<s>
Extensible	B-Application
programming	I-Application
is	O
a	O
term	O
used	O
in	O
computer	B-General_Concept
science	I-General_Concept
to	O
describe	O
a	O
style	O
of	O
computer	O
programming	O
that	O
focuses	O
on	O
mechanisms	O
to	O
extend	O
the	O
programming	O
language	O
,	O
compiler	B-Language
and	O
runtime	B-Device
environment	I-Device
.	O
</s>
<s>
Extensible	B-Application
programming	I-Application
languages	I-Application
,	O
supporting	O
this	O
style	O
of	O
programming	O
,	O
were	O
an	O
active	O
area	O
of	O
work	O
in	O
the	O
1960s	O
,	O
but	O
the	O
movement	O
was	O
marginalized	O
in	O
the	O
1970s	O
.	O
</s>
<s>
Extensible	B-Application
programming	I-Application
has	O
become	O
a	O
topic	O
of	O
renewed	O
interest	O
in	O
the	O
21st	O
century	O
.	O
</s>
<s>
The	O
first	O
paper	O
usually	O
associated	O
with	O
the	O
extensible	B-Application
programming	I-Application
language	I-Application
movement	O
is	O
M	O
.	O
Douglas	O
McIlroy	O
's	O
1960	O
paper	O
on	O
macros	O
for	O
higher-level	O
programming	O
languages	O
.	O
</s>
<s>
Another	O
early	O
description	O
of	O
the	O
principle	O
of	O
extensibility	O
occurs	O
in	O
Brooker	O
and	O
Morris	O
's	O
1960	O
paper	O
on	O
the	O
Compiler-Compiler	B-Language
.	O
</s>
<s>
The	O
Forth	B-Application
programming	I-Application
language	I-Application
was	O
an	O
exception	O
,	O
but	O
it	O
went	O
essentially	O
unnoticed	O
.	O
</s>
<s>
As	O
typically	O
envisioned	O
,	O
an	O
extensible	B-Application
programming	I-Application
language	I-Application
consisted	O
of	O
a	O
base	O
language	O
providing	O
elementary	O
computing	O
facilities	O
,	O
and	O
a	O
meta-language	O
capable	O
of	O
modifying	O
the	O
base	O
language	O
.	O
</s>
<s>
The	O
Lisp	B-Language
language	I-Language
community	O
remained	O
separate	O
from	O
the	O
extensible	B-Application
language	I-Application
community	O
,	O
apparently	O
because	O
,	O
as	O
one	O
researcher	O
observed	O
,	O
</s>
<s>
...	O
this	O
can	O
be	O
seen	O
very	O
easily	O
from	O
the	O
fact	O
that	O
Lisp	B-Language
has	O
been	O
used	O
as	O
an	O
extendible	O
language	O
for	O
years.Harrison	O
,	O
M.C.	O
,	O
in	O
"	O
Panel	O
on	O
the	O
Concept	O
of	O
Extensibility	O
"	O
,	O
pp	O
.	O
</s>
<s>
At	O
the	O
1969	O
conference	O
,	O
Simula	B-Language
was	O
presented	O
as	O
an	O
extensible	B-Application
programming	I-Application
language	I-Application
.	O
</s>
<s>
Standish	O
described	O
three	O
classes	O
of	O
language	O
extension	O
,	O
which	O
he	O
called	O
paraphrase	B-General_Concept
,	O
orthophrase	O
,	O
and	O
metaphrase	O
(	O
otherwise	O
paraphrase	B-General_Concept
and	O
metaphrase	O
being	O
translation	O
terms	O
)	O
.	O
</s>
<s>
Paraphrase	B-General_Concept
defines	O
a	O
facility	O
by	O
showing	O
how	O
to	O
exchange	O
it	O
for	O
something	O
previously	O
defined	O
(	O
or	O
to	O
be	O
defined	O
)	O
.	O
</s>
<s>
Orthophrase	O
corresponds	O
to	O
the	O
modern	O
notion	O
of	O
plug-ins	B-Application
.	O
</s>
<s>
It	O
corresponds	O
to	O
the	O
modern	O
notion	O
of	O
reflection	B-Language
.	O
</s>
<s>
(	O
Note	O
that	O
shielding	O
the	O
programmer	O
from	O
lower-level	O
details	O
is	O
the	O
intent	O
of	O
the	O
abstraction	B-Application
movement	O
that	O
supplanted	O
the	O
extensibility	O
movement	O
.	O
)	O
</s>
<s>
Despite	O
the	O
earlier	O
presentation	O
of	O
Simula	B-Language
as	O
extensible	O
,	O
by	O
1975	O
,	O
Standish	O
's	O
survey	O
does	O
not	O
seem	O
in	O
practice	O
to	O
have	O
included	O
the	O
newer	O
abstraction-based	O
technologies	O
(	O
though	O
he	O
used	O
a	O
very	O
general	O
definition	O
of	O
extensibility	O
that	O
technically	O
could	O
have	O
included	O
them	O
)	O
.	O
</s>
<s>
A	O
1978	O
history	O
of	O
programming	O
abstraction	B-Application
from	O
the	O
invention	O
of	O
the	O
computer	O
to	O
the	O
(	O
then	O
)	O
present	O
day	O
made	O
no	O
mention	O
of	O
macros	O
,	O
and	O
gave	O
no	O
hint	O
that	O
the	O
extensible	B-Application
languages	I-Application
movement	O
had	O
ever	O
occurred	O
.	O
</s>
<s>
Macros	O
were	O
tentatively	O
admitted	O
into	O
the	O
abstraction	B-Application
movement	O
by	O
the	O
late	O
1980s	O
(	O
perhaps	O
due	O
to	O
the	O
advent	O
of	O
hygienic	B-Application
macros	I-Application
)	O
,	O
by	O
being	O
granted	O
the	O
pseudonym	O
syntactic	O
abstractions	B-Application
.	O
</s>
<s>
In	O
the	O
modern	O
sense	O
,	O
a	O
system	O
that	O
supports	O
extensible	B-Application
programming	I-Application
will	O
provide	O
all	O
of	O
the	O
features	O
described	O
below	O
.	O
</s>
<s>
This	O
simply	O
means	O
that	O
the	O
source	O
language(s )	O
to	O
be	O
compiled	B-Language
must	O
not	O
be	O
closed	O
,	O
fixed	O
,	O
or	O
static	O
.	O
</s>
<s>
Languages	O
which	O
allow	O
the	O
addition	O
of	O
constructs	O
with	O
user	O
defined	O
syntax	O
include	O
Racket	B-Operating_System
,	O
Camlp4	B-Language
,	O
OpenC++	O
,	O
Seed7	B-Language
,	O
Red	B-Language
,	O
Rebol	B-Application
,	O
and	O
Felix	O
.	O
</s>
<s>
In	O
extensible	B-Application
programming	I-Application
,	O
a	O
compiler	B-Language
is	O
not	O
a	O
monolithic	O
program	O
that	O
converts	O
source	O
code	O
input	O
into	O
binary	O
executable	O
output	O
.	O
</s>
<s>
The	O
compiler	B-Language
itself	O
must	O
be	O
extensible	O
to	O
the	O
point	O
that	O
it	O
is	O
really	O
a	O
collection	O
of	O
plugins	B-Application
that	O
assist	O
with	O
the	O
translation	O
of	O
source	O
language	O
input	O
into	O
anything	O
.	O
</s>
<s>
For	O
example	O
,	O
an	O
extensible	O
compiler	B-Language
will	O
support	O
the	O
generation	O
of	O
object	O
code	O
,	O
code	O
documentation	O
,	O
re-formatted	O
source	O
code	O
,	O
or	O
any	O
other	O
desired	O
output	O
.	O
</s>
<s>
The	O
architecture	O
of	O
the	O
compiler	B-Language
must	O
permit	O
its	O
users	O
to	O
"	O
get	O
inside	O
"	O
the	O
compilation	B-Language
process	O
and	O
provide	O
alternative	O
processing	O
tasks	O
at	O
every	O
reasonable	O
step	O
in	O
the	O
compilation	B-Language
process	O
.	O
</s>
<s>
For	O
just	O
the	O
task	O
of	O
translating	O
source	O
code	O
into	O
something	O
that	O
can	O
be	O
executed	O
on	O
a	O
computer	O
,	O
an	O
extensible	O
compiler	B-Language
should	O
:	O
</s>
<s>
allow	O
new	O
kinds	O
of	O
nodes	O
in	O
the	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
(	O
AST	O
)	O
,	O
</s>
<s>
At	O
runtime	O
,	O
extensible	B-Application
programming	I-Application
systems	O
must	O
permit	O
languages	O
to	O
extend	O
the	O
set	O
of	O
operations	O
that	O
it	O
permits	O
.	O
</s>
<s>
Extensible	B-Application
programming	I-Application
systems	O
should	O
regard	O
programs	O
as	O
data	O
to	O
be	O
processed	O
.	O
</s>
<s>
The	O
visual	O
display	O
and	O
editing	O
of	O
programs	O
to	O
users	O
should	O
be	O
a	O
translation	O
function	O
,	O
supported	O
by	O
the	O
extensible	O
compiler	B-Language
,	O
that	O
translates	O
the	O
program	O
data	O
into	O
forms	O
more	O
suitable	O
for	O
viewing	O
or	O
editing	O
.	O
</s>
<s>
Extensible	B-Application
programming	I-Application
systems	O
must	O
support	O
the	O
debugging	O
of	O
programs	O
using	O
the	O
constructs	O
of	O
the	O
original	O
source	O
language	O
regardless	O
of	O
the	O
extensions	O
or	O
transformation	O
the	O
program	O
has	O
undergone	O
in	O
order	O
to	O
make	O
it	O
executable	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
language	O
supports	O
a	O
data	O
structure	O
for	O
a	O
business	O
process	O
or	O
work	B-Protocol
flow	I-Protocol
,	O
it	O
must	O
be	O
possible	O
for	O
the	O
debugger	O
to	O
display	O
that	O
data	O
structure	O
as	O
a	O
fishbone	O
chart	O
or	O
other	O
form	O
provided	O
by	O
a	O
plugin	B-Application
.	O
</s>
