2007	O
.	O
</s>
<s>
Academic	O
use	O
of	O
Scheme	B-Language
for	O
teaching	O
computer	B-General_Concept
science	I-General_Concept
seems	O
to	O
have	O
declined	O
somewhat	O
.	O
</s>
<s>
Some	O
universities	O
are	O
no	O
longer	O
using	O
Scheme	B-Language
in	O
their	O
computer	B-General_Concept
science	I-General_Concept
introductory	O
courses	O
;	O
MIT	O
now	O
uses	O
Python	B-Language
instead	O
of	O
Scheme	B-Language
for	O
its	O
undergraduate	O
computer	B-General_Concept
science	I-General_Concept
program	O
and	O
MITx	O
massive	O
open	O
online	O
course	O
.	O
</s>
<s>
There	O
are	O
several	O
new	O
dialects	O
of	O
Lisp	B-Language
:	O
Arc	B-Operating_System
,	O
Hy	B-Language
,	O
Nu	B-Application
,	O
Liskell	O
,	O
and	O
LFE	B-Language
(	O
Lisp	B-Language
Flavored	O
Erlang	O
)	O
.	O
</s>
<s>
The	O
parser	O
for	O
Julia	B-Application
is	O
implemented	O
in	O
Femtolisp	B-Application
,	O
a	O
dialect	O
of	O
Scheme	B-Language
(	O
Julia	B-Application
is	O
inspired	O
by	O
Scheme	B-Language
,	O
which	O
in	O
turn	O
is	O
a	O
Lisp	B-Language
dialect	O
)	O
.	O
</s>
<s>
In	O
October	O
2019	O
,	O
Paul	O
Graham	O
released	O
,	O
"	O
a	O
new	O
dialect	O
of	O
Lisp.	O
"	O
</s>
<s>
Common	B-Language
Lisp	I-Language
and	O
Scheme	B-Language
represent	O
two	O
major	O
streams	O
of	O
Lisp	B-Language
development	O
.	O
</s>
<s>
Common	B-Language
Lisp	I-Language
is	O
a	O
successor	O
to	O
Maclisp	B-Language
.	O
</s>
<s>
The	O
primary	O
influences	O
were	O
Lisp	B-Language
Machine	I-Language
Lisp	I-Language
,	O
Maclisp	B-Language
,	O
NIL	B-Language
,	O
S-1	B-Language
Lisp	I-Language
,	O
Spice	B-Language
Lisp	I-Language
,	O
and	O
Scheme	B-Language
.	O
</s>
<s>
It	O
has	O
many	O
of	O
the	O
features	O
of	O
Lisp	B-Language
Machine	I-Language
Lisp	I-Language
(	O
a	O
large	O
Lisp	B-Language
dialect	O
used	O
to	O
program	O
Lisp	B-Operating_System
Machines	I-Operating_System
)	O
,	O
but	O
was	O
designed	O
to	O
be	O
efficiently	O
implementable	O
on	O
any	O
personal	O
computer	O
or	O
workstation	B-Device
.	O
</s>
<s>
Common	B-Language
Lisp	I-Language
is	O
a	O
general-purpose	O
programming	O
language	O
and	O
thus	O
has	O
a	O
large	O
language	O
standard	O
including	O
many	O
built-in	O
data	O
types	O
,	O
functions	O
,	O
macros	O
and	O
other	O
language	O
elements	O
,	O
and	O
an	O
object	O
system	O
(	O
Common	B-Application
Lisp	I-Application
Object	I-Application
System	I-Application
)	O
.	O
</s>
<s>
Common	B-Language
Lisp	I-Language
also	O
borrowed	O
certain	O
features	O
from	O
Scheme	B-Language
such	O
as	O
lexical	O
scoping	B-Language
and	O
lexical	B-Language
closures	I-Language
.	O
</s>
<s>
Common	B-Language
Lisp	I-Language
implementations	O
are	O
available	O
for	O
targeting	O
different	O
platforms	O
such	O
as	O
the	O
LLVM	B-Application
,	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
,	O
</s>
<s>
x86-64	O
,	O
PowerPC	O
,	O
Alpha	O
,	O
ARM	O
,	O
Motorola	O
68000	O
,	O
and	O
MIPS	O
,	O
and	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
Windows	O
,	O
macOS	O
,	O
Linux	O
,	O
Solaris	O
,	O
FreeBSD	O
,	O
NetBSD	O
,	O
OpenBSD	O
,	O
Dragonfly	O
BSD	O
,	O
and	O
Heroku	O
.	O
</s>
<s>
Scheme	B-Language
is	O
a	O
statically	O
scoped	O
and	O
properly	O
tail-recursive	B-Language
dialect	O
of	O
the	O
Lisp	B-Language
programming	I-Language
language	I-Language
invented	O
by	O
Guy	O
L	O
.	O
Steele	O
,	O
Jr.	O
and	O
Gerald	O
Jay	O
Sussman	O
.	O
</s>
<s>
Designed	O
about	O
a	O
decade	O
earlier	O
than	O
Common	B-Language
Lisp	I-Language
,	O
Scheme	B-Language
is	O
a	O
more	O
minimalist	O
design	O
.	O
</s>
<s>
It	O
has	O
a	O
much	O
smaller	O
set	O
of	O
standard	O
features	O
but	O
with	O
certain	O
implementation	O
features	O
(	O
such	O
as	O
tail-call	B-Language
optimization	I-Language
and	O
full	O
continuations	O
)	O
not	O
specified	O
in	O
Common	B-Language
Lisp	I-Language
.	O
</s>
<s>
A	O
wide	O
variety	O
of	O
programming	O
paradigms	O
,	O
including	O
imperative	O
,	O
functional	B-Language
,	O
and	O
message	O
passing	O
styles	O
,	O
find	O
convenient	O
expression	O
in	O
Scheme	B-Language
.	O
</s>
<s>
Scheme	B-Language
continues	O
to	O
evolve	O
with	O
a	O
series	O
of	O
standards	O
(	O
Revisedn	O
Report	O
on	O
the	O
Algorithmic	B-Language
Language	I-Language
Scheme	B-Language
)	O
and	O
a	O
series	O
of	O
Scheme	B-Language
Requests	I-Language
for	I-Language
Implementation	I-Language
.	O
</s>
<s>
Clojure	B-Language
is	O
a	O
recent	O
dialect	O
of	O
Lisp	B-Language
that	O
targets	O
mainly	O
the	O
Java	B-Language
virtual	I-Language
machine	I-Language
,	O
and	O
the	O
Common	O
Language	O
Runtime	B-Library
(	O
CLR	O
)	O
,	O
the	O
Python	B-Language
VM	O
,	O
the	O
Ruby	B-Language
VM	O
YARV	B-Language
,	O
and	O
compiling	B-Language
to	O
JavaScript	B-Language
.	O
</s>
<s>
Clojure	B-Language
draws	O
considerable	O
influences	O
from	O
Haskell	B-Language
and	O
places	O
a	O
very	O
strong	O
emphasis	O
on	O
immutability	O
.	O
</s>
<s>
Clojure	B-Language
provides	O
access	O
to	O
Java	O
frameworks	O
and	O
libraries	O
,	O
with	O
optional	O
type	O
hints	O
and	O
type	O
inference	O
,	O
so	O
that	O
calls	O
to	O
Java	O
can	O
avoid	O
reflection	O
and	O
enable	O
fast	O
primitive	O
operations	O
.	O
</s>
<s>
Clojure	B-Language
is	O
not	O
designed	O
to	O
be	O
backwards	B-General_Concept
compatible	I-General_Concept
with	O
other	O
Lisp	B-Language
dialects	O
.	O
</s>
<s>
Further	O
,	O
Lisp	B-Language
dialects	O
are	O
used	O
as	O
scripting	B-Language
languages	I-Language
in	O
many	O
applications	O
,	O
with	O
the	O
best-known	O
being	O
Emacs	B-Operating_System
Lisp	I-Operating_System
in	O
the	O
Emacs	B-Application
editor	O
,	O
AutoLISP	B-Algorithm
and	O
later	O
Visual	B-Algorithm
Lisp	I-Algorithm
in	O
AutoCAD	B-Algorithm
,	O
Nyquist	O
in	O
Audacity	B-Language
,	O
and	O
Scheme	B-Language
in	O
LilyPond	B-Application
.	O
</s>
<s>
The	O
potential	O
small	O
size	O
of	O
a	O
useful	O
Scheme	B-Language
interpreter	B-Application
makes	O
it	O
particularly	O
popular	O
for	O
embedded	O
scripting	O
.	O
</s>
<s>
Examples	O
include	O
SIOD	B-Language
and	O
TinyScheme	B-Language
,	O
both	O
of	O
which	O
have	O
been	O
successfully	O
embedded	O
in	O
the	B-Application
GIMP	I-Application
image	O
processor	O
under	O
the	O
generic	O
name	O
"	O
Script-fu	B-Application
"	O
.	O
</s>
<s>
LIBREP	O
,	O
a	O
Lisp	B-Language
interpreter	B-Application
by	O
John	O
Harper	O
originally	O
based	O
on	O
the	O
Emacs	B-Operating_System
Lisp	I-Operating_System
language	O
,	O
has	O
been	O
embedded	O
in	O
the	O
Sawfish	B-Language
window	B-Application
manager	I-Application
.	O
</s>
<s>
Lisp	B-Language
has	O
officially	O
standardized	O
dialects	O
:	O
R6RS	O
Scheme	B-Language
,	O
R7RS	O
Scheme	B-Language
,	O
IEEE	O
Scheme	B-Language
,	O
ANSI	B-Language
Common	I-Language
Lisp	I-Language
and	O
ISO	O
ISLISP	B-Language
.	O
</s>
<s>
Paul	O
Graham	O
identifies	O
nine	O
important	O
aspects	O
of	O
Lisp	B-Language
that	O
distinguished	O
it	O
from	O
existing	O
languages	O
like	O
Fortran	B-Application
:	O
</s>
<s>
Lisp	B-Language
was	O
the	O
first	O
language	O
where	O
the	O
structure	O
of	O
program	O
code	O
is	O
represented	O
faithfully	O
and	O
directly	O
in	O
a	O
standard	O
data	B-General_Concept
structure	I-General_Concept
—	O
a	O
quality	O
much	O
later	O
dubbed	O
"	O
homoiconicity	O
"	O
.	O
</s>
<s>
Thus	O
,	O
Lisp	B-Language
functions	O
can	O
be	O
manipulated	O
,	O
altered	O
or	O
even	O
created	O
within	O
a	O
Lisp	B-Language
program	O
without	O
lower-level	O
manipulations	O
.	O
</s>
<s>
This	O
is	O
generally	O
considered	O
one	O
of	O
the	O
main	O
advantages	O
of	O
the	O
language	O
with	O
regard	O
to	O
its	O
expressive	O
power	O
,	O
and	O
makes	O
the	O
language	O
suitable	O
for	O
syntactic	O
macros	O
and	O
meta-circular	B-Application
evaluation	I-Application
.	O
</s>
<s>
A	O
conditional	B-Language
using	O
an	O
if	B-Language
–	I-Language
then	I-Language
–	I-Language
else	I-Language
syntax	O
was	O
invented	O
by	O
McCarthy	O
for	O
a	O
chess	O
program	O
written	O
in	O
Fortran	B-Application
.	O
</s>
<s>
He	O
proposed	O
its	O
inclusion	O
in	O
ALGOL	B-Language
,	O
but	O
it	O
was	O
not	O
made	O
part	O
of	O
the	O
Algol	B-Language
58	I-Language
specification	O
.	O
</s>
<s>
For	O
Lisp	B-Language
,	O
McCarthy	O
used	O
the	O
more	O
general	O
cond-structure	O
.	O
</s>
<s>
Algol	B-Language
60	I-Language
took	O
up	O
if	B-Language
–	I-Language
then	I-Language
–	I-Language
else	I-Language
and	O
popularized	O
it	O
.	O
</s>
<s>
Lisp	B-Language
deeply	O
influenced	O
Alan	O
Kay	O
,	O
the	O
leader	O
of	O
the	O
research	O
team	O
that	O
developed	O
Smalltalk	B-Application
at	O
Xerox	O
PARC	O
;	O
and	O
in	O
turn	O
Lisp	B-Language
was	O
influenced	O
by	O
Smalltalk	B-Application
,	O
with	O
later	O
dialects	O
adopting	O
object-oriented	O
programming	O
features	O
(	O
inheritance	O
classes	O
,	O
encapsulating	O
instances	O
,	O
message	O
passing	O
,	O
etc	O
.	O
)	O
</s>
<s>
The	O
Flavors	B-Language
object	O
system	O
introduced	O
the	O
concept	O
of	O
multiple	B-Application
inheritance	I-Application
and	O
the	O
mixin	B-Language
.	O
</s>
<s>
The	O
Common	B-Application
Lisp	I-Application
Object	I-Application
System	I-Application
provides	O
multiple	B-Application
inheritance	I-Application
,	O
multimethods	O
with	O
multiple	O
dispatch	O
,	O
and	O
first-class	O
generic	O
functions	O
,	O
yielding	O
a	O
flexible	O
and	O
powerful	O
form	O
of	O
dynamic	O
dispatch	O
.	O
</s>
<s>
It	O
has	O
served	O
as	O
the	O
template	O
for	O
many	O
subsequent	O
Lisp	B-Language
(	O
including	O
Scheme	B-Language
)	O
object	O
systems	O
,	O
which	O
are	O
often	O
implemented	O
via	O
a	O
metaobject	O
protocol	O
,	O
a	O
reflective	B-Language
meta-circular	B-Application
design	I-Application
in	O
which	O
the	O
object	O
system	O
is	O
defined	O
in	O
terms	O
of	O
itself	O
:	O
Lisp	B-Language
was	O
only	O
the	O
second	O
language	O
after	O
Smalltalk	B-Application
(	O
and	O
is	O
still	O
one	O
of	O
the	O
very	O
few	O
languages	O
)	O
to	O
possess	O
such	O
a	O
metaobject	O
system	O
.	O
</s>
<s>
Many	O
years	O
later	O
,	O
Alan	O
Kay	O
suggested	O
that	O
as	O
a	O
result	O
of	O
the	O
confluence	O
of	O
these	O
features	O
,	O
only	O
Smalltalk	B-Application
and	O
Lisp	B-Language
could	O
be	O
regarded	O
as	O
properly	O
conceived	O
object-oriented	O
programming	O
systems	O
.	O
</s>
<s>
Lisp	B-Language
introduced	O
the	O
concept	O
of	O
automatic	B-General_Concept
garbage	I-General_Concept
collection	I-General_Concept
,	O
in	O
which	O
the	O
system	O
walks	O
the	O
heap	O
looking	O
for	O
unused	O
memory	O
.	O
</s>
<s>
Progress	O
in	O
modern	O
sophisticated	O
garbage	B-General_Concept
collection	I-General_Concept
algorithms	O
such	O
as	O
generational	O
garbage	B-General_Concept
collection	I-General_Concept
was	O
stimulated	O
by	O
its	O
use	O
in	O
Lisp	B-Language
.	O
</s>
<s>
Largely	O
because	O
of	O
its	O
resource	O
requirements	O
with	O
respect	O
to	O
early	O
computing	O
hardware	O
(	O
including	O
early	O
microprocessors	O
)	O
,	O
Lisp	B-Language
did	O
not	O
become	O
as	O
popular	O
outside	O
of	O
the	O
AI	B-Application
community	O
as	O
Fortran	B-Application
and	O
the	O
ALGOL-descended	O
C	B-Language
language	I-Language
.	O
</s>
<s>
Because	O
of	O
its	O
suitability	O
to	O
complex	O
and	O
dynamic	O
applications	O
,	O
Lisp	B-Language
enjoyed	O
some	O
resurgence	O
of	O
popular	O
interest	O
in	O
the	O
2010s	O
.	O
</s>
<s>
Note	O
:	O
This	O
article	O
's	O
examples	O
are	O
written	O
in	O
Common	B-Language
Lisp	I-Language
(	O
though	O
most	O
are	O
also	O
valid	O
in	O
Scheme	B-Language
)	O
.	O
</s>
<s>
Lisp	B-Language
is	O
an	O
expression	O
oriented	O
language	O
.	O
</s>
<s>
When	O
an	O
expression	O
is	O
evaluated	O
,	O
it	O
produces	O
a	O
value	O
(	O
in	O
Common	B-Language
Lisp	I-Language
,	O
possibly	O
multiple	O
values	O
)	O
,	O
which	O
can	O
then	O
be	O
embedded	O
into	O
other	O
expressions	O
.	O
</s>
<s>
McCarthy	O
's	O
1958	O
paper	O
introduced	O
two	O
types	O
of	O
syntax	O
:	O
Symbolic	O
expressions	O
(	O
S-expressions	B-Protocol
,	O
sexps	B-Protocol
)	O
,	O
which	O
mirror	O
the	O
internal	O
representation	O
of	O
code	O
and	O
data	O
;	O
and	O
Meta	B-Application
expressions	O
(	O
M-expressions	B-Language
)	O
,	O
which	O
express	O
functions	O
of	O
S-expressions	B-Protocol
.	O
</s>
<s>
M-expressions	B-Language
never	O
found	O
favor	O
,	O
and	O
almost	O
all	O
Lisps	B-Language
today	O
use	O
S-expressions	B-Protocol
to	O
manipulate	O
both	O
code	O
and	O
data	O
.	O
</s>
<s>
The	O
use	O
of	O
parentheses	O
is	O
Lisp	B-Language
's	O
most	O
immediately	O
obvious	O
difference	O
from	O
other	O
programming	O
language	O
families	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
students	O
have	O
long	O
given	O
Lisp	B-Language
nicknames	O
such	O
as	O
Lost	O
In	O
Stupid	O
Parentheses	O
,	O
or	O
Lots	O
of	O
Irritating	O
Superfluous	O
Parentheses	O
.	O
</s>
<s>
However	O
,	O
the	O
S-expression	B-Protocol
syntax	O
is	O
also	O
responsible	O
for	O
much	O
of	O
Lisp	B-Language
's	O
power	O
:	O
the	O
syntax	O
is	O
simple	O
and	O
consistent	O
,	O
which	O
facilitates	O
manipulation	O
by	O
computer	O
.	O
</s>
<s>
However	O
,	O
the	O
syntax	O
of	O
Lisp	B-Language
is	O
not	O
limited	O
to	O
traditional	O
parentheses	O
notation	O
.	O
</s>
<s>
For	O
example	O
,	O
XMLisp	O
is	O
a	O
Common	B-Language
Lisp	I-Language
extension	O
that	O
employs	O
the	O
metaobject	O
protocol	O
to	O
integrate	O
S-expressions	B-Protocol
with	O
the	O
Extensible	B-Protocol
Markup	I-Protocol
Language	I-Protocol
(	O
XML	B-Protocol
)	O
.	O
</s>
<s>
Because	O
Lisp	B-Language
functions	O
are	O
written	O
as	O
lists	O
,	O
they	O
can	O
be	O
processed	O
exactly	O
like	O
data	O
.	O
</s>
<s>
This	O
allows	O
easy	O
writing	O
of	O
programs	O
which	O
manipulate	O
other	O
programs	O
(	O
metaprogramming	B-Application
)	O
.	O
</s>
<s>
Many	O
Lisp	B-Language
dialects	O
exploit	O
this	O
feature	O
using	O
macro	O
systems	O
,	O
which	O
enables	O
extension	O
of	O
the	O
language	O
almost	O
without	O
limit	O
.	O
</s>
<s>
A	O
Lisp	B-Language
list	O
is	O
written	O
with	O
its	O
elements	O
separated	O
by	O
whitespace	O
,	O
and	O
surrounded	O
by	O
parentheses	O
.	O
</s>
<s>
These	O
values	O
are	O
implicitly	O
typed	O
:	O
they	O
are	O
respectively	O
two	O
integers	O
and	O
a	O
Lisp-specific	O
data	O
type	O
called	O
a	O
"	O
symbol	O
"	O
,	O
and	O
do	O
not	O
have	O
to	O
be	O
declared	O
as	O
such	O
.	O
</s>
<s>
This	O
is	O
the	O
only	O
entity	O
in	O
Lisp	B-Language
which	O
is	O
both	O
an	O
atom	O
and	O
a	O
list	O
.	O
</s>
<s>
The	O
first	O
element	O
in	O
the	O
list	O
is	O
the	O
name	O
of	O
a	O
function	O
,	O
the	O
name	O
of	O
a	O
macro	O
,	O
a	O
lambda	B-General_Concept
expression	I-General_Concept
or	O
the	O
name	O
of	O
a	O
"	O
special	O
operator	O
"	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
Lisp	B-Language
has	O
no	O
notion	O
of	O
operators	O
as	O
implemented	O
in	O
Algol-derived	O
languages	O
.	O
</s>
<s>
Arithmetic	O
operators	O
in	O
Lisp	B-Language
are	O
variadic	B-Language
functions	I-Language
(	O
or	O
n-ary	O
)	O
,	O
able	O
to	O
take	O
any	O
number	O
of	O
arguments	O
.	O
</s>
<s>
"	O
Special	O
operators	O
"	O
(	O
sometimes	O
called	O
"	O
special	O
forms	O
"	O
)	O
provide	O
Lisp	B-Language
's	O
control	O
structure	O
.	O
</s>
<s>
If	O
the	O
first	O
argument	O
is	O
non-nil	O
,	O
it	O
evaluates	O
to	O
the	O
second	O
argument	O
;	O
otherwise	O
,	O
it	O
evaluates	O
to	O
the	O
third	O
argument	O
.	O
</s>
<s>
Lisp	B-Language
also	O
provides	O
logical	O
operators	O
and	O
,	O
or	O
and	O
not	O
.	O
</s>
<s>
The	O
and	O
and	O
or	O
operators	O
do	O
short-circuit	O
evaluation	O
and	O
will	O
return	O
their	O
first	O
nil	B-Language
and	O
non-nil	O
argument	O
respectively	O
.	O
</s>
<s>
Lambda	B-General_Concept
expressions	I-General_Concept
are	O
treated	O
no	O
differently	O
from	O
named	O
functions	O
;	O
they	O
are	O
invoked	O
the	O
same	O
way	O
.	O
</s>
<s>
Here	O
,	O
we	O
're	O
doing	O
a	O
function	O
application	O
:	O
we	O
execute	O
the	O
anonymous	B-General_Concept
function	I-General_Concept
by	O
passing	O
to	O
it	O
the	O
value	O
5	O
.	O
</s>
<s>
Named	O
functions	O
are	O
created	O
by	O
storing	O
a	O
lambda	B-General_Concept
expression	I-General_Concept
in	O
a	O
symbol	O
using	O
the	O
defun	O
macro	O
.	O
</s>
<s>
In	O
the	O
original	O
LISP	B-Language
there	O
were	O
two	O
fundamental	O
data	O
types	O
:	O
atoms	O
and	O
lists	O
.	O
</s>
<s>
A	O
symbol	O
was	O
essentially	O
a	O
unique	O
named	O
item	O
,	O
written	O
as	O
an	O
alphanumeric	O
string	O
in	O
source	O
code	O
,	O
and	O
used	O
either	O
as	O
a	O
variable	O
name	O
or	O
as	O
a	O
data	O
item	O
in	O
symbolic	B-Algorithm
processing	I-Algorithm
.	O
</s>
<s>
As	O
more	O
data	O
types	O
were	O
introduced	O
in	O
later	O
Lisp	B-Language
dialects	O
,	O
and	O
programming	B-Language
styles	I-Language
evolved	O
,	O
the	O
concept	O
of	O
an	O
atom	O
lost	O
importance	O
.	O
</s>
<s>
Many	O
dialects	O
still	O
retained	O
the	O
predicate	O
atom	O
for	O
legacy	B-General_Concept
compatibility	I-General_Concept
,	O
defining	O
it	O
true	O
for	O
any	O
object	O
which	O
is	O
not	O
a	O
cons	B-Language
.	O
</s>
<s>
A	O
Lisp	B-Language
list	O
is	O
implemented	O
as	O
a	O
singly	O
linked	B-Data_Structure
list	I-Data_Structure
.	O
</s>
<s>
Each	O
cell	O
of	O
this	O
list	O
is	O
called	O
a	O
cons	B-Language
(	O
in	O
Scheme	B-Language
,	O
a	O
pair	O
)	O
and	O
is	O
composed	O
of	O
two	O
pointers	O
,	O
called	O
the	O
car	B-Language
and	I-Language
cdr	I-Language
.	O
</s>
<s>
These	O
are	O
respectively	O
equivalent	O
to	O
the	O
and	O
fields	O
discussed	O
in	O
the	O
article	O
linked	B-Data_Structure
list	I-Data_Structure
.	O
</s>
<s>
Of	O
the	O
many	O
data	B-General_Concept
structures	I-General_Concept
that	O
can	O
be	O
built	O
out	O
of	O
cons	B-Language
cells	I-Language
,	O
one	O
of	O
the	O
most	O
basic	O
is	O
called	O
a	O
proper	O
list	O
.	O
</s>
<s>
A	O
proper	O
list	O
is	O
either	O
the	O
special	O
(	O
empty	O
list	O
)	O
symbol	O
,	O
or	O
a	O
cons	B-Language
in	O
which	O
the	O
points	O
to	O
a	O
datum	O
(	O
which	O
may	O
be	O
another	O
cons	B-Language
structure	O
,	O
such	O
as	O
a	O
list	O
)	O
,	O
and	O
the	O
points	O
to	O
another	O
proper	O
list	O
.	O
</s>
<s>
If	O
a	O
given	O
cons	B-Language
is	O
taken	O
to	O
be	O
the	O
head	O
of	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
,	O
then	O
its	O
car	O
points	O
to	O
the	O
first	O
element	O
of	O
the	O
list	O
,	O
and	O
its	O
cdr	O
points	O
to	O
the	O
rest	O
of	O
the	O
list	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	O
and	O
functions	O
are	O
also	O
called	O
and	O
when	O
referring	O
to	O
conses	O
which	O
are	O
part	O
of	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
(	O
rather	O
than	O
,	O
say	O
,	O
a	O
tree	O
)	O
.	O
</s>
<s>
Thus	O
,	O
a	O
Lisp	B-Language
list	O
is	O
not	O
an	O
atomic	O
object	O
,	O
as	O
an	O
instance	O
of	O
a	O
container	O
class	O
in	O
C++	B-Language
or	O
Java	O
would	O
be	O
.	O
</s>
<s>
A	O
variable	O
that	O
refers	O
to	O
a	O
given	O
list	O
is	O
simply	O
a	O
pointer	O
to	O
the	O
first	O
cons	B-Language
in	O
the	O
list	O
.	O
</s>
<s>
Traversal	O
of	O
a	O
list	O
can	O
be	O
done	O
by	O
cdring	O
down	O
the	O
list	O
;	O
that	O
is	O
,	O
taking	O
successive	O
cdrs	O
to	O
visit	O
each	O
cons	B-Language
of	O
the	O
list	O
;	O
or	O
by	O
using	O
any	O
of	O
several	O
higher-order	B-Language
functions	I-Language
to	O
map	O
a	O
function	O
over	O
a	O
list	O
.	O
</s>
<s>
Because	O
conses	O
and	O
lists	O
are	O
so	O
universal	O
in	O
Lisp	B-Language
systems	O
,	O
it	O
is	O
a	O
common	O
misconception	O
that	O
they	O
are	O
Lisp	B-Language
's	O
only	O
data	B-General_Concept
structures	I-General_Concept
.	O
</s>
<s>
In	O
fact	O
,	O
all	O
but	O
the	O
most	O
simplistic	O
Lisps	B-Language
have	O
other	O
data	B-General_Concept
structures	I-General_Concept
,	O
such	O
as	O
vectors	O
(	O
arrays	O
)	O
,	O
hash	B-Algorithm
tables	I-Algorithm
,	O
structures	O
,	O
and	O
so	O
forth	B-Application
.	O
</s>
<s>
Parenthesized	O
S-expressions	B-Protocol
represent	O
linked	B-Data_Structure
list	I-Data_Structure
structures	O
.	O
</s>
<s>
There	O
are	O
several	O
ways	O
to	O
represent	O
the	O
same	O
list	O
as	O
an	O
S-expression	B-Protocol
.	O
</s>
<s>
A	O
cons	B-Language
can	O
be	O
written	O
in	O
dotted-pair	O
notation	O
as	O
,	O
where	O
is	O
the	O
car	O
and	O
the	O
cdr	O
.	O
</s>
<s>
Lisp	B-Language
provides	O
many	O
built-in	O
procedures	O
for	O
accessing	O
and	O
controlling	O
lists	O
.	O
</s>
<s>
Because	O
of	O
the	O
way	O
that	O
lists	O
are	O
constructed	O
from	O
cons	B-Language
pairs	I-Language
,	O
the	O
procedure	O
can	O
be	O
used	O
to	O
add	O
an	O
element	O
to	O
the	O
front	O
of	O
a	O
list	O
.	O
</s>
<s>
Lisp	B-Language
lists	O
,	O
being	O
simple	O
linked	B-Data_Structure
lists	I-Data_Structure
,	O
can	O
share	O
structure	O
with	O
one	O
another	O
.	O
</s>
<s>
For	O
instance	O
,	O
after	O
the	O
execution	O
of	O
the	O
following	O
Common	B-Language
Lisp	I-Language
code	O
:	O
</s>
<s>
It	O
is	O
not	O
a	O
copy	O
;	O
the	O
cons	B-Language
cells	I-Language
pointing	O
to	O
and	O
are	O
in	O
the	O
same	O
memory	O
locations	O
for	O
both	O
lists	O
.	O
</s>
<s>
This	O
can	O
be	O
a	O
source	O
of	O
bugs	B-Error_Name
,	O
and	O
functions	O
which	O
alter	O
their	O
arguments	O
are	O
documented	O
as	O
destructive	O
for	O
this	O
very	O
reason	O
.	O
</s>
<s>
Aficionados	O
of	O
functional	B-Language
programming	I-Language
avoid	O
destructive	O
functions	O
.	O
</s>
<s>
In	O
the	O
Scheme	B-Language
dialect	O
,	O
which	O
favors	O
the	O
functional	B-Language
style	O
,	O
the	O
names	O
of	O
destructive	O
functions	O
are	O
marked	O
with	O
a	O
cautionary	O
exclamation	O
point	O
,	O
or	O
"	O
bang	O
"	O
—	O
such	O
as	O
(	O
read	O
set	O
car	O
bang	O
)	O
,	O
which	O
replaces	O
the	O
car	O
of	O
a	O
cons	B-Language
.	O
</s>
<s>
In	O
the	O
Common	B-Language
Lisp	I-Language
dialect	O
,	O
destructive	O
functions	O
are	O
commonplace	O
;	O
the	O
equivalent	O
of	O
is	O
named	O
for	O
"	O
replace	O
car	O
"	O
.	O
</s>
<s>
This	O
function	O
is	O
rarely	O
seen	O
,	O
however	O
,	O
as	O
Common	B-Language
Lisp	I-Language
includes	O
a	O
special	O
facility	O
,	O
,	O
to	O
make	O
it	O
easier	O
to	O
define	O
and	O
use	O
destructive	O
functions	O
.	O
</s>
<s>
A	O
frequent	O
style	O
in	O
Common	B-Language
Lisp	I-Language
is	O
to	O
write	O
code	O
functionally	O
(	O
without	O
destructive	O
calls	O
)	O
when	O
prototyping	O
,	O
then	O
to	O
add	O
destructive	O
calls	O
as	O
an	O
optimization	O
where	O
it	O
is	O
safe	O
to	O
do	O
so	O
.	O
</s>
<s>
Lisp	B-Language
evaluates	O
expressions	O
which	O
are	O
entered	O
by	O
the	O
user	O
.	O
</s>
<s>
However	O
,	O
most	O
other	O
forms	O
evaluate	O
to	O
themselves	O
:	O
if	O
entering	O
into	O
Lisp	B-Language
,	O
it	O
returns	O
.	O
</s>
<s>
Both	O
Common	B-Language
Lisp	I-Language
and	O
Scheme	B-Language
also	O
support	O
the	O
backquote	O
operator	O
(	O
termed	O
quasiquote	O
in	O
Scheme	B-Language
)	O
,	O
entered	O
with	O
the	O
character	O
(	O
grave	O
accent	O
)	O
.	O
</s>
<s>
Self-evaluating	O
forms	O
and	O
quoted	O
forms	O
are	O
Lisp	B-Language
's	O
equivalent	O
of	O
literals	O
.	O
</s>
<s>
Modifying	O
a	O
quoted	O
form	O
like	O
this	O
is	O
generally	O
considered	O
bad	O
style	O
,	O
and	O
is	O
defined	O
by	O
ANSI	B-Language
Common	I-Language
Lisp	I-Language
as	O
erroneous	O
(	O
resulting	O
in	O
"	O
undefined	O
"	O
behavior	O
in	O
compiled	B-Language
files	O
,	O
because	O
the	O
file-compiler	O
can	O
coalesce	O
similar	O
constants	O
,	O
put	O
them	O
in	O
write-protected	O
memory	O
,	O
etc	O
.	O
</s>
<s>
Lisp	B-Language
's	O
formalization	O
of	O
quotation	O
has	O
been	O
noted	O
by	O
Douglas	O
Hofstadter	O
(	O
in	O
Gödel	O
,	O
Escher	O
,	O
Bach	O
)	O
and	O
others	O
as	O
an	O
example	O
of	O
the	O
philosophical	O
idea	O
of	O
self-reference	O
.	O
</s>
<s>
The	O
Lisp	B-Language
family	O
splits	O
over	O
the	O
use	O
of	O
dynamic	O
or	O
static	O
(	O
a.k.a.	O
</s>
<s>
lexical	O
)	O
scope	B-Language
.	O
</s>
<s>
Clojure	B-Language
,	O
Common	B-Language
Lisp	I-Language
and	O
Scheme	B-Language
make	O
use	O
of	O
static	O
scoping	B-Language
by	O
default	O
,	O
while	O
newLISP	B-Language
,	O
Picolisp	B-Language
and	O
the	O
embedded	O
languages	O
in	O
Emacs	B-Application
and	O
AutoCAD	B-Algorithm
use	O
dynamic	O
scoping	B-Language
.	O
</s>
<s>
Since	O
version	O
24.1	O
,	O
Emacs	B-Application
uses	O
both	O
dynamic	O
and	O
lexical	O
scoping	B-Language
.	O
</s>
<s>
A	O
fundamental	O
distinction	O
between	O
Lisp	B-Language
and	O
other	O
languages	O
is	O
that	O
in	O
Lisp	B-Language
,	O
the	O
textual	O
representation	O
of	O
a	O
program	O
is	O
simply	O
a	O
human-readable	O
description	O
of	O
the	O
same	O
internal	O
data	B-General_Concept
structures	I-General_Concept
(	O
linked	B-Data_Structure
lists	I-Data_Structure
,	O
symbols	O
,	O
number	O
,	O
characters	O
,	O
etc	O
.	O
)	O
</s>
<s>
as	O
would	O
be	O
used	O
by	O
the	O
underlying	O
Lisp	B-Language
system	O
.	O
</s>
<s>
Lisp	B-Language
uses	O
this	O
to	O
implement	O
a	O
very	O
powerful	O
macro	O
system	O
.	O
</s>
<s>
Like	O
other	O
macro	O
languages	O
such	O
as	O
the	O
one	O
defined	O
by	O
the	O
C	B-Language
preprocessor	I-Language
(	O
the	O
macro	O
preprocessor	O
for	O
the	O
C	B-Language
,	O
Objective-C	B-Language
and	O
C++	B-Language
programming	I-Language
languages	I-Language
)	O
,	O
a	O
macro	O
returns	O
code	O
that	O
can	O
then	O
be	O
compiled	B-Language
.	O
</s>
<s>
However	O
,	O
unlike	O
C	B-Language
preprocessor	I-Language
macros	O
,	O
the	O
macros	O
are	O
Lisp	B-Language
functions	O
and	O
so	O
can	O
exploit	O
the	O
full	O
power	O
of	O
Lisp	B-Language
.	O
</s>
<s>
Further	O
,	O
because	O
Lisp	B-Language
code	O
has	O
the	O
same	O
structure	O
as	O
lists	O
,	O
macros	O
can	O
be	O
built	O
with	O
any	O
of	O
the	O
list-processing	O
functions	O
in	O
the	O
language	O
.	O
</s>
<s>
In	O
short	O
,	O
anything	O
that	O
Lisp	B-Language
can	O
do	O
to	O
a	O
data	B-General_Concept
structure	I-General_Concept
,	O
Lisp	B-Language
macros	O
can	O
do	O
to	O
code	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Common	B-Application
Lisp	I-Application
Object	I-Application
System	I-Application
can	O
be	O
implemented	O
cleanly	O
as	O
a	O
language	O
extension	O
using	O
macros	O
.	O
</s>
<s>
This	O
is	O
in	O
stark	O
contrast	O
to	O
most	O
other	O
languages	O
;	O
for	O
example	O
,	O
Java	O
does	O
not	O
support	O
multiple	B-Application
inheritance	I-Application
and	O
there	O
is	O
no	O
reasonable	O
way	O
to	O
add	O
it	O
.	O
</s>
<s>
In	O
simplistic	O
Lisp	B-Language
implementations	O
,	O
this	O
list	O
structure	O
is	O
directly	O
interpreted	B-Application
to	O
run	O
the	O
program	O
;	O
a	O
function	O
is	O
literally	O
a	O
piece	O
of	O
list	O
structure	O
which	O
is	O
traversed	O
by	O
the	O
interpreter	B-Application
in	O
executing	O
it	O
.	O
</s>
<s>
However	O
,	O
most	O
substantial	O
Lisp	B-Language
systems	O
also	O
include	O
a	O
compiler	B-Language
.	O
</s>
<s>
The	O
compiler	B-Language
translates	O
list	O
structure	O
into	O
machine	B-Language
code	I-Language
or	O
bytecode	O
for	O
execution	O
.	O
</s>
<s>
This	O
code	O
can	O
run	O
as	O
fast	O
as	O
code	O
compiled	B-Language
in	O
conventional	O
languages	O
such	O
as	O
C	B-Language
.	O
</s>
<s>
Macros	O
expand	O
before	O
the	O
compilation	B-Language
step	O
,	O
and	O
thus	O
offer	O
some	O
interesting	O
options	O
.	O
</s>
<s>
If	O
a	O
program	O
needs	O
a	O
precomputed	O
table	O
,	O
then	O
a	O
macro	O
might	O
create	O
the	O
table	O
at	O
compile	B-Application
time	I-Application
,	O
so	O
the	O
compiler	B-Language
need	O
only	O
output	O
the	O
table	O
and	O
need	O
not	O
call	O
code	O
to	O
create	O
the	O
table	O
at	O
run	B-Library
time	I-Library
.	O
</s>
<s>
Some	O
Lisp	B-Language
implementations	O
even	O
have	O
a	O
mechanism	O
,	O
eval-when	O
,	O
that	O
allows	O
code	O
to	O
be	O
present	O
during	O
compile	B-Application
time	I-Application
(	O
when	O
a	O
macro	O
would	O
need	O
it	O
)	O
,	O
but	O
not	O
present	O
in	O
the	O
emitted	O
module	O
.	O
</s>
<s>
Lisp	B-Language
languages	I-Language
are	O
often	O
used	O
with	O
an	O
interactive	O
command	B-Application
line	I-Application
,	O
which	O
may	O
be	O
combined	O
with	O
an	O
integrated	B-Application
development	I-Application
environment	I-Application
(	O
IDE	O
)	O
.	O
</s>
<s>
The	O
user	O
types	O
in	O
expressions	O
at	O
the	O
command	B-Application
line	I-Application
,	O
or	O
directs	O
the	O
IDE	O
to	O
transmit	O
them	O
to	O
the	O
Lisp	B-Language
system	O
.	O
</s>
<s>
Lisp	B-Language
reads	O
the	O
entered	O
expressions	O
,	O
evaluates	O
them	O
,	O
and	O
prints	O
the	O
result	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	O
Lisp	B-Language
command	B-Application
line	I-Application
is	O
called	O
a	O
read	B-Application
–	I-Application
eval	I-Application
–	I-Application
print	I-Application
loop	I-Application
(	O
REPL	B-Application
)	O
.	O
</s>
<s>
The	O
basic	O
operation	O
of	O
the	O
REPL	B-Application
is	O
as	O
follows	O
.	O
</s>
<s>
This	O
is	O
a	O
simplistic	O
description	O
which	O
omits	O
many	O
elements	O
of	O
a	O
real	O
Lisp	B-Language
,	O
such	O
as	O
quoting	O
and	O
macros	O
.	O
</s>
<s>
The	O
function	O
accepts	O
textual	O
S-expressions	B-Protocol
as	O
input	O
,	O
and	O
parses	O
them	O
into	O
an	O
internal	O
data	B-General_Concept
structure	I-General_Concept
.	O
</s>
<s>
For	O
instance	O
,	O
if	O
you	O
type	O
the	O
text	O
at	O
the	O
prompt	O
,	O
translates	O
this	O
into	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
with	O
three	O
elements	O
:	O
the	O
symbol	O
,	O
the	O
number	O
1	O
,	O
and	O
the	O
number	O
2	O
.	O
</s>
<s>
It	O
so	O
happens	O
that	O
this	O
list	O
is	O
also	O
a	O
valid	O
piece	O
of	O
Lisp	B-Language
code	O
;	O
that	O
is	O
,	O
it	O
can	O
be	O
evaluated	O
.	O
</s>
<s>
The	O
function	O
evaluates	O
the	O
data	O
,	O
returning	O
zero	O
or	O
more	O
other	O
Lisp	B-Language
data	O
as	O
a	O
result	O
.	O
</s>
<s>
Evaluation	O
does	O
not	O
have	O
to	O
mean	O
interpretation	O
;	O
some	O
Lisp	B-Language
systems	O
compile	B-Language
every	O
expression	O
to	O
native	O
machine	B-Language
code	I-Language
.	O
</s>
<s>
An	O
expression	O
which	O
evaluated	O
to	O
a	O
piece	O
of	O
list	O
structure	O
would	O
require	O
that	O
traverse	O
the	O
list	O
and	O
print	O
it	O
out	O
as	O
an	O
S-expression	B-Protocol
.	O
</s>
<s>
To	O
implement	O
a	O
Lisp	B-Language
REPL	B-Application
,	O
it	O
is	O
necessary	O
only	O
to	O
implement	O
these	O
three	O
functions	O
and	O
an	O
infinite-loop	O
function	O
.	O
</s>
<s>
This	O
done	O
,	O
a	O
basic	O
REPL	B-Application
is	O
one	O
line	O
of	O
code	O
:	O
.	O
</s>
<s>
The	O
Lisp	B-Language
REPL	B-Application
typically	O
also	O
provides	O
input	O
editing	O
,	O
an	O
input	O
history	O
,	O
error	O
handling	O
and	O
an	O
interface	O
to	O
the	O
debugger	O
.	O
</s>
<s>
Lisp	B-Language
is	O
usually	O
evaluated	O
eagerly	O
.	O
</s>
<s>
In	O
Common	B-Language
Lisp	I-Language
,	O
arguments	O
are	O
evaluated	O
in	O
applicative	O
order	O
( 	O
 '	O
leftmost	O
innermost	O
 '	O
)	O
,	O
while	O
in	O
Scheme	B-Language
order	O
of	O
arguments	O
is	O
undefined	O
,	O
leaving	O
room	O
for	O
optimization	O
by	O
a	O
compiler	B-Language
.	O
</s>
<s>
Lisp	B-Language
originally	O
had	O
very	O
few	O
control	O
structures	O
,	O
but	O
many	O
more	O
were	O
added	O
during	O
the	O
language	O
's	O
evolution	O
.	O
</s>
<s>
(	O
Lisp	B-Language
's	O
original	O
conditional	B-Language
operator	O
,	O
,	O
is	O
the	O
precursor	O
to	O
later	O
structures	O
.	O
)	O
</s>
<s>
Programmers	O
in	O
the	O
Scheme	B-Language
dialect	O
often	O
express	O
loops	O
using	O
tail	B-Language
recursion	I-Language
.	O
</s>
<s>
Scheme	B-Language
's	O
commonality	O
in	O
academic	O
computer	B-General_Concept
science	I-General_Concept
has	O
led	O
some	O
students	O
to	O
believe	O
that	O
tail	B-Language
recursion	I-Language
is	O
the	O
only	O
,	O
or	O
the	O
most	O
common	O
,	O
way	O
to	O
write	O
iterations	B-Algorithm
in	O
Lisp	B-Language
,	O
but	O
this	O
is	O
incorrect	O
.	O
</s>
<s>
All	O
oft-seen	O
Lisp	B-Language
dialects	O
have	O
imperative-style	O
iteration	B-Algorithm
constructs	O
,	O
from	O
Scheme	B-Language
's	O
loop	O
to	O
Common	B-Language
Lisp	I-Language
's	O
complex	O
expressions	O
.	O
</s>
<s>
Moreover	O
,	O
the	O
key	O
issue	O
that	O
makes	O
this	O
an	O
objective	O
rather	O
than	O
subjective	O
matter	O
is	O
that	O
Scheme	B-Language
makes	O
specific	O
requirements	O
for	O
the	O
handling	O
of	O
tail	B-Language
calls	I-Language
,	O
and	O
thus	O
the	O
reason	O
that	O
the	O
use	O
of	O
tail	B-Language
recursion	I-Language
is	O
generally	O
encouraged	O
for	O
Scheme	B-Language
is	O
that	O
the	O
practice	O
is	O
expressly	O
supported	O
by	O
the	O
language	O
definition	O
.	O
</s>
<s>
By	O
contrast	O
,	O
ANSI	B-Language
Common	I-Language
Lisp	I-Language
does	O
not	O
require	O
the	O
optimization	O
commonly	O
termed	O
a	O
tail	B-Language
call	I-Language
elimination	I-Language
.	O
</s>
<s>
Thus	O
,	O
the	O
fact	O
that	O
tail	B-Language
recursive	I-Language
style	O
as	O
a	O
casual	O
replacement	O
for	O
the	O
use	O
of	O
more	O
traditional	O
iteration	B-Algorithm
constructs	O
(	O
such	O
as	O
,	O
or	O
)	O
is	O
discouraged	O
in	O
Common	B-Language
Lisp	I-Language
is	O
not	O
just	O
a	O
matter	O
of	O
stylistic	O
preference	O
,	O
but	O
potentially	O
one	O
of	O
efficiency	O
(	O
since	O
an	O
apparent	O
tail	B-Language
call	I-Language
in	O
Common	B-Language
Lisp	I-Language
may	O
not	O
compile	B-Language
as	O
a	O
simple	O
jump	B-General_Concept
)	O
and	O
program	O
correctness	O
(	O
since	O
tail	B-Language
recursion	I-Language
may	O
increase	O
stack	O
use	O
in	O
Common	B-Language
Lisp	I-Language
,	O
risking	O
stack	B-Error_Name
overflow	I-Error_Name
)	O
.	O
</s>
<s>
Some	O
Lisp	B-Language
control	O
structures	O
are	O
special	O
operators	O
,	O
equivalent	O
to	O
other	O
languages	O
 '	O
syntactic	O
keywords	O
.	O
</s>
<s>
Expressions	O
using	O
these	O
operators	O
have	O
the	O
same	O
surface	O
appearance	O
as	O
function	O
calls	O
,	O
but	O
differ	O
in	O
that	O
the	O
arguments	O
are	O
not	O
necessarily	O
evaluated	O
—	O
or	O
,	O
in	O
the	O
case	O
of	O
an	O
iteration	B-Algorithm
expression	O
,	O
may	O
be	O
evaluated	O
more	O
than	O
once	O
.	O
</s>
<s>
In	O
contrast	O
to	O
most	O
other	O
major	O
programming	O
languages	O
,	O
Lisp	B-Language
allows	O
implementing	O
control	O
structures	O
using	O
the	O
language	O
.	O
</s>
<s>
Several	O
control	O
structures	O
are	O
implemented	O
as	O
Lisp	B-Language
macros	O
,	O
and	O
can	O
even	O
be	O
macro-expanded	O
by	O
the	O
programmer	O
who	O
wants	O
to	O
know	O
how	O
they	O
work	O
.	O
</s>
<s>
Both	O
Common	B-Language
Lisp	I-Language
and	O
Scheme	B-Language
have	O
operators	O
for	O
non-local	O
control	O
flow	O
.	O
</s>
<s>
Scheme	B-Language
supports	O
re-entrant	O
continuations	O
using	O
the	O
procedure	O
,	O
which	O
allows	O
a	O
program	O
to	O
save	O
(	O
and	O
later	O
restore	O
)	O
a	O
particular	O
place	O
in	O
execution	O
.	O
</s>
<s>
Common	B-Language
Lisp	I-Language
does	O
not	O
support	O
re-entrant	O
continuations	O
,	O
but	O
does	O
support	O
several	O
ways	O
of	O
handling	O
escape	O
continuations	O
.	O
</s>
<s>
Often	O
,	O
the	O
same	O
algorithm	O
can	O
be	O
expressed	O
in	O
Lisp	B-Language
in	O
either	O
an	O
imperative	O
or	O
a	O
functional	B-Language
style	O
.	O
</s>
<s>
As	O
noted	O
above	O
,	O
Scheme	B-Language
tends	O
to	O
favor	O
the	O
functional	B-Language
style	O
,	O
using	O
tail	B-Language
recursion	I-Language
and	O
continuations	O
to	O
express	O
control	O
flow	O
.	O
</s>
<s>
The	O
style	O
preferred	O
by	O
many	O
Common	B-Language
Lisp	I-Language
programmers	O
may	O
seem	O
more	O
familiar	O
to	O
programmers	O
used	O
to	O
structured	O
languages	O
such	O
as	O
C	B-Language
,	O
while	O
that	O
preferred	O
by	O
Schemers	O
more	O
closely	O
resembles	O
pure-functional	O
languages	O
such	O
as	O
Haskell	B-Language
.	O
</s>
<s>
Because	O
of	O
Lisp	B-Language
's	O
early	O
heritage	O
in	O
list	B-Language
processing	I-Language
,	O
it	O
has	O
a	O
wide	O
array	O
of	O
higher-order	B-Language
functions	I-Language
relating	O
to	O
iteration	B-Algorithm
over	O
sequences	O
.	O
</s>
<s>
In	O
many	O
cases	O
where	O
an	O
explicit	O
loop	O
would	O
be	O
needed	O
in	O
other	O
languages	O
(	O
like	O
a	O
loop	O
in	O
C	B-Language
)	O
in	O
Lisp	B-Language
the	O
same	O
task	O
can	O
be	O
accomplished	O
with	O
a	O
higher-order	B-Language
function	I-Language
.	O
</s>
<s>
(	O
The	O
same	O
is	O
true	O
of	O
many	O
functional	B-Language
programming	I-Language
languages	I-Language
.	O
)	O
</s>
<s>
A	O
good	O
example	O
is	O
a	O
function	O
which	O
in	O
Scheme	B-Language
is	O
called	O
and	O
in	O
Common	B-Language
Lisp	I-Language
is	O
called	O
.	O
</s>
<s>
Here	O
are	O
examples	O
of	O
Common	B-Language
Lisp	I-Language
code	O
.	O
</s>
<s>
Lisp	B-Protocol
syntax	I-Protocol
lends	O
itself	O
naturally	O
to	O
recursion	O
.	O
</s>
<s>
An	O
alternative	O
implementation	O
takes	O
less	O
stack	O
space	O
than	O
the	O
previous	O
version	O
if	O
the	O
underlying	O
Lisp	B-Language
system	O
optimizes	O
tail	B-Language
recursion	I-Language
:	O
</s>
<s>
Contrast	O
the	O
examples	O
above	O
with	O
an	O
iterative	B-Algorithm
version	O
which	O
uses	O
Common	B-Language
Lisp	I-Language
's	O
macro	O
:	O
</s>
<s>
(	O
Lisp	B-Language
's	O
built-in	O
reverse	O
function	O
does	O
the	O
same	O
thing	O
.	O
)	O
</s>
<s>
Various	O
object	O
systems	O
and	O
models	O
have	O
been	O
built	O
on	O
top	O
of	O
,	O
alongside	O
,	O
or	O
into	O
Lisp	B-Language
,	O
including	O
:	O
</s>
<s>
The	O
Common	B-Application
Lisp	I-Application
Object	I-Application
System	I-Application
,	O
CLOS	B-Application
,	O
is	O
an	O
integral	O
part	O
of	O
ANSI	B-Language
Common	I-Language
Lisp	I-Language
.	O
</s>
<s>
CLOS	B-Application
descended	O
from	O
New	B-Language
Flavors	I-Language
and	O
CommonLOOPS	B-Language
.	O
</s>
<s>
ANSI	B-Language
Common	I-Language
Lisp	I-Language
was	O
the	O
first	O
standardized	O
object-oriented	O
programming	O
language	O
(	O
1994	O
,	O
ANSI	O
X3J13	B-Language
)	O
.	O
</s>
<s>
Flavors	B-Language
,	O
built	O
at	O
MIT	O
,	O
and	O
its	O
descendant	O
New	B-Language
Flavors	I-Language
(	O
developed	O
by	O
Symbolics	O
)	O
.	O
</s>
<s>
KR	O
(	O
short	O
for	O
Knowledge	O
Representation	O
)	O
,	O
a	O
constraints-based	O
object	O
system	O
developed	O
to	O
aid	O
the	O
writing	O
of	O
Garnet	O
,	O
a	O
GUI	O
library	O
for	O
Common	B-Language
Lisp	I-Language
.	O
</s>
<s>
Several	O
operating	B-General_Concept
systems	I-General_Concept
,	O
including	O
language-based	B-Operating_System
systems	I-Operating_System
,	O
are	O
based	O
on	O
Lisp	B-Language
(	O
use	O
Lisp	B-Language
features	O
,	O
conventions	O
,	O
methods	O
,	O
data	B-General_Concept
structures	I-General_Concept
,	O
etc	O
.	O
</s>
<s>
)	O
,	O
or	O
are	O
written	O
in	O
Lisp	B-Language
,	O
including	O
:	O
</s>
<s>
Genera	B-Application
,	O
renamed	O
Open	B-Application
Genera	I-Application
,	O
by	O
Symbolics	O
;	O
Medley	O
,	O
written	O
in	O
Interlisp	B-Language
,	O
originally	O
a	O
family	O
of	O
graphical	O
operating	B-General_Concept
systems	I-General_Concept
that	O
ran	O
on	O
Xerox	O
's	O
later	O
Star	B-Operating_System
workstations	B-Device
;	O
Mezzano	O
;	O
Interim	O
;	O
ChrysaLisp	O
,	O
by	O
developers	O
of	O
Tao	O
Systems	O
 '	O
TAOS	O
.	O
</s>
