<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
dynamic	B-Language
programming	I-Language
language	I-Language
is	O
a	O
class	O
of	O
high-level	B-Language
programming	I-Language
languages	I-Language
,	O
which	O
at	B-Library
runtime	I-Library
execute	O
many	O
common	O
programming	O
behaviours	O
that	O
static	O
programming	O
languages	O
perform	O
during	O
compilation	B-Language
.	O
</s>
<s>
Although	O
similar	O
behaviors	O
can	O
be	O
emulated	O
in	O
nearly	O
any	O
language	O
,	O
with	O
varying	O
degrees	O
of	O
difficulty	O
,	O
complexity	O
and	O
performance	O
costs	O
,	O
dynamic	B-Language
languages	I-Language
provide	O
direct	O
tools	O
to	O
make	O
use	O
of	O
them	O
.	O
</s>
<s>
Many	O
of	O
these	O
features	O
were	O
first	O
implemented	O
as	O
native	O
features	O
in	O
the	O
Lisp	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
Most	O
dynamic	B-Language
languages	I-Language
are	O
also	O
dynamically	O
typed	O
,	O
but	O
not	O
all	O
are	O
.	O
</s>
<s>
Dynamic	B-Language
languages	I-Language
are	O
frequently	O
(	O
but	O
not	O
always	O
)	O
referred	O
to	O
as	O
scripting	B-Language
languages	I-Language
,	O
although	O
that	O
term	O
in	O
its	O
narrowest	O
sense	O
refers	O
to	O
languages	O
specific	O
to	O
a	O
given	O
run-time	O
environment	O
.	O
</s>
<s>
Some	O
dynamic	B-Language
languages	I-Language
offer	O
an	O
eval	O
function	O
.	O
</s>
<s>
This	O
function	O
takes	O
a	O
string	O
or	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
containing	O
code	O
in	O
the	O
language	O
and	O
executes	O
it	O
.	O
</s>
<s>
Erik	O
Meijer	O
and	O
Peter	O
Drayton	O
distinguish	O
the	O
runtime	B-Application
code	I-Application
generation	I-Application
offered	O
by	O
eval	O
from	O
the	O
dynamic	B-Operating_System
loading	I-Operating_System
offered	O
by	O
shared	O
libraries	O
,	O
and	O
warn	O
that	O
in	O
many	O
cases	O
eval	O
is	O
used	O
merely	O
to	O
implement	O
higher-order	B-Language
functions	I-Language
(	O
by	O
passing	O
functions	O
as	O
strings	O
)	O
or	O
deserialization	B-Application
.	O
</s>
<s>
A	O
type	O
or	O
object	O
system	O
can	O
typically	O
be	O
modified	O
during	O
runtime	B-Library
in	O
a	O
dynamic	B-Language
language	I-Language
.	O
</s>
<s>
This	O
can	O
mean	O
generating	O
new	O
objects	O
from	O
a	O
runtime	B-Library
definition	O
or	O
based	O
on	O
mixins	B-Language
of	O
existing	O
types	O
or	O
objects	O
.	O
</s>
<s>
This	O
can	O
also	O
refer	O
to	O
changing	O
the	O
inheritance	B-Language
or	O
type	O
tree	O
,	O
and	O
thus	O
altering	O
the	O
way	O
that	O
existing	O
types	O
behave	O
(	O
especially	O
with	O
respect	O
to	O
the	O
invocation	O
of	O
methods	B-Language
)	O
.	O
</s>
<s>
As	O
a	O
lot	O
of	O
dynamic	B-Language
languages	I-Language
come	O
with	O
a	O
dynamic	O
type	O
system	O
,	O
runtime	B-Library
inference	O
of	O
types	O
based	O
on	O
values	O
for	O
internal	O
interpretation	O
marks	O
a	O
common	O
task	O
.	O
</s>
<s>
Static	O
programming	O
languages	O
(	O
possibly	O
indirectly	O
)	O
require	O
developers	O
to	O
define	O
the	O
size	O
of	O
utilized	O
memory	O
before	O
compilation	B-Language
(	O
unless	O
working	O
around	O
with	O
pointer	O
logic	O
)	O
.	O
</s>
<s>
Consistent	O
with	O
object	O
runtime	B-Library
alteration	O
,	O
dynamic	B-Language
languages	I-Language
implicitly	O
need	O
to	O
(	O
re	O
-	O
)	O
allocate	O
memory	O
based	O
on	O
program	O
individual	O
operations	O
.	O
</s>
<s>
Reflection	B-Language
is	O
common	O
in	O
many	O
dynamic	B-Language
languages	I-Language
,	O
and	O
typically	O
involves	O
analysis	B-Application
of	O
the	O
types	O
and	O
metadata	O
of	O
generic	O
or	O
polymorphic	B-Application
data	O
.	O
</s>
<s>
It	O
can	O
,	O
however	O
,	O
also	O
include	O
full	O
evaluation	O
and	O
modification	O
of	O
a	O
program	O
's	O
code	O
as	O
data	O
,	O
such	O
as	O
the	O
features	O
that	O
Lisp	B-Language
provides	O
in	O
analyzing	O
S-expressions	B-Protocol
.	O
</s>
<s>
A	O
limited	O
number	O
of	O
dynamic	B-Language
programming	I-Language
languages	I-Language
provide	O
features	O
which	O
combine	O
code	B-Application
introspection	I-Application
(	O
the	O
ability	O
to	O
examine	O
classes	O
,	O
functions	O
,	O
and	O
keywords	O
to	O
know	O
what	O
they	O
are	O
,	O
what	O
they	O
do	O
and	O
what	O
they	O
know	O
)	O
and	O
eval	O
in	O
a	O
feature	O
called	O
macros	O
.	O
</s>
<s>
Most	O
programmers	O
today	O
who	O
are	O
aware	O
of	O
the	O
term	O
macro	O
have	O
encountered	O
them	O
in	O
C	B-Language
or	O
C++	B-Language
,	O
where	O
they	O
are	O
a	O
static	O
feature	O
which	O
is	O
built	O
in	O
a	O
small	O
subset	O
of	O
the	O
language	O
,	O
and	O
are	O
capable	O
only	O
of	O
string	O
substitutions	O
on	O
the	O
text	O
of	O
the	O
program	O
.	O
</s>
<s>
In	O
dynamic	B-Language
languages	I-Language
,	O
however	O
,	O
they	O
provide	O
access	O
to	O
the	O
inner	O
workings	O
of	O
the	O
compiler	B-Language
,	O
and	O
full	O
access	O
to	O
the	O
interpreter	O
,	O
virtual	O
machine	O
,	O
or	O
runtime	B-Library
,	O
allowing	O
the	O
definition	O
of	O
language-like	O
constructs	O
which	O
can	O
optimize	O
code	O
or	O
modify	O
the	O
syntax	O
or	O
grammar	O
of	O
the	O
language	O
.	O
</s>
<s>
Assembly	B-Language
,	O
C	B-Language
,	O
C++	B-Language
,	O
early	O
Java	B-Language
,	O
and	O
Fortran	B-Application
do	O
not	O
generally	O
fit	O
into	O
this	O
category	O
.	O
</s>
<s>
The	O
following	O
examples	O
show	O
dynamic	O
features	O
using	O
the	O
language	O
Common	B-Language
Lisp	I-Language
and	O
its	O
Common	B-Application
Lisp	I-Application
Object	I-Application
System	I-Application
(	O
CLOS	B-Application
)	O
.	O
</s>
<s>
The	O
print	O
method	O
gets	O
redefined	O
such	O
that	O
it	O
assembles	O
several	O
methods	B-Language
into	O
the	O
effective	O
method	O
.	O
</s>
<s>
The	O
effective	O
method	O
gets	O
assembled	O
based	O
on	O
the	O
class	O
of	O
the	O
argument	O
and	O
the	O
at	B-Library
runtime	I-Library
available	O
and	O
applicable	O
methods	B-Language
.	O
</s>
<s>
Popular	O
dynamic	B-Language
programming	I-Language
languages	I-Language
include	O
JavaScript	B-Language
,	O
Python	B-Language
,	O
Ruby	B-Language
,	O
PHP	B-Application
,	O
Lua	B-Language
and	O
Perl	B-Language
.	O
</s>
<s>
The	O
following	O
are	O
generally	O
considered	O
dynamic	B-Language
languages	I-Language
:	O
</s>
