<s>
Smalltalk	B-Application
is	O
a	O
purely	O
object	B-Language
oriented	I-Language
programming	I-Language
language	I-Language
(	O
OOP	O
)	O
,	O
created	O
in	O
the	O
1970s	O
for	O
educational	O
use	O
,	O
specifically	O
for	O
constructionist	O
learning	O
,	O
at	O
Xerox	O
PARC	O
by	O
Learning	O
Research	O
Group	O
(	O
LRG	O
)	O
scientists	O
,	O
including	O
Alan	O
Kay	O
,	O
Dan	O
Ingalls	O
,	O
Adele	O
Goldberg	O
,	O
Ted	O
Kaehler	O
,	O
Diana	O
Merry	O
,	O
and	O
Scott	O
Wallace	O
.	O
</s>
<s>
In	O
Smalltalk	B-Application
,	O
executing	O
programs	O
are	O
built	O
of	O
opaque	O
,	O
atomic	O
,	O
so-called	O
objects	O
,	O
which	O
are	O
instances	O
of	O
template-code	O
stored	O
in	O
classes	O
,	O
and	O
these	O
objects	O
intercommunicate	O
by	O
passing	O
of	O
messages	O
,	O
via	O
an	O
intermediary	O
virtual	B-Architecture
machine	I-Architecture
environment	O
(	O
VM	O
)	O
.	O
</s>
<s>
A	O
relatively	O
small	O
number	O
of	O
objects	O
,	O
called	O
primitives	O
,	O
are	O
not	O
amenable	O
to	O
live	O
redefinition	O
,	O
sometimes	O
being	O
defined	O
independently	O
of	O
the	O
Smalltalk	B-Application
programming	B-Application
environment	I-Application
.	O
</s>
<s>
Having	O
undergone	O
significant	O
industry	O
development	O
toward	O
other	O
uses	O
,	O
including	O
business	O
and	O
database	O
functions	O
,	O
Smalltalk	B-Application
is	O
still	O
in	O
use	O
today	O
.	O
</s>
<s>
When	O
first	O
publicly	O
released	O
,	O
Smalltalk-80	B-Application
presented	O
innovative	O
and	O
foundational	O
ideas	O
for	O
the	O
nascent	O
field	O
of	O
object-oriented	B-Language
programming	I-Language
(	O
OOP	O
)	O
.	O
</s>
<s>
Since	O
inception	O
,	O
the	O
language	O
provided	O
interactive	B-Application
programming	I-Application
via	O
an	O
integrated	B-Application
development	I-Application
environment	I-Application
.	O
</s>
<s>
This	O
requires	O
reflection	B-Language
and	O
late	O
binding	O
in	O
the	O
language	O
execution	B-Application
of	I-Application
code	I-Application
.	O
</s>
<s>
Later	O
development	O
has	O
led	O
to	O
at	O
least	O
one	O
instance	O
of	O
Smalltalk	B-Application
execution	O
environment	O
which	O
lacks	O
such	O
an	O
integrated	O
graphical	B-Application
user	I-Application
interface	I-Application
or	O
front-end	O
.	O
</s>
<s>
Smalltalk-like	O
languages	O
are	O
in	O
active	O
development	O
and	O
have	O
gathered	O
loyal	O
communities	O
of	O
users	O
around	O
them	O
.	O
</s>
<s>
ANSI	O
Smalltalk	B-Application
was	O
ratified	O
in	O
1998	O
and	O
represents	O
the	O
standard	O
version	O
of	O
Smalltalk	B-Application
.	O
</s>
<s>
Smalltalk	B-Application
took	O
second	O
place	O
for	O
"	O
most	O
loved	O
programming	O
language	O
"	O
in	O
the	O
Stack	B-Error_Name
Overflow	I-Error_Name
Developer	O
Survey	O
in	O
2017	O
,	O
but	O
it	O
was	O
not	O
among	O
the	O
26	O
most	O
loved	O
programming	O
languages	O
of	O
the	O
2018	O
survey	O
.	O
</s>
<s>
There	O
are	O
a	O
large	O
number	O
of	O
Smalltalk	B-Application
variants	O
.	O
</s>
<s>
The	O
unqualified	O
word	O
Smalltalk	B-Application
is	O
often	O
used	O
to	O
indicate	O
the	O
Smalltalk-80	B-Application
language	O
and	O
compatible	O
VM	O
,	O
the	O
first	O
version	O
to	O
be	O
made	O
publicly	O
available	O
and	O
created	O
in	O
1980	O
.	O
</s>
<s>
The	O
first	O
hardware-environments	O
which	O
ran	O
the	O
Smalltalk	B-Application
VMs	O
were	O
Xerox	B-Device
Alto	I-Device
computers	O
.	O
</s>
<s>
Smalltalk	B-Application
was	O
the	O
product	O
of	O
research	O
led	O
by	O
Alan	O
Kay	O
at	O
Xerox	O
Palo	O
Alto	B-Device
Research	O
Center	O
(	O
PARC	O
)	O
;	O
Alan	O
Kay	O
designed	O
most	O
of	O
the	O
early	O
Smalltalk	B-Application
versions	O
,	O
Adele	O
Goldberg	O
wrote	O
most	O
of	O
the	O
documentation	O
,	O
and	O
Dan	O
Ingalls	O
implemented	O
most	O
of	O
the	O
early	O
versions	O
.	O
</s>
<s>
The	O
first	O
version	O
,	O
termed	O
Smalltalk-71	O
,	O
was	O
created	O
by	O
Kay	O
in	O
a	O
few	O
mornings	O
on	O
a	O
bet	O
that	O
a	O
programming	O
language	O
based	O
on	O
the	O
idea	O
of	O
message	B-Architecture
passing	I-Architecture
inspired	O
by	O
Simula	B-Language
could	O
be	O
implemented	O
in	O
"	O
a	O
page	O
of	O
code	O
"	O
.	O
</s>
<s>
A	O
later	O
variant	O
used	O
for	O
research	O
work	O
is	O
now	O
termed	O
Smalltalk-72	B-Application
and	O
influenced	O
the	O
development	O
of	O
the	O
Actor	B-Application
model	I-Application
.	O
</s>
<s>
Its	O
syntax	O
and	O
execution	O
model	O
were	O
very	O
different	O
from	O
modern	O
Smalltalk	B-Application
variants	O
.	O
</s>
<s>
After	O
significant	O
revisions	O
which	O
froze	O
some	O
aspects	O
of	O
execution	O
semantics	O
to	O
gain	O
performance	O
(	O
by	O
adopting	O
a	O
Simula-like	O
class	B-Language
inheritance	I-Language
model	O
of	O
execution	O
)	O
,	O
Smalltalk-76	O
was	O
created	O
.	O
</s>
<s>
This	O
system	O
had	O
a	O
development	B-Application
environment	I-Application
featuring	O
most	O
of	O
the	O
now	O
familiar	O
tools	O
,	O
including	O
a	O
class	O
library	O
code	O
browser/editor	O
.	O
</s>
<s>
Smalltalk-80	B-Application
added	O
metaclasses	B-Language
,	O
to	O
help	O
maintain	O
the	O
"	O
everything	O
is	O
an	O
object	O
"	O
(	O
except	O
variables	O
)	O
paradigm	O
by	O
associating	O
properties	O
and	O
behavior	O
with	O
individual	O
classes	O
,	O
and	O
even	O
primitives	O
such	O
as	O
integer	O
and	O
boolean	O
values	O
(	O
for	O
example	O
,	O
to	O
support	O
different	O
ways	O
to	O
create	O
instances	O
)	O
.	O
</s>
<s>
Smalltalk-80	B-Application
was	O
the	O
first	O
language	O
variant	O
made	O
available	O
outside	O
of	O
PARC	O
,	O
first	O
as	O
Smalltalk-80	B-Application
Version	O
1	O
,	O
being	O
given	O
to	O
a	O
small	O
number	O
of	O
firms	O
(	O
Hewlett-Packard	O
,	O
Apple	O
Computer	O
,	O
Tektronix	O
,	O
and	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
)	O
and	O
universities	O
(	O
UC	O
Berkeley	O
)	O
for	O
peer	O
review	O
and	O
implementing	O
on	O
their	O
platforms	O
.	O
</s>
<s>
Later	O
,	O
in	O
1983	O
,	O
a	O
general	O
availability	O
implementation	O
,	O
named	O
Smalltalk-80	B-Application
Version	O
2	O
,	O
was	O
released	O
as	O
an	O
image	B-Operating_System
(	O
platform-independent	B-Operating_System
file	O
with	O
object	O
definitions	O
)	O
and	O
a	O
virtual	B-Architecture
machine	I-Architecture
specification	O
.	O
</s>
<s>
ANSI	O
Smalltalk	B-Application
has	O
been	O
the	O
standard	O
language	O
reference	O
since	O
1998	O
.	O
</s>
<s>
Two	O
currently	O
popular	O
Smalltalk	B-Application
implementation	O
variants	O
are	O
descendants	O
of	O
those	O
original	O
Smalltalk-80	B-Application
images	O
.	O
</s>
<s>
Squeak	B-Operating_System
is	O
an	O
open	B-Application
source	I-Application
implementation	O
derived	O
from	O
Smalltalk-80	B-Application
Version	O
1	O
by	O
way	O
of	O
Apple	O
Smalltalk	B-Application
.	O
</s>
<s>
VisualWorks	B-Application
is	O
derived	O
from	O
Smalltalk-80	B-Application
version	O
2	O
by	O
way	O
of	O
Smalltalk-80	B-Application
2.5	O
and	O
ObjectWorks	O
(	O
both	O
products	O
of	O
ParcPlace	O
Systems	O
,	O
a	O
Xerox	O
PARC	O
spin-off	O
company	O
formed	O
to	O
bring	O
Smalltalk	B-Application
to	O
the	O
market	O
)	O
.	O
</s>
<s>
As	O
an	O
interesting	O
link	O
between	O
generations	O
,	O
in	O
2001	O
Vassili	O
Bykov	O
implemented	O
Hobbes	O
,	O
a	O
virtual	B-Architecture
machine	I-Architecture
running	O
Smalltalk-80	B-Application
inside	O
VisualWorks	B-Application
.	O
</s>
<s>
(	O
Dan	O
Ingalls	O
later	O
ported	O
Hobbes	O
to	O
Squeak	B-Operating_System
.	O
)	O
</s>
<s>
During	O
the	O
late	O
1980s	O
to	O
mid-1990s	O
,	O
Smalltalk	B-Application
environments	O
—	O
including	O
support	O
,	O
training	O
and	O
add-ons	O
—	O
were	O
sold	O
by	O
two	O
competing	O
organizations	O
:	O
ParcPlace	O
Systems	O
and	O
Digitalk	O
,	O
both	O
California	O
based	O
.	O
</s>
<s>
Both	O
firms	O
struggled	O
to	O
take	O
Smalltalk	B-Application
mainstream	O
due	O
to	O
Smalltalk	B-Application
's	O
substantial	O
memory	O
needs	O
,	O
limited	O
run-time	O
performance	O
,	O
and	O
initial	O
lack	O
of	O
supported	O
connectivity	O
to	O
SQL-based	O
relational	B-Application
database	I-Application
servers	O
.	O
</s>
<s>
While	O
the	O
high	O
price	O
of	O
ParcPlace	O
Smalltalk	B-Application
limited	O
its	O
market	O
penetration	O
to	O
mid-sized	O
and	O
large	O
commercial	O
organizations	O
,	O
the	O
Digitalk	O
products	O
initially	O
tried	O
to	O
reach	O
a	O
wider	O
audience	O
with	O
a	O
lower	O
price	O
.	O
</s>
<s>
IBM	O
initially	O
supported	O
the	O
Digitalk	O
product	O
,	O
but	O
then	O
entered	O
the	O
market	O
with	O
a	O
Smalltalk	B-Application
product	O
in	O
1995	O
called	O
VisualAge/Smalltalk	O
.	O
</s>
<s>
Enfin	O
became	O
far	O
more	O
popular	O
in	O
Europe	O
,	O
as	O
IBM	O
introduced	O
it	O
into	O
IT	O
shops	O
before	O
their	O
development	O
of	O
IBM	O
Smalltalk	B-Application
(	O
later	O
VisualAge	B-Language
)	O
.	O
</s>
<s>
Enfin	O
was	O
later	O
acquired	O
by	O
Cincom	B-Application
Systems	I-Application
,	O
and	O
is	O
now	O
sold	O
under	O
the	O
name	O
ObjectStudio	O
,	O
and	O
is	O
part	O
of	O
the	O
Cincom	B-Application
Smalltalk	B-Application
product	O
suite	O
.	O
</s>
<s>
The	O
merged	O
firm	O
never	O
managed	O
to	O
find	O
an	O
effective	O
response	O
to	O
Java	B-Language
as	O
to	O
market	O
positioning	O
,	O
and	O
by	O
1997	O
its	O
owners	O
were	O
looking	O
to	O
sell	O
the	O
business	O
.	O
</s>
<s>
In	O
1999	O
,	O
Seagull	O
Software	O
acquired	O
the	O
ObjectShare	O
Java	B-Language
development	O
lab	O
(	O
including	O
the	O
original	O
Smalltalk/V	B-Application
and	O
Visual	B-Application
Smalltalk	I-Application
development	O
team	O
)	O
,	O
and	O
still	O
owns	O
VisualSmalltalk	O
,	O
although	O
worldwide	O
distribution	O
rights	O
for	O
the	O
Smalltalk	B-Application
product	O
remained	O
with	O
ObjectShare	O
who	O
then	O
sold	O
them	O
to	O
Cincom	B-Application
.	O
</s>
<s>
VisualWorks	B-Application
was	O
sold	O
to	O
Cincom	B-Application
and	O
is	O
now	O
part	O
of	O
Cincom	B-Application
Smalltalk	B-Application
.	O
</s>
<s>
Cincom	B-Application
has	O
backed	O
Smalltalk	B-Application
strongly	O
,	O
releasing	O
multiple	O
new	O
versions	O
of	O
VisualWorks	B-Application
and	O
ObjectStudio	O
each	O
year	O
since	O
1999	O
.	O
</s>
<s>
Cincom	B-Application
,	O
GemTalk	O
,	O
and	O
Instantiations	O
,	O
continue	O
to	O
sell	O
Smalltalk	B-Application
environments	O
.	O
</s>
<s>
IBM	O
has	O
'	O
end	O
of	O
life	O
'd	O
VisualAge	B-Language
Smalltalk	B-Application
having	O
in	O
the	O
late	O
1990s	O
decided	O
to	O
back	O
Java	B-Language
instead	O
and	O
it	O
is	O
,	O
,	O
supported	O
by	O
Instantiations	O
,	O
Inc	O
.	O
who	O
renamed	O
the	O
product	O
VA	B-Language
Smalltalk	I-Language
(	O
VAST	O
Platform	O
)	O
and	O
continue	O
to	O
release	O
new	O
versions	O
yearly	O
.	O
</s>
<s>
The	O
open	O
Squeak	B-Operating_System
implementation	O
has	O
an	O
active	O
community	O
of	O
developers	O
,	O
including	O
many	O
of	O
the	O
original	O
Smalltalk	B-Application
community	O
,	O
and	O
has	O
recently	O
been	O
used	O
to	O
provide	O
the	O
Etoys	B-Application
environment	O
on	O
the	O
OLPC	O
project	O
,	O
a	O
toolkit	O
for	O
developing	O
collaborative	O
applications	O
Croquet	B-Protocol
Project	I-Protocol
,	O
and	O
the	O
Open	B-Protocol
Cobalt	I-Protocol
virtual	O
world	O
application	O
.	O
</s>
<s>
GNU	B-Application
Smalltalk	I-Application
is	O
a	O
free	B-Application
software	I-Application
implementation	O
of	O
a	O
derivative	O
of	O
Smalltalk-80	B-Application
from	O
the	O
GNU	B-Operating_System
project	O
.	O
</s>
<s>
Pharo	B-Language
Smalltalk	B-Application
is	O
a	O
fork	O
of	O
Squeak	B-Operating_System
oriented	O
toward	O
research	O
and	O
use	O
in	O
commercial	O
environments	O
.	O
</s>
<s>
A	O
significant	O
development	O
,	O
that	O
has	O
spread	O
across	O
all	O
Smalltalk	B-Application
environments	O
as	O
of	O
2016	O
,	O
is	O
the	O
increasing	O
usage	O
of	O
two	O
web	O
frameworks	O
,	O
Seaside	B-Application
and	O
AIDA/Web	B-Protocol
,	O
to	O
simplify	O
the	O
building	O
of	O
complex	O
web	O
applications	O
.	O
</s>
<s>
Seaside	B-Application
has	O
seen	O
considerable	O
market	O
interest	O
with	O
Cincom	B-Application
,	O
Gemstone	B-Operating_System
,	O
and	O
Instantiations	O
incorporating	O
and	O
extending	O
it	O
.	O
</s>
<s>
Smalltalk	B-Application
was	O
one	O
of	O
many	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
based	O
on	O
Simula	B-Language
.	O
</s>
<s>
Smalltalk	B-Application
is	O
also	O
one	O
of	O
the	O
most	O
influential	O
programming	O
languages	O
.	O
</s>
<s>
Virtually	O
all	O
of	O
the	O
object-oriented	B-Language
languages	I-Language
that	O
came	O
after	O
—	O
Flavors	B-Language
,	O
CLOS	B-Application
,	O
Objective-C	B-Language
,	O
Java	B-Language
,	O
Python	B-Language
,	O
Ruby	B-Language
,	O
and	O
many	O
others	O
—	O
were	O
influenced	O
by	O
Smalltalk	B-Application
.	O
</s>
<s>
Smalltalk	B-Application
was	O
also	O
one	O
of	O
the	O
most	O
popular	O
languages	O
for	O
agile	O
software	O
development	O
methods	B-Language
,	O
rapid	O
application	O
development	O
(	O
RAD	O
)	O
or	O
prototyping	O
,	O
and	O
software	O
design	O
patterns	O
.	O
</s>
<s>
The	O
highly	O
productive	O
environment	O
provided	O
by	O
Smalltalk	B-Application
platforms	O
made	O
them	O
ideal	O
for	O
rapid	O
,	O
iterative	O
development	O
.	O
</s>
<s>
Smalltalk	B-Application
emerged	O
from	O
a	O
larger	O
program	O
of	O
Advanced	O
Research	O
Projects	O
Agency	O
(	O
ARPA	O
)	O
funded	O
research	O
that	O
in	O
many	O
ways	O
defined	O
the	O
modern	O
world	O
of	O
computing	O
.	O
</s>
<s>
In	O
addition	O
to	O
Smalltalk	B-Application
,	O
working	O
prototypes	O
of	O
things	O
such	O
as	O
hypertext	O
,	O
GUIs	B-Application
,	O
multimedia	O
,	O
the	O
mouse	O
,	O
telepresence	B-Application
,	O
and	O
the	O
Internet	O
were	O
developed	O
by	O
ARPA	O
researchers	O
in	O
the	O
1960s	O
.	O
</s>
<s>
Alan	O
Kay	O
(	O
one	O
of	O
the	O
inventors	O
of	O
Smalltalk	B-Application
)	O
also	O
described	O
a	O
tablet	O
computer	O
he	O
called	O
the	O
Dynabook	B-General_Concept
which	O
resembles	O
modern	O
tablet	O
computers	O
like	O
the	O
iPad	O
.	O
</s>
<s>
Smalltalk	B-Application
environments	O
were	O
often	O
the	O
first	O
to	O
develop	O
what	O
are	O
now	O
common	O
object-oriented	B-Language
software	O
design	O
patterns	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
popular	O
is	O
the	O
model	O
–	O
view	O
–	O
controller	O
(	O
MVC	O
)	O
pattern	O
for	O
user	B-Application
interface	I-Application
design	O
.	O
</s>
<s>
It	O
's	O
ideal	O
for	O
software	B-Application
development	I-Application
environments	I-Application
,	O
where	O
there	O
are	O
various	O
views	O
(	O
e.g.	O
,	O
entity-relation	O
,	O
dataflow	O
,	O
object	O
model	O
,	O
etc	O
.	O
)	O
</s>
<s>
Also	O
,	O
for	O
simulations	O
or	O
games	O
where	O
the	O
underlying	O
model	O
may	O
be	O
viewed	O
from	O
various	O
angles	O
and	O
levels	O
of	O
abstraction	B-Application
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
MVC	O
pattern	O
,	O
the	O
Smalltalk	B-Application
language	O
and	O
environment	O
were	O
highly	O
influential	O
in	O
the	O
history	O
of	O
the	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
and	O
the	O
what	B-Application
you	I-Application
see	I-Application
is	I-Application
what	I-Application
you	I-Application
get	I-Application
(	O
WYSIWYG	B-Application
)	O
user	B-Application
interface	I-Application
,	O
font	O
editors	O
,	O
and	O
desktop	O
metaphors	O
for	O
UI	O
design	O
.	O
</s>
<s>
The	O
powerful	O
built-in	O
debugging	O
and	O
object	O
inspection	O
tools	O
that	O
came	O
with	O
Smalltalk	B-Application
environments	O
set	O
the	O
standard	O
for	O
all	O
the	O
integrated	B-Application
development	I-Application
environments	I-Application
,	O
starting	O
with	O
Lisp	B-Operating_System
Machine	I-Operating_System
environments	O
,	O
that	O
came	O
after	O
.	O
</s>
<s>
As	O
in	O
other	O
object-oriented	B-Language
languages	I-Language
,	O
the	O
central	O
concept	O
in	O
Smalltalk-80	B-Application
(	O
but	O
not	O
in	O
Smalltalk-72	B-Application
)	O
is	O
that	O
of	O
an	O
object	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
GUI	B-Application
's	O
window	O
class	O
might	O
declare	O
that	O
windows	O
have	O
properties	O
such	O
as	O
the	O
label	O
,	O
the	O
position	O
and	O
whether	O
the	O
window	O
is	O
visible	O
or	O
not	O
.	O
</s>
<s>
A	O
Smalltalk	B-Application
object	O
can	O
do	O
exactly	O
three	O
things	O
:	O
</s>
<s>
Receive	B-Architecture
a	I-Architecture
message	I-Architecture
from	O
itself	O
or	O
another	O
object	O
.	O
</s>
<s>
Alan	O
Kay	O
has	O
commented	O
that	O
despite	O
the	O
attention	O
given	O
to	O
objects	O
,	O
messaging	O
is	O
the	O
most	O
important	O
concept	O
in	O
Smalltalk	B-Application
:	O
"	O
The	O
big	O
idea	O
is	O
'	O
messaging'—that	O
is	O
what	O
the	O
kernel	O
of	O
Smalltalk/Squeak	O
is	O
all	O
about	O
(	O
and	O
it	O
's	O
something	O
that	O
was	O
never	O
quite	O
completed	O
in	O
our	O
Xerox	O
PARC	O
phase	O
)	O
.	O
"	O
</s>
<s>
Unlike	O
most	O
other	O
languages	O
,	O
Smalltalk	B-Application
objects	O
can	O
be	O
modified	O
while	O
the	O
system	O
is	O
running	O
.	O
</s>
<s>
Live	O
coding	O
and	O
applying	O
fixes	O
‘	O
on-the-fly	O
’	O
is	O
a	O
dominant	O
programming	O
methodology	O
for	O
Smalltalk	B-Application
and	O
is	O
one	O
of	O
the	O
main	O
reasons	O
for	O
its	O
efficiency	O
.	O
</s>
<s>
Smalltalk	B-Application
is	O
a	O
"	O
pure	O
"	O
object-oriented	B-Language
programming	I-Language
language	I-Language
,	O
meaning	O
that	O
,	O
unlike	O
C++	B-Language
and	O
Java	B-Language
,	O
there	O
is	O
no	O
difference	O
between	O
values	O
which	O
are	O
objects	O
and	O
values	O
which	O
are	O
primitive	O
types	O
.	O
</s>
<s>
In	O
Smalltalk	B-Application
,	O
primitive	O
values	O
such	O
as	O
integers	O
,	O
booleans	O
and	O
characters	O
are	O
also	O
objects	O
,	O
in	O
the	O
sense	O
that	O
they	O
are	O
instances	O
of	O
corresponding	O
classes	O
,	O
and	O
operations	O
on	O
them	O
are	O
invoked	O
by	O
sending	O
messages	O
.	O
</s>
<s>
This	O
fact	O
is	O
summarized	O
in	O
the	O
commonly	O
heard	O
phrase	O
"	O
In	O
Smalltalk	B-Application
everything	O
is	O
an	O
object	O
"	O
,	O
which	O
may	O
be	O
more	O
accurately	O
expressed	O
as	O
"	O
all	O
values	O
are	O
objects	O
"	O
,	O
as	O
variables	O
are	O
not	O
.	O
</s>
<s>
Each	O
class	O
is	O
an	O
instance	O
of	O
the	O
metaclass	B-Language
of	O
that	O
class	O
.	O
</s>
<s>
Metaclasses	B-Language
in	O
turn	O
are	O
also	O
objects	O
,	O
and	O
are	O
all	O
instances	O
of	O
a	O
class	O
called	O
Metaclass	B-Language
.	O
</s>
<s>
Code	O
Smalltalk	B-Application
's	O
way	O
of	O
expressing	O
anonymous	B-General_Concept
functions	I-General_Concept
—	O
are	O
also	O
objects	O
.	O
</s>
<s>
Reflection	B-Language
is	O
a	O
term	O
that	O
computer	O
scientists	O
apply	O
to	O
software	O
programs	O
that	O
have	O
the	O
ability	O
to	O
inspect	O
their	O
own	O
structure	O
,	O
for	O
example	O
their	O
parse	O
tree	O
or	O
data	O
types	O
of	O
input	O
and	O
output	O
parameters	O
.	O
</s>
<s>
Reflection	B-Language
is	O
a	O
feature	O
of	O
dynamic	O
,	O
interactive	O
languages	O
such	O
as	O
Smalltalk	B-Application
and	O
Lisp	B-Language
.	O
</s>
<s>
Interactive	O
programs	O
with	O
reflection	B-Language
(	O
either	O
interpreted	O
or	O
compiled	O
)	O
maintain	O
the	O
state	O
of	O
all	O
in-memory	O
objects	O
,	O
including	O
the	O
code	O
object	O
itself	O
,	O
which	O
are	O
generated	O
during	O
parsing/compilation	O
and	O
are	O
programmatically	O
accessible	O
and	O
modifiable	O
.	O
</s>
<s>
Reflection	B-Language
is	O
also	O
a	O
feature	O
of	O
having	O
a	O
meta-model	O
as	O
Smalltalk	B-Application
does	O
.	O
</s>
<s>
Smalltalk-80	B-Application
is	O
a	O
totally	O
reflective	O
system	O
.	O
</s>
<s>
Smalltalk-80	B-Application
provides	O
both	O
structural	O
and	O
computational	B-Language
reflection	I-Language
.	O
</s>
<s>
Smalltalk	B-Application
is	O
a	O
structurally	O
reflective	O
system	O
which	O
structure	O
is	O
defined	O
by	O
Smalltalk-80	B-Application
objects	O
.	O
</s>
<s>
The	O
classes	O
and	O
methods	B-Language
that	O
define	O
the	O
system	O
are	O
also	O
objects	O
and	O
fully	O
part	O
of	O
the	O
system	O
that	O
they	O
help	O
define	O
.	O
</s>
<s>
The	O
Smalltalk	B-Application
compiler	O
compiles	O
textual	O
source	O
code	O
into	O
method	O
objects	O
,	O
typically	O
instances	O
of	O
CompiledMethod	O
.	O
</s>
<s>
The	O
system	O
is	O
extended	O
by	O
running	O
Smalltalk-80	B-Application
code	O
that	O
creates	O
or	O
defines	O
classes	O
and	O
methods	B-Language
.	O
</s>
<s>
In	O
this	O
way	O
a	O
Smalltalk-80	B-Application
system	O
is	O
a	O
"	O
living	O
"	O
system	O
,	O
carrying	O
around	O
the	O
ability	O
to	O
extend	O
itself	O
at	O
run	B-Library
time	I-Library
.	O
</s>
<s>
Since	O
the	O
classes	O
are	O
objects	O
,	O
they	O
can	O
be	O
asked	O
questions	O
such	O
as	O
"	O
what	O
methods	B-Language
do	O
you	O
implement	O
?	O
"	O
</s>
<s>
So	O
objects	O
can	O
easily	O
be	O
inspected	O
,	O
copied	O
,	O
(	O
de	O
)	O
serialized	B-Application
and	O
so	O
on	O
with	O
generic	O
code	O
that	O
applies	O
to	O
any	O
object	O
in	O
the	O
system	O
.	O
</s>
<s>
Smalltalk-80	B-Application
also	O
provides	O
computational	B-Language
reflection	I-Language
,	O
the	O
ability	O
to	O
observe	O
the	O
computational	O
state	O
of	O
the	O
system	O
.	O
</s>
<s>
In	O
languages	O
derived	O
from	O
the	O
original	O
Smalltalk-80	B-Application
the	O
current	O
activation	O
of	O
a	O
method	O
is	O
accessible	O
as	O
an	O
object	O
named	O
via	O
a	O
pseudo-variable	O
(	O
one	O
of	O
the	O
six	O
reserved	O
words	O
)	O
,	O
thisContext	O
.	O
</s>
<s>
These	O
facilities	O
make	O
it	O
possible	O
to	O
implement	O
co-routines	B-Architecture
or	O
Prolog-like	O
back-tracking	O
without	O
modifying	O
the	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
One	O
of	O
the	O
more	O
interesting	O
uses	O
of	O
this	O
is	O
in	O
the	O
Seaside	B-Application
web	I-Application
framework	I-Application
which	O
relieves	O
the	O
programmer	O
of	O
dealing	O
with	O
the	O
complexity	O
of	O
a	O
Web	O
Browser	O
's	O
back	O
button	O
by	O
storing	O
continuations	O
for	O
each	O
edited	O
page	O
and	O
switching	O
between	O
them	O
as	O
the	O
user	O
navigates	O
a	O
web	O
site	O
.	O
</s>
<s>
Programming	O
the	O
web	O
server	O
using	O
Seaside	B-Application
can	O
then	O
be	O
done	O
using	O
a	O
more	O
conventional	O
programming	O
style	O
.	O
</s>
<s>
An	O
example	O
of	O
how	O
Smalltalk	B-Application
can	O
use	O
reflection	B-Language
is	O
the	O
mechanism	O
for	O
handling	O
errors	O
.	O
</s>
<s>
When	O
an	O
object	O
is	O
sent	O
a	O
message	O
that	O
it	O
does	O
not	O
implement	O
,	O
the	O
virtual	B-Architecture
machine	I-Architecture
sends	O
the	O
object	O
the	O
doesNotUnderstand	O
:	O
message	O
with	O
a	O
reification	O
of	O
the	O
message	O
as	O
an	O
argument	O
.	O
</s>
<s>
In	O
an	O
interactive	O
Smalltalk	B-Application
system	O
the	O
default	O
implementation	O
of	O
doesNotUnderstand	O
:	O
is	O
one	O
that	O
opens	O
an	O
error	O
window	O
(	O
a	O
Notifier	O
)	O
reporting	O
the	O
error	O
to	O
the	O
user	O
.	O
</s>
<s>
Through	O
this	O
and	O
the	O
reflective	O
facilities	O
the	O
user	O
can	O
examine	O
the	O
context	O
in	O
which	O
the	O
error	O
occurred	O
,	O
redefine	O
the	O
offending	O
code	O
,	O
and	O
continue	O
,	O
all	O
within	O
the	O
system	O
,	O
using	O
Smalltalk-80	B-Application
'	O
s	O
reflective	O
facilities	O
.	O
</s>
<s>
Such	O
proxies	O
can	O
then	O
be	O
used	O
to	O
implement	O
a	O
number	O
of	O
facilities	O
such	O
as	O
distributed	O
Smalltalk	B-Application
where	O
messages	O
are	O
exchanged	O
between	O
multiple	O
Smalltalk	B-Application
systems	O
,	O
database	O
interfaces	O
where	O
objects	O
are	O
transparently	O
faulted	O
out	O
of	O
a	O
database	O
,	O
promises	B-Operating_System
,	O
etc	O
.	O
</s>
<s>
The	O
design	O
of	O
distributed	O
Smalltalk	B-Application
influenced	O
such	O
systems	O
as	O
CORBA	B-Architecture
.	O
</s>
<s>
Smalltalk-80	B-Application
syntax	O
is	O
rather	O
minimalist	O
,	O
based	O
on	O
only	O
a	O
handful	O
of	O
declarations	O
and	O
reserved	O
words	O
.	O
</s>
<s>
In	O
fact	O
,	O
only	O
six	O
"	O
keywords	O
"	O
are	O
reserved	O
in	O
Smalltalk	B-Application
:	O
true	O
,	O
false	O
,	O
nil	O
,	O
self	B-Operating_System
,	O
super	O
,	O
and	O
thisContext	O
.	O
</s>
<s>
self	B-Operating_System
and	O
super	O
refer	O
to	O
the	O
receiver	O
of	O
a	O
message	O
within	O
a	O
method	O
activated	O
in	O
response	O
to	O
that	O
message	O
,	O
but	O
sends	O
to	O
super	O
are	O
looked	O
up	O
in	O
the	O
superclass	O
of	O
the	O
method	O
's	O
defining	O
class	O
rather	O
than	O
the	O
class	O
of	O
the	O
receiver	O
,	O
which	O
allows	O
methods	B-Language
in	O
subclasses	O
to	O
invoke	O
methods	B-Language
of	O
the	O
same	O
name	O
in	O
superclasses	O
.	O
</s>
<s>
The	O
only	O
built-in	O
language	O
constructs	O
are	O
message	B-Architecture
sends	I-Architecture
,	O
assignment	O
,	O
method	O
return	O
and	O
literal	O
syntax	O
for	O
some	O
objects	O
.	O
</s>
<s>
From	O
its	O
origins	O
as	O
a	O
language	O
for	O
children	O
of	O
all	O
ages	O
,	O
standard	O
Smalltalk	B-Application
syntax	O
uses	O
punctuation	O
in	O
a	O
manner	O
more	O
like	O
English	O
than	O
mainstream	O
coding	O
languages	O
.	O
</s>
<s>
The	O
remainder	O
of	O
the	O
language	O
,	O
including	O
control	O
structures	O
for	O
conditional	O
evaluation	O
and	O
iteration	O
,	O
is	O
implemented	O
on	O
top	O
of	O
the	O
built-in	O
constructs	O
by	O
the	O
standard	O
Smalltalk	B-Application
class	O
library	O
.	O
</s>
<s>
The	O
adage	O
that	O
"	O
Smalltalk	B-Application
syntax	O
fits	O
on	O
a	O
postcard	B-Device
"	O
may	O
have	O
originated	O
in	O
Alan	O
Kay	O
's	O
original	O
conception	O
of	O
the	O
language	O
,	O
as	O
related	O
by	O
him	O
in	O
practically	O
every	O
of	O
tens	O
or	O
hundreds	O
of	O
public	O
lectures	O
,	O
op	O
.	O
</s>
<s>
cit.	O
,	O
or	O
perhaps	O
it	O
could	O
refer	O
to	O
a	O
code	O
snippet	O
by	O
Ralph	O
Johnson	O
,	O
demonstrating	O
all	O
the	O
basic	O
standard	O
syntactic	O
elements	O
of	O
methods	B-Language
:	O
</s>
<s>
The	O
following	O
examples	O
illustrate	O
the	O
most	O
common	O
objects	O
which	O
can	O
be	O
written	O
as	O
literal	O
values	O
in	O
Smalltalk-80	B-Application
methods	B-Language
.	O
</s>
<s>
In	O
addition	O
to	O
strings	O
,	O
Smalltalk	B-Application
has	O
a	O
class	O
of	O
character	O
sequence	O
objects	O
called	O
Symbol	O
.	O
</s>
<s>
Symbols	O
are	O
written	O
as	O
#	O
followed	O
by	O
a	O
string	B-Language
literal	I-Language
.	O
</s>
<s>
Many	O
Smalltalk	B-Application
dialects	O
implement	O
additional	O
syntaxes	O
for	O
other	O
objects	O
,	O
but	O
the	O
ones	O
above	O
are	O
the	O
essentials	O
supported	O
by	O
all	O
.	O
</s>
<s>
The	O
two	O
kinds	O
of	O
variables	O
commonly	O
used	O
in	O
Smalltalk	B-Application
are	O
instance	O
variables	O
and	O
temporary	O
variables	O
.	O
</s>
<s>
For	O
example	O
,	O
VisualWorks	B-Application
has	O
class	O
shared	O
variables	O
and	O
namespace	O
shared	O
variables	O
,	O
while	O
Squeak	B-Operating_System
and	O
many	O
other	O
implementations	O
have	O
class	O
variables	O
,	O
pool	O
variables	O
and	O
global	O
variables	O
.	O
</s>
<s>
Temporary	O
variable	O
declarations	O
in	O
Smalltalk	B-Application
are	O
variables	O
declared	O
inside	O
a	O
method	O
(	O
see	O
below	O
)	O
.	O
</s>
<s>
The	O
string	O
is	O
an	O
object	O
(	O
a	O
sequence	O
of	O
characters	O
between	O
single	O
quotes	O
is	O
the	O
syntax	O
for	O
literal	B-Language
strings	I-Language
)	O
,	O
created	O
by	O
the	O
compiler	O
at	O
compile	O
time	O
.	O
</s>
<s>
In	O
the	O
original	O
Parc	O
Place	O
image	B-Operating_System
,	O
the	O
glyph	O
of	O
the	O
underscore	O
character	O
⟨_⟩	O
appeared	O
as	O
a	O
left-facing	O
arrow	O
⟨	O
←	O
⟩	O
(	O
like	O
in	O
the	O
1963	O
version	O
of	O
the	O
ASCII	B-Protocol
code	I-Protocol
)	O
.	O
</s>
<s>
Smalltalk	B-Application
originally	O
accepted	O
this	O
left-arrow	O
as	O
the	O
only	O
assignment	O
operator	O
.	O
</s>
<s>
Most	O
modern	O
Smalltalk	B-Application
implementations	O
accept	O
either	O
the	O
underscore	O
or	O
the	O
colon-equals	O
syntax	O
.	O
</s>
<s>
The	O
message	O
is	O
the	O
most	O
fundamental	O
language	O
construct	O
in	O
Smalltalk	B-Application
.	O
</s>
<s>
Even	O
control	O
structures	O
are	O
implemented	O
as	O
message	B-Architecture
sends	I-Architecture
.	O
</s>
<s>
Smalltalk	B-Application
adopts	O
by	O
default	O
a	O
dynamic	O
dispatch	O
and	O
single	O
dispatch	O
strategy	O
(	O
as	O
opposed	O
to	O
multiple	O
dispatch	O
,	O
used	O
by	O
some	O
other	O
object-oriented	B-Language
languages	I-Language
)	O
.	O
</s>
<s>
The	O
message	O
result	O
,	O
or	O
in	O
Smalltalk	B-Application
parlance	O
,	O
the	O
answer	O
is	O
supposed	O
to	O
be	O
16	O
.	O
</s>
<s>
For	O
example	O
,	O
an	O
expression	O
to	O
create	O
a	O
rectangle	O
using	O
a	O
C++	B-Language
or	O
Java-like	O
syntax	O
might	O
be	O
written	O
as	O
:	O
</s>
<s>
By	O
contrast	O
,	O
in	O
Smalltalk	B-Application
,	O
this	O
code	O
would	O
be	O
written	O
as	O
:	O
</s>
<s>
Notice	O
,	O
that	O
the	O
Smalltalk-80	B-Application
language	O
itself	O
does	O
not	O
imply	O
the	O
meaning	O
of	O
those	O
operators	O
.	O
</s>
<s>
A	O
side	O
effect	O
of	O
this	O
mechanism	O
is	O
operator	O
overloading	B-Application
.	O
</s>
<s>
An	O
expression	O
can	O
include	O
multiple	O
message	B-Architecture
sends	I-Architecture
.	O
</s>
<s>
Note	O
that	O
because	O
the	O
meaning	O
of	O
binary	O
messages	O
is	O
not	O
hardwired	O
into	O
Smalltalk-80	B-Application
syntax	O
,	O
all	O
of	O
them	O
are	O
considered	O
to	O
have	O
equal	O
precedence	O
and	O
are	O
evaluated	O
simply	O
from	O
left	O
to	O
right	O
.	O
</s>
<s>
Because	O
of	O
this	O
,	O
the	O
meaning	O
of	O
Smalltalk	B-Application
expressions	O
using	O
binary	O
messages	O
can	O
be	O
different	O
from	O
their	O
"	O
traditional	O
"	O
interpretation	O
:	O
</s>
<s>
Unary	O
messages	O
can	O
be	O
chained	B-Language
by	O
writing	O
them	O
one	O
after	O
another	O
:	O
</s>
<s>
A	O
block	O
of	O
code	O
(	O
an	O
anonymous	B-General_Concept
function	I-General_Concept
)	O
can	O
be	O
expressed	O
as	O
a	O
literal	O
value	O
(	O
which	O
is	O
an	O
object	O
,	O
since	O
all	O
values	O
are	O
objects	O
)	O
.	O
</s>
<s>
This	O
means	O
that	O
the	O
Smalltalk	B-Application
code	O
:	O
</s>
<s>
or	O
expressed	O
in	O
lambda	B-General_Concept
terms	O
as	O
:	O
</s>
<s>
Or	O
in	O
lambda	B-General_Concept
terms	O
as	O
:	O
</s>
<s>
The	O
resulting	O
block	O
object	O
can	O
form	O
a	O
closure	B-Language
:	O
it	O
can	O
access	O
the	O
variables	O
of	O
its	O
enclosing	O
lexical	B-Language
scopes	O
at	O
any	O
time	O
.	O
</s>
<s>
Code	O
that	O
would	O
typically	O
be	O
written	O
with	O
loops	O
in	O
some	O
languages	O
can	O
be	O
written	O
concisely	O
in	O
Smalltalk	B-Application
using	O
blocks	O
,	O
sometimes	O
in	O
a	O
single	O
line	O
.	O
</s>
<s>
But	O
more	O
importantly	O
blocks	O
allow	O
control	O
structure	O
to	O
be	O
expressed	O
using	O
messages	O
and	O
polymorphism	B-Application
,	O
since	O
blocks	O
defer	O
computation	O
and	O
polymorphism	B-Application
can	O
be	O
used	O
to	O
select	O
alternatives	O
.	O
</s>
<s>
Note	O
that	O
this	O
is	O
related	O
to	O
functional	B-Language
programming	I-Language
,	O
wherein	O
patterns	O
of	O
computation	O
(	O
here	O
selection	O
)	O
are	O
abstracted	B-Application
into	O
higher-order	B-Language
functions	I-Language
.	O
</s>
<s>
For	O
example	O
,	O
the	O
message	O
select	O
:	O
on	O
a	O
Collection	O
is	O
equivalent	O
to	O
the	O
higher-order	B-Language
function	I-Language
filter	B-Application
on	O
an	O
appropriate	O
functor	B-Language
.	O
</s>
<s>
Control	O
structures	O
do	O
not	O
have	O
special	O
syntax	O
in	O
Smalltalk	B-Application
.	O
</s>
<s>
Blocks	O
are	O
also	O
used	O
to	O
implement	O
user-defined	O
control	O
structures	O
,	O
enumerators	O
,	O
visitors	O
,	O
exception	B-General_Concept
handling	I-General_Concept
,	O
pluggable	O
behavior	O
and	O
many	O
other	O
patterns	O
.	O
</s>
<s>
The	O
exception	B-General_Concept
handling	I-General_Concept
mechanism	O
uses	O
blocks	O
as	O
handlers	O
(	O
similar	O
to	O
CLOS-style	O
exception	B-General_Concept
handling	I-General_Concept
)	O
:	O
</s>
<s>
The	O
exception	B-General_Concept
handler	I-General_Concept
's	O
"	O
ex	O
"	O
argument	O
provides	O
access	O
to	O
the	O
state	O
of	O
the	O
suspended	O
operation	O
(	O
stack	O
frame	O
,	O
line-number	O
,	O
receiver	O
and	O
arguments	O
etc	O
.	O
)	O
</s>
<s>
In	O
other	O
words	O
:	O
classes	O
are	O
first-class	O
objects	O
in	O
Smalltalk	B-Application
which	O
can	O
receive	O
messages	O
just	O
like	O
any	O
other	O
object	O
and	O
can	O
be	O
created	O
dynamically	O
at	O
execution	B-Library
time	I-Library
.	O
</s>
<s>
Note	O
that	O
objects	O
are	O
responsible	O
for	O
determining	O
dynamically	O
at	B-Library
runtime	I-Library
which	O
method	O
to	O
execute	O
in	O
response	O
to	O
a	O
message	O
—	O
while	O
in	O
many	O
languages	O
this	O
may	O
be	O
(	O
sometimes	O
,	O
or	O
even	O
always	O
)	O
determined	O
statically	O
at	O
compile	O
time	O
.	O
</s>
<s>
For	O
Smalltalk	B-Application
,	O
the	O
program	O
is	O
extremely	O
simple	O
to	O
write	O
.	O
</s>
<s>
The	O
following	O
code	O
,	O
the	O
message	O
"	O
show	O
:	O
"	O
is	O
sent	O
to	O
the	O
object	O
"	O
Transcript	O
"	O
with	O
the	O
String	B-Language
literal	I-Language
'	O
Hello	O
,	O
world!	O
 '	O
</s>
<s>
Most	O
popular	O
programming	O
systems	O
separate	O
static	O
program	O
code	O
(	O
in	O
the	O
form	O
of	O
class	O
definitions	O
,	O
functions	O
or	O
procedures	O
)	O
from	O
dynamic	O
,	O
or	O
run	B-Library
time	I-Library
,	O
program	O
state	O
(	O
such	O
as	O
objects	O
or	O
other	O
forms	O
of	O
program	O
data	O
)	O
.	O
</s>
<s>
Image	B-Operating_System
based	O
systems	O
do	O
n't	O
force	O
losing	O
all	O
that	O
just	O
because	O
a	O
computer	O
is	O
turned	O
off	O
,	O
or	O
an	O
OS	O
updates	O
.	O
</s>
<s>
Many	O
Smalltalk	B-Application
systems	O
,	O
however	O
,	O
do	O
not	O
differentiate	O
between	O
program	O
data	O
(	O
objects	O
)	O
and	O
code	O
(	O
classes	O
)	O
.	O
</s>
<s>
Thus	O
,	O
most	O
Smalltalk	B-Application
systems	O
store	O
the	O
entire	O
program	O
state	O
(	O
including	O
both	O
Class	O
and	O
non-Class	O
objects	O
)	O
in	O
an	O
image	B-Operating_System
file	O
.	O
</s>
<s>
The	O
image	B-Operating_System
can	O
then	O
be	O
loaded	O
by	O
the	O
Smalltalk	B-Application
virtual	B-Architecture
machine	I-Architecture
to	O
restore	O
a	O
Smalltalk-like	O
system	O
to	O
a	O
prior	O
state	O
.	O
</s>
<s>
Smalltalk	B-Application
images	O
are	O
similar	O
to	O
(	O
restartable	O
)	O
core	B-Error_Name
dumps	I-Error_Name
and	O
can	O
provide	O
the	O
same	O
functionality	O
as	O
core	B-Error_Name
dumps	I-Error_Name
,	O
such	O
as	O
delayed	O
or	O
remote	O
debugging	O
with	O
full	O
access	O
to	O
the	O
program	O
state	O
at	O
the	O
time	O
of	O
error	O
.	O
</s>
<s>
Other	O
languages	O
that	O
model	O
application	O
code	O
as	O
a	O
form	O
of	O
data	O
,	O
such	O
as	O
Lisp	B-Language
,	O
often	O
use	O
image-based	O
persistence	O
as	O
well	O
(	O
see	O
EMACS	B-Application
,	O
for	O
example	O
)	O
.	O
</s>
<s>
For	O
one	O
thing	O
,	O
developers	O
may	O
often	O
want	O
to	O
hide	O
implementation	O
details	O
and	O
not	O
make	O
them	O
available	O
in	O
a	O
run	B-Library
time	I-Library
environment	O
.	O
</s>
<s>
For	O
reasons	O
of	O
legality	O
and	O
maintenance	O
,	O
allowing	O
anyone	O
to	O
modify	O
a	O
program	O
at	O
run	B-Library
time	I-Library
inevitably	O
introduces	O
complexity	O
and	O
potential	O
errors	O
that	O
would	O
not	O
be	O
possible	O
with	O
a	O
compiled	O
system	O
that	O
exposes	O
no	O
source	O
code	O
in	O
the	O
run	B-Library
time	I-Library
environment	O
.	O
</s>
<s>
Everything	O
in	O
Smalltalk-80	B-Application
,	O
unless	O
customised	O
to	O
avoid	O
the	O
possibility	O
,	O
is	O
available	O
for	O
modification	O
from	O
within	O
a	O
running	O
program	O
.	O
</s>
<s>
This	O
means	O
that	O
,	O
for	O
example	O
,	O
the	O
IDE	B-Application
can	O
be	O
changed	O
in	O
a	O
running	O
system	O
without	O
restarting	O
it	O
.	O
</s>
<s>
In	O
some	O
implementations	O
,	O
the	O
syntax	O
of	O
the	O
language	O
or	O
the	O
garbage	B-General_Concept
collection	I-General_Concept
implementation	O
can	O
also	O
be	O
changed	O
on	O
the	O
fly	O
.	O
</s>
<s>
Even	O
the	O
statement	O
true	O
become	O
:	O
false	O
is	O
valid	O
in	O
Smalltalk	B-Application
,	O
although	O
executing	O
it	O
is	O
not	O
recommended	O
except	O
for	O
demonstration	O
purposes	O
(	O
see	O
virtual	B-Architecture
machine	I-Architecture
,	O
image-based	O
persistence	O
,	O
and	O
backups	B-Protocol
)	O
.	O
</s>
<s>
Smalltalk	B-Application
programs	O
are	O
usually	O
compiled	O
to	O
bytecode	O
,	O
which	O
is	O
then	O
interpreted	O
by	O
a	O
virtual	B-Architecture
machine	I-Architecture
or	O
dynamically	O
translated	O
into	O
machine-native	O
code	O
.	O
</s>
<s>
(	O
OS	O
VM	O
)	O
is	O
a	O
notable	O
implementation	O
of	O
the	O
Smalltalk	B-Application
virtual	B-Architecture
machine	I-Architecture
on	O
which	O
many	O
modern	O
Smalltalk	B-Application
implementations	O
are	O
based	O
or	O
derived	O
from	O
.	O
</s>
<s>
OS	O
VM	O
itself	O
is	O
transpiled	B-Language
from	O
a	O
set	O
of	O
Smalltalk	B-Application
source	O
code	O
files	O
(	O
using	O
a	O
subset	O
of	O
Smalltalk	B-Application
called	O
Slang	O
)	O
to	O
native	O
C	B-Language
language	I-Language
source	O
code	O
(	O
by	O
using	O
a	O
transpiler	B-Language
called	O
VMMaker	O
)	O
,	O
which	O
is	O
in	O
turn	O
compiled	O
against	O
specific	O
platform	O
and	O
architecture	O
of	O
the	O
hardware	O
practically	O
enabling	O
cross-platform	B-Operating_System
execution	O
of	O
the	O
Smalltalk	B-Application
images	O
.	O
</s>
<s>
The	O
source	O
code	O
is	O
available	O
on	O
GitHub	O
and	O
distributed	O
under	O
MIT	B-License
License	I-License
.	O
</s>
<s>
The	O
known	O
Smalltalk	B-Application
implementations	O
based	O
on	O
the	O
OS	O
VM	O
are	O
:	O
</s>
<s>
Strongtalk	B-Language
,	O
an	O
open-source	B-Application
(	O
since	O
2006	O
)	O
Windows-only	O
version	O
,	O
offers	O
optional	O
strong	O
typing	O
;	O
initially	O
created	O
at	O
Sun	O
Microsystem	O
Labs	O
.	O
</s>
