<s>
Tcl	B-Operating_System
(	O
pronounced	O
"	O
tickle	O
"	O
or	O
as	O
an	O
initialism	O
)	O
is	O
a	O
high-level	B-Language
,	O
general-purpose	B-Language
,	O
interpreted	B-Application
,	O
dynamic	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
Tcl	B-Operating_System
casts	O
everything	O
into	O
the	O
mold	O
of	O
a	O
command	B-Application
,	O
even	O
programming	O
constructs	O
like	O
variable	O
assignment	O
and	O
procedure	O
definition	O
.	O
</s>
<s>
Tcl	B-Operating_System
supports	O
multiple	O
programming	O
paradigms	O
,	O
including	O
object-oriented	B-Language
,	O
imperative	B-Application
and	O
functional	B-Language
programming	I-Language
or	O
procedural	B-Application
styles	O
.	O
</s>
<s>
It	O
is	O
commonly	O
used	O
embedded	O
into	O
C	B-Language
applications	O
,	O
for	O
rapid	O
prototyping	O
,	O
scripted	O
applications	O
,	O
GUIs	B-Application
,	O
and	O
testing	O
.	O
</s>
<s>
Tcl	B-Operating_System
interpreters	B-Application
are	O
available	O
for	O
many	O
operating	B-General_Concept
systems	I-General_Concept
,	O
allowing	O
Tcl	B-Operating_System
code	O
to	O
run	O
on	O
a	O
wide	O
variety	O
of	O
systems	O
.	O
</s>
<s>
Because	O
Tcl	B-Operating_System
is	O
a	O
very	O
compact	O
language	O
,	O
it	O
is	O
used	O
on	O
embedded	B-Architecture
systems	I-Architecture
platforms	O
,	O
both	O
in	O
its	O
full	O
form	O
and	O
in	O
several	O
other	O
small-footprint	O
versions	O
.	O
</s>
<s>
The	O
popular	O
combination	O
of	O
Tcl	B-Operating_System
with	O
the	O
Tk	B-Application
extension	O
is	O
referred	O
to	O
as	O
Tcl/Tk	B-Application
,	O
and	O
enables	O
building	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
(	O
GUI	B-Application
)	O
natively	O
in	O
Tcl	B-Operating_System
.	O
</s>
<s>
Tcl/Tk	B-Application
is	O
included	O
in	O
the	O
standard	O
Python	B-Language
installation	O
in	O
the	O
form	O
of	O
Tkinter	B-Language
.	O
</s>
<s>
The	O
Tcl	B-Operating_System
programming	I-Operating_System
language	I-Operating_System
was	O
created	O
in	O
the	O
spring	O
of	O
1988	O
by	O
John	O
Ousterhout	O
while	O
he	O
was	O
working	O
at	O
the	O
University	O
of	O
California	O
,	O
Berkeley	O
.	O
</s>
<s>
Originally	O
"	O
born	O
out	O
of	O
frustration	O
"	O
,	O
according	O
to	O
the	O
author	O
,	O
with	O
programmers	O
devising	O
their	O
own	O
languages	O
intended	O
to	O
be	O
embedded	O
into	O
Electronic	O
design	O
automation	O
(	O
EDA	O
)	O
Magic	B-Application
application	O
where	O
John	O
had	O
professional	O
focus	O
.	O
</s>
<s>
Later	O
Tcl	B-Operating_System
gained	O
acceptance	O
on	O
its	O
own	O
.	O
</s>
<s>
Ousterhout	O
was	O
awarded	O
the	O
ACM	O
Software	O
System	O
Award	O
in	O
1997	O
for	O
Tcl/Tk	B-Application
.	O
</s>
<s>
The	O
name	O
originally	O
comes	O
from	O
Tool	B-Operating_System
Command	I-Operating_System
Language	I-Operating_System
,	O
but	O
is	O
conventionally	O
spelled	O
"	O
Tcl	B-Operating_System
"	O
rather	O
than	O
"	O
TCL	B-Operating_System
"	O
.	O
</s>
<s>
Date	O
Event	O
January	O
1990	O
Tcl	B-Operating_System
announced	O
beyond	O
Berkeley	O
(	O
Winter	O
USENIX	B-Operating_System
)	O
.	O
</s>
<s>
June	O
1990	O
Expect	O
announced	O
(	O
Summer	O
USENIX	B-Operating_System
)	O
.	O
</s>
<s>
January	O
1991	O
First	O
announcement	O
of	O
Tk	B-Application
(	O
Winter	O
USENIX	B-Operating_System
)	O
.	O
</s>
<s>
June	O
1993	O
First	O
Tcl/Tk	B-Application
conference	O
(	O
Berkeley	O
)	O
.	O
</s>
<s>
August	O
1997	O
Tcl	B-Operating_System
8.0	O
introduced	O
a	O
bytecode	O
compiler	O
.	O
</s>
<s>
April	O
1999	O
Tcl	B-Operating_System
8.1	O
introduces	O
full	O
Unicode	O
support	O
and	O
advanced	O
regular	B-Language
expressions	I-Language
.	O
</s>
<s>
August	O
1999	O
Tcl	B-Operating_System
8.2	O
introduces	O
Tcl	B-Operating_System
Extension	O
Architecture	O
(	O
TEA	B-Language
)	O
August	O
2000	O
Tcl	B-Operating_System
Core	O
Team	O
formed	O
,	O
moving	O
Tcl	B-Operating_System
to	O
a	O
more	O
community-oriented	O
development	O
model	O
.	O
</s>
<s>
September	O
2002	O
Ninth	O
Tcl/Tk	B-Application
conference	O
(	O
Vancouver	O
)	O
.	O
</s>
<s>
Tcl	B-Operating_System
8.4.0	O
released	O
.	O
</s>
<s>
December	O
2007	O
Tcl	B-Operating_System
8.5	O
added	O
new	O
datatypes	O
,	O
a	O
new	O
extension	O
repository	O
,	O
bignums	B-Algorithm
,	O
lambdas	B-General_Concept
.	O
</s>
<s>
December	O
2012	O
Tcl	B-Operating_System
8.6	O
added	O
built-in	O
dynamic	O
object	B-Language
system	I-Language
,	O
TclOO	O
,	O
and	O
stackless	O
evaluation	O
.	O
</s>
<s>
Tcl	B-Operating_System
conferences	O
and	O
workshops	O
are	O
held	O
in	O
both	O
the	O
United	O
States	O
and	O
Europe	O
.	O
</s>
<s>
All	O
operations	O
are	O
commands	B-Application
,	O
including	O
language	O
structures	O
.	O
</s>
<s>
Commands	B-Application
commonly	O
accept	O
a	O
variable	O
number	O
of	O
arguments	O
(	O
are	O
variadic	B-Language
)	O
.	O
</s>
<s>
Fully	O
dynamic	O
,	O
class-based	O
object	B-Language
system	I-Language
,	O
TclOO	O
,	O
including	O
advanced	O
features	O
such	O
as	O
meta-classes	O
,	O
filters	O
,	O
and	O
mixins	B-Language
.	O
</s>
<s>
Event-driven	B-Application
interface	I-Application
to	O
sockets	B-Protocol
and	O
files	B-Operating_System
.	O
</s>
<s>
Variable	O
visibility	O
restricted	O
to	O
lexical	O
(	O
static	O
)	O
scope	B-Language
by	O
default	O
,	O
but	O
uplevel	O
and	O
upvar	O
allowing	O
procs	O
to	O
interact	O
with	O
the	O
enclosing	O
functions	O
 '	O
scopes	O
.	O
</s>
<s>
All	O
commands	B-Application
defined	O
by	O
Tcl	B-Operating_System
itself	O
generate	O
error	O
messages	O
on	O
incorrect	O
usage	O
.	O
</s>
<s>
Extensibility	B-Architecture
,	O
via	O
C	B-Language
,	O
C++	B-Language
,	O
Java	B-Language
,	O
Python	B-Language
,	O
and	O
Tcl	B-Operating_System
.	O
</s>
<s>
Cross-platform	B-Operating_System
:	O
Windows	B-Library
API	I-Library
;	O
Unix	B-Application
,	O
Linux	B-Application
,	O
Macintosh	B-Device
etc	O
.	O
</s>
<s>
Close	O
,	O
cross-platform	B-Operating_System
integration	O
with	O
windowing	O
(	O
GUI	B-Application
)	O
interface	O
Tk	B-Application
.	O
</s>
<s>
Freely	O
distributable	O
source	O
code	O
under	O
a	O
BSD	B-Operating_System
license	I-Operating_System
.	O
</s>
<s>
Safe-Tcl	O
is	O
a	O
subset	O
of	O
Tcl	B-Operating_System
that	O
has	O
restricted	O
features	O
so	O
that	O
Tcl	B-Operating_System
scripts	O
cannot	O
harm	O
their	O
hosting	O
machine	O
or	O
application	O
.	O
</s>
<s>
File	O
system	O
access	O
is	O
limited	O
and	O
arbitrary	O
system	B-Application
commands	I-Application
are	O
prevented	O
from	O
execution	O
.	O
</s>
<s>
It	O
uses	O
a	O
dual	O
interpreter	B-Application
model	O
with	O
the	O
untrusted	O
interpreter	B-Application
running	O
code	O
in	O
an	O
untrusted	O
script	O
.	O
</s>
<s>
Safe-Tcl	O
can	O
be	O
included	O
in	O
e-mail	O
when	O
the	O
application/safe	O
-tcl	O
and	O
multipart/enabled	O
-mail	O
are	O
supported	O
.	O
</s>
<s>
The	O
functionality	O
of	O
Safe-Tcl	O
has	O
since	O
been	O
incorporated	O
as	O
part	O
of	O
the	O
standard	O
Tcl/Tk	B-Application
releases	O
.	O
</s>
<s>
The	O
syntax	O
and	O
semantics	O
of	O
Tcl	B-Operating_System
are	O
covered	O
by	O
twelve	O
rules	O
known	O
as	O
the	O
Dodekalogue	O
.	O
</s>
<s>
A	O
Tcl	B-Operating_System
script	O
consists	O
of	O
a	O
series	O
of	O
command	B-Application
invocations	O
.	O
</s>
<s>
A	O
command	B-Application
invocation	O
is	O
a	O
list	O
of	O
words	O
separated	O
by	O
whitespace	O
and	O
terminated	O
by	O
a	O
newline	O
or	O
semicolon	O
.	O
</s>
<s>
The	O
first	O
word	O
is	O
the	O
name	O
of	O
a	O
command	B-Application
,	O
which	O
may	O
be	O
built	O
into	O
the	O
language	O
,	O
found	O
in	O
an	O
available	O
library	B-Library
,	O
or	O
defined	O
in	O
the	O
script	O
itself	O
.	O
</s>
<s>
The	O
subsequent	O
words	O
serve	O
as	O
arguments	O
to	O
the	O
command	B-Application
:	O
</s>
<s>
The	O
following	O
example	O
uses	O
the	O
puts	O
(	O
short	O
for	O
"	O
put	O
string	O
"	O
)	O
command	B-Application
to	O
display	O
a	O
string	O
of	O
text	O
on	O
the	O
host	O
console	O
:	O
</s>
<s>
Variables	O
and	O
the	O
results	O
of	O
other	O
commands	B-Application
can	O
be	O
substituted	O
into	O
strings	O
,	O
such	O
as	O
in	O
this	O
example	O
which	O
uses	O
the	O
set	O
and	O
expr	O
commands	B-Application
to	O
store	O
the	O
result	O
of	O
a	O
calculation	O
in	O
a	O
variable	O
(	O
note	O
that	O
Tcl	B-Operating_System
does	O
not	O
use	O
=	O
as	O
an	O
assignment	O
operator	O
)	O
,	O
and	O
then	O
uses	O
puts	O
to	O
print	O
the	O
result	O
together	O
with	O
some	O
explanatory	O
text	O
:	O
</s>
<s>
Comments	O
can	O
appear	O
anywhere	O
the	O
interpreter	B-Application
is	O
expecting	O
a	O
command	B-Application
name	O
.	O
</s>
<s>
As	O
seen	O
in	O
these	O
examples	O
,	O
there	O
is	O
one	O
basic	O
construct	O
in	O
the	O
language	O
:	O
the	O
command	B-Application
.	O
</s>
<s>
Quoting	O
mechanisms	O
and	O
substitution	O
rules	O
determine	O
how	O
the	O
arguments	O
to	O
each	O
command	B-Application
are	O
processed	O
.	O
</s>
<s>
One	O
special	O
substitution	O
occurs	O
before	O
the	O
parsing	O
of	O
any	O
commands	B-Application
or	O
arguments	O
.	O
</s>
<s>
If	B-Language
the	O
final	O
character	O
on	O
a	O
line	O
(	O
i.e.	O
,	O
immediately	O
before	O
a	O
newline	O
)	O
is	O
a	O
backslash	O
,	O
then	O
the	O
backslash-newline	O
combination	O
(	O
and	O
any	O
spaces	O
or	O
tabs	O
immediately	O
following	O
the	O
newline	O
)	O
are	O
replaced	O
by	O
a	O
single	O
space	O
.	O
</s>
<s>
Such	O
a	O
word	O
can	O
thus	O
contain	O
whitespace	O
and	O
semicolons	O
without	O
those	O
characters	O
being	O
interpreted	B-Application
as	O
having	O
any	O
special	O
meaning	O
(	O
i.e.	O
,	O
they	O
are	O
treated	O
as	O
normal	O
text	O
characters	O
)	O
.	O
</s>
<s>
Command	B-Application
substitution	O
replaces	O
the	O
contents	O
of	O
balanced	O
square	O
brackets	O
with	O
the	O
result	O
of	O
evaluating	O
the	O
script	O
contained	O
inside	O
.	O
</s>
<s>
From	O
Tcl	B-Operating_System
8.5	O
onwards	O
,	O
any	O
word	O
may	O
be	O
prefixed	O
by	O
{*},	O
which	O
causes	O
the	O
word	O
to	O
be	O
split	O
apart	O
into	O
its	O
constituent	O
sub-words	O
for	O
the	O
purposes	O
of	O
building	O
the	O
command	B-Application
invocation	O
(	O
similar	O
to	O
the	O
,	O
@	O
sequence	O
of	O
Lisp	B-Language
's	O
quasiquote	O
feature	O
)	O
.	O
</s>
<s>
As	O
a	O
consequence	O
of	O
these	O
rules	O
,	O
the	O
result	O
of	O
any	O
command	B-Application
may	O
be	O
used	O
as	O
an	O
argument	O
to	O
any	O
other	O
command	B-Application
.	O
</s>
<s>
Note	O
that	O
,	O
unlike	O
in	O
Unix	B-Device
command	I-Device
shells	I-Device
,	O
Tcl	B-Operating_System
does	O
not	O
reparse	O
any	O
string	O
unless	O
explicitly	O
directed	O
to	O
do	O
so	O
,	O
which	O
makes	O
interactive	O
use	O
more	O
cumbersome	O
,	O
but	O
scripted	O
use	O
more	O
predictable	O
(	O
e.g.	O
,	O
the	O
presence	O
of	O
spaces	O
in	O
filenames	O
does	O
not	O
cause	O
difficulties	O
)	O
.	O
</s>
<s>
The	O
double	O
equality	O
sign	O
(=	O
=	O
)	O
is	O
the	O
test	O
for	O
equality	O
which	O
is	O
used	O
in	O
expression	O
contexts	O
such	O
as	O
the	O
expr	O
command	B-Application
and	O
in	O
the	O
first	O
argument	O
to	O
if	B-Language
.	O
</s>
<s>
(	O
Both	O
commands	B-Application
are	O
part	O
of	O
the	O
standard	O
library	B-Library
;	O
they	O
have	O
no	O
special	O
place	O
in	O
the	O
library	B-Library
and	O
can	O
be	O
replaced	O
if	B-Language
desired	O
.	O
)	O
</s>
<s>
The	O
majority	O
of	O
Tcl	B-Operating_System
commands	B-Application
,	O
especially	O
in	O
the	O
standard	O
library	B-Library
,	O
are	O
variadic	B-Language
,	O
and	O
the	O
proc	O
(	O
the	O
constructor	O
for	O
scripted	O
command	B-Application
procedures	O
)	O
allows	O
one	O
to	O
define	O
default	O
values	O
for	O
unspecified	O
arguments	O
and	O
a	O
catch-all	O
argument	O
to	O
allow	O
the	O
code	O
to	O
process	O
arbitrary	O
numbers	O
of	O
arguments	O
.	O
</s>
<s>
Tcl	B-Operating_System
is	O
not	O
statically	O
typed	O
:	O
each	O
variable	O
may	O
contain	O
integers	O
,	O
floats	O
,	O
strings	O
,	O
lists	O
,	O
command	B-Application
names	O
,	O
dictionaries	B-Application
,	O
or	O
any	O
other	O
value	O
;	O
values	O
are	O
reinterpreted	O
(	O
subject	O
to	O
syntactic	O
constraints	O
)	O
as	O
other	O
types	O
on	O
demand	O
.	O
</s>
<s>
However	O
,	O
values	O
are	O
immutable	B-Application
and	O
operations	O
that	O
appear	O
to	O
change	O
them	O
actually	O
just	O
return	O
a	O
new	O
value	O
instead	O
.	O
</s>
<s>
The	O
most	O
important	O
commands	B-Application
that	O
refer	O
to	O
program	O
execution	O
and	O
data	O
operations	O
are	O
:	O
</s>
<s>
set	O
writes	O
a	O
new	O
value	O
to	O
a	O
variable	O
(	O
creates	O
a	O
variable	O
if	B-Language
did	O
not	O
exist	O
)	O
.	O
</s>
<s>
If	B-Language
used	O
only	O
with	O
one	O
argument	O
,	O
it	O
returns	O
the	O
value	O
of	O
the	O
given	O
variable	O
(	O
it	O
must	O
exist	O
in	O
this	O
case	O
)	O
.	O
</s>
<s>
proc	O
defines	O
a	O
new	O
command	B-Application
,	O
whose	O
execution	O
results	O
in	O
executing	O
a	O
given	O
Tcl	B-Operating_System
script	O
,	O
written	O
as	O
a	O
set	O
of	O
commands	B-Application
.	O
</s>
<s>
The	O
usual	O
execution	O
control	O
commands	B-Application
are	O
:	O
</s>
<s>
if	B-Language
executes	O
given	O
script	O
body	O
(	O
second	O
argument	O
)	O
,	O
if	B-Language
the	O
condition	O
(	O
first	O
argument	O
)	O
is	O
satisfied	O
.	O
</s>
<s>
Those	O
above	O
looping	O
commands	B-Application
can	O
be	O
additionally	O
controlled	O
by	O
the	O
following	O
commands	B-Application
:	O
</s>
<s>
continue	O
interrupts	O
the	O
body	O
execution	O
,	O
but	O
the	O
control	O
is	O
still	O
given	O
back	O
to	O
the	O
looping	O
command	B-Application
.	O
</s>
<s>
expr	O
passes	O
the	O
argument	O
to	O
a	O
separate	O
expression	O
interpreter	B-Application
and	O
returns	O
the	O
evaluated	O
value	O
.	O
</s>
<s>
Note	O
that	O
the	O
same	O
interpreter	B-Application
is	O
used	O
also	O
for	O
"	O
conditional	B-Language
"	O
expression	O
for	O
if	B-Language
and	O
looping	O
commands	B-Application
.	O
</s>
<s>
list	O
creates	O
a	O
list	O
comprising	O
all	O
the	O
arguments	O
,	O
or	O
an	O
empty	O
string	O
if	B-Language
no	O
argument	O
is	O
specified	O
.	O
</s>
<s>
The	O
lindex	O
command	B-Application
may	O
be	O
used	O
on	O
the	O
result	O
to	O
re-extract	O
the	O
original	O
arguments	O
.	O
</s>
<s>
array	B-Application
manipulates	O
array	B-Application
variables	O
.	O
</s>
<s>
dict	O
manipulates	O
dictionary	B-Application
(	O
since	O
8.5	O
)	O
,	O
which	O
are	O
lists	O
with	O
an	O
even	O
number	O
of	O
elements	O
where	O
every	O
two	O
elements	O
are	O
interpreted	B-Application
as	O
a	O
key/value	O
pair	O
.	O
</s>
<s>
regexp	B-Language
matches	O
a	O
regular	B-Language
expression	I-Language
against	O
a	O
string	O
.	O
</s>
<s>
regsub	O
Performs	O
substitutions	O
based	O
on	O
regular	B-Language
expression	I-Language
pattern	O
matching	O
.	O
</s>
<s>
uplevel	O
is	O
a	O
command	B-Application
that	O
allows	O
a	O
command	B-Application
script	O
to	O
be	O
executed	O
in	O
a	O
scope	B-Language
other	O
than	O
the	O
current	O
innermost	O
scope	B-Language
on	O
the	O
stack	O
.	O
</s>
<s>
namespace	O
lets	O
you	O
create	O
,	O
access	O
,	O
and	O
destroy	O
separate	O
contexts	O
for	O
commands	B-Application
and	O
variables	O
.	O
</s>
<s>
apply	O
applies	O
an	O
anonymous	B-General_Concept
function	I-General_Concept
(	O
since	O
8.5	O
)	O
.	O
</s>
<s>
coroutine	B-Architecture
,	O
yield	O
,	O
and	O
yieldto	O
create	O
and	O
produce	O
values	O
from	O
coroutines	B-Architecture
(	O
since	O
8.6	O
)	O
.	O
</s>
<s>
zlib	O
provides	O
access	O
to	O
the	O
compression	O
and	O
checksumming	O
facilities	O
of	O
the	O
Zlib	O
library	B-Library
(	O
since	O
8.6	O
)	O
.	O
</s>
<s>
uplevel	O
allows	O
a	O
command	B-Application
script	O
to	O
be	O
executed	O
in	O
a	O
scope	B-Language
other	O
than	O
the	O
current	O
innermost	O
scope	B-Language
on	O
the	O
stack	O
.	O
</s>
<s>
Because	O
the	O
command	B-Application
script	O
may	O
itself	O
call	O
procedures	O
that	O
use	O
the	O
uplevel	O
command	B-Application
,	O
this	O
has	O
the	O
net	O
effect	O
of	O
transforming	O
the	O
call	B-General_Concept
stack	I-General_Concept
into	O
a	O
call	O
tree	O
.	O
</s>
<s>
It	O
was	O
originally	O
implemented	O
to	O
permit	O
Tcl	B-Operating_System
procedures	O
to	O
reimplement	O
built-in	O
commands	B-Application
(	O
like	O
for	O
,	O
if	B-Language
or	O
while	O
)	O
and	O
still	O
have	O
the	O
ability	O
to	O
manipulate	O
local	O
variables	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
following	O
Tcl	B-Operating_System
script	O
is	O
a	O
reimplementation	O
of	O
the	O
for	O
command	B-Application
(	O
omitting	O
exception	B-General_Concept
handling	I-General_Concept
)	O
:	O
</s>
<s>
The	O
upvar	O
command	B-Application
simplifies	O
the	O
implementation	O
of	O
call-by-name	O
procedure	O
calling	O
and	O
also	O
makes	O
it	O
easier	O
to	O
build	O
new	O
control	O
constructs	O
as	O
Tcl	B-Operating_System
procedures	O
.	O
</s>
<s>
A	O
decr	O
command	B-Application
that	O
works	O
like	O
the	O
built-in	O
incr	O
command	B-Application
except	O
it	O
subtracts	O
the	O
value	O
from	O
the	O
variable	O
instead	O
of	O
adding	O
it	O
:	O
</s>
<s>
Tcl	B-Operating_System
8.6	O
added	O
a	O
built-in	O
dynamic	O
object	B-Language
system	I-Language
,	O
TclOO	O
,	O
in	O
2012	O
.	O
</s>
<s>
Class-based	O
object	B-Language
system	I-Language
.	O
</s>
<s>
Tcl	B-Operating_System
did	O
not	O
have	O
object	B-Language
oriented	I-Language
(	O
OO	O
)	O
syntax	O
until	O
2012	O
,	O
so	O
various	O
extension	O
packages	O
emerged	O
to	O
enable	O
object-oriented	B-Language
programming	I-Language
.	O
</s>
<s>
They	O
are	O
widespread	O
in	O
existing	O
Tcl	B-Operating_System
source	O
code	O
.	O
</s>
<s>
TclOO	O
was	O
not	O
only	O
added	O
to	O
build	O
a	O
strong	O
object	B-Language
oriented	I-Language
system	O
,	O
but	O
also	O
to	O
enable	O
extension	O
packages	O
to	O
build	O
object	B-Language
oriented	I-Language
abstractions	O
using	O
it	O
as	O
a	O
foundation	O
.	O
</s>
<s>
After	O
the	O
release	O
of	O
TclOO	O
,	O
incr	B-Application
Tcl	I-Application
was	O
updated	O
to	O
use	O
TclOO	O
as	O
its	O
foundation	O
.	O
</s>
<s>
is	O
a	O
pure-Tcl	O
implementation	O
of	O
an	O
HTTP	O
protocol	O
server	O
.	O
</s>
<s>
It	O
runs	O
as	O
a	O
script	O
on	O
top	O
of	O
a	O
vanilla	O
Tcl	B-Operating_System
interpreter	B-Application
.	O
</s>
<s>
is	O
an	O
open	O
source	O
programming	O
system	O
for	O
Apache	B-Application
HTTP	I-Application
Server	I-Application
that	O
allows	O
developers	O
to	O
use	O
Tcl	B-Operating_System
as	O
a	O
scripting	O
language	O
for	O
creating	O
dynamic	O
web	O
applications	O
.	O
</s>
<s>
Rivet	O
is	O
similar	O
to	O
PHP	B-Application
,	O
ASP	B-General_Concept
,	O
and	O
JSP	B-Language
.	O
</s>
<s>
Rivet	O
can	O
use	O
any	O
of	O
the	O
thousands	O
of	O
publicly	O
available	O
Tcl	B-Operating_System
packages	O
that	O
offer	O
countless	O
features	O
such	O
as	O
database	O
interaction	O
(	O
Oracle	O
,	O
PostgreSQL	B-Application
,	O
MySQL	B-Application
,	O
SQLite	B-Language
,	O
etc	O
.	O
</s>
<s>
)	O
,	O
or	O
interfaces	O
to	O
popular	O
applications	O
such	O
as	O
the	O
GD	B-Language
Graphics	I-Language
Library	I-Language
.	O
</s>
<s>
Tcl	B-Operating_System
interfaces	O
natively	O
with	O
the	O
C	B-Language
language	I-Language
.	O
</s>
<s>
This	O
is	O
because	O
it	O
was	O
originally	O
written	O
to	O
be	O
a	O
framework	O
for	O
providing	O
a	O
syntactic	O
front-end	O
to	O
commands	B-Application
written	O
in	O
C	B-Language
,	O
and	O
all	O
commands	B-Application
in	O
the	O
language	O
(	O
including	O
things	O
that	O
might	O
otherwise	O
be	O
keywords	O
,	O
such	O
as	O
if	B-Language
or	O
while	O
)	O
are	O
implemented	O
this	O
way	O
.	O
</s>
<s>
Each	O
command	B-Application
implementation	O
function	O
is	O
passed	O
an	O
array	B-Application
of	O
values	O
that	O
describe	O
the	O
(	O
already	O
substituted	O
)	O
arguments	O
to	O
the	O
command	B-Application
,	O
and	O
is	O
free	O
to	O
interpret	O
those	O
values	O
as	O
it	O
sees	O
fit	O
.	O
</s>
<s>
Digital	O
logic	O
simulators	O
often	O
include	O
a	O
Tcl	B-Operating_System
scripting	O
interface	O
for	O
simulating	O
Verilog	B-Language
,	O
VHDL	B-Language
and	O
SystemVerilog	O
hardware	O
languages	O
.	O
</s>
<s>
SWIG	B-Application
,	O
)	O
to	O
automatically	O
generate	O
the	O
necessary	O
code	O
to	O
connect	O
arbitrary	O
C	B-Language
functions	O
and	O
the	O
Tcl	B-Operating_System
runtime	O
,	O
and	O
does	O
the	O
reverse	O
,	O
allowing	O
embedding	O
of	O
arbitrary	O
C	B-Language
code	O
inside	O
a	O
Tcl	B-Operating_System
script	O
and	O
compiling	O
it	O
at	O
runtime	O
into	O
a	O
DLL	O
.	O
</s>
<s>
The	O
Tcl	B-Operating_System
language	O
has	O
always	O
allowed	O
for	O
extension	O
packages	O
,	O
which	O
provide	O
additional	O
functionality	O
,	O
such	O
as	O
a	O
GUI	B-Application
,	O
terminal-based	O
application	O
automation	O
,	O
database	O
access	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
Tk	B-Application
The	O
most	O
popular	O
Tcl	B-Operating_System
extension	O
is	O
the	O
Tk	B-Application
toolkit	I-Application
,	O
which	O
provides	O
a	O
graphical	B-Application
user	I-Application
interface	I-Application
library	B-Library
for	O
a	O
variety	O
of	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Each	O
GUI	B-Application
consists	O
of	O
one	O
or	O
more	O
frames	O
.	O
</s>
<s>
Expect	O
One	O
of	O
the	O
other	O
very	O
popular	O
Tcl	B-Operating_System
extensions	O
is	O
Expect	O
extension	O
.	O
</s>
<s>
The	O
early	O
close	O
relationship	O
of	O
Expect	O
with	O
Tcl	B-Operating_System
is	O
largely	O
responsible	O
for	O
the	O
popularity	O
of	O
Tcl	B-Operating_System
in	O
prolific	O
areas	O
of	O
use	O
such	O
as	O
in	O
Unix	B-Application
testing	O
,	O
where	O
Expect	O
was	O
(	O
and	O
still	O
is	O
today	O
)	O
employed	O
very	O
successfully	O
to	O
automate	O
telnet	O
,	O
ssh	O
,	O
and	O
serial	O
sessions	O
to	O
perform	O
many	O
repetitive	O
tasks	O
(	O
i.e.	O
,	O
scripting	O
of	O
formerly	O
interactive-only	O
applications	O
)	O
.	O
</s>
<s>
Tcl	B-Operating_System
was	O
the	O
only	O
way	O
to	O
run	O
Expect	O
,	O
so	O
Tcl	B-Operating_System
became	O
very	O
popular	O
in	O
these	O
areas	O
of	O
industry	O
.	O
</s>
<s>
Tile/Ttk	O
Tile/Ttk	O
is	O
a	O
styles	O
and	O
theming	O
widget	O
collection	O
that	O
can	O
replace	O
most	O
of	O
the	O
widgets	O
in	O
Tk	B-Application
with	O
variants	O
that	O
are	O
truly	O
platform	O
native	O
through	O
calls	O
to	O
an	O
operating	B-General_Concept
system	I-General_Concept
's	O
API	B-General_Concept
.	O
</s>
<s>
Themes	O
covered	O
in	O
this	O
way	O
are	O
Windows	B-Application
XP	I-Application
,	O
Windows	O
Classic	O
,	O
Qt	B-Language
(	O
that	O
hooks	O
into	O
the	O
X11	B-Operating_System
KDE	B-Application
environment	O
libraries	O
)	O
and	O
Aqua	B-Application
(	O
Mac	O
OS	O
X	B-Operating_System
)	O
.	O
</s>
<s>
Themes	O
created	O
this	O
way	O
include	O
Classic	O
Tk	B-Application
,	O
Step	O
,	O
Alt/Revitalized	O
,	O
Plastik	O
and	O
Keramik	O
.	O
</s>
<s>
Under	O
Tcl	B-Operating_System
8.4	O
,	O
this	O
package	O
is	O
known	O
as	O
Tile	O
,	O
while	O
in	O
Tcl	B-Operating_System
8.5	O
it	O
has	O
been	O
folded	O
into	O
the	O
core	O
distribution	O
of	O
Tk	B-Application
(	O
as	O
Ttk	O
)	O
.	O
</s>
<s>
Tix	O
Tix	O
,	O
the	O
Tk	B-Application
Interface	O
eXtension	O
,	O
is	O
a	O
set	O
of	O
user	O
interface	O
components	O
that	O
expand	O
the	O
capabilities	O
of	O
Tcl/Tk	B-Application
and	O
Python	B-Language
applications	O
.	O
</s>
<s>
It	O
is	O
an	O
open	O
source	O
software	O
package	O
maintained	O
by	O
volunteers	O
in	O
the	O
Tix	O
Project	O
Group	O
and	O
released	O
under	O
a	O
BSD-style	B-Operating_System
license	I-Operating_System
.	O
</s>
<s>
Itcl/IncrTcl	O
Itcl	B-Application
is	O
an	O
object	B-Language
system	I-Language
for	O
Tcl	B-Operating_System
,	O
and	O
is	O
normally	O
named	O
as	O
[	O
incr	O
Tcl ]	O
(	O
that	O
being	O
the	O
way	O
to	O
increment	O
in	O
Tcl	B-Operating_System
,	O
similar	O
in	O
fashion	O
to	O
the	O
name	O
C++	B-Language
)	O
.	O
</s>
<s>
Tcllib	O
Tcllib	O
is	O
a	O
set	O
of	O
scripted	O
packages	O
for	O
Tcl	B-Operating_System
that	O
can	O
be	O
used	O
with	O
no	O
compilation	O
steps	O
.	O
</s>
<s>
Tklib	O
Tklib	O
is	O
a	O
collection	O
of	O
utility	O
modules	O
for	O
Tk	B-Application
,	O
and	O
a	O
companion	O
to	O
Tcllib	O
.	O
</s>
<s>
TclTLS	O
TclTLS	O
is	O
OpenSSL	B-Language
extension	O
to	O
Tcl	B-Operating_System
.	O
</s>
<s>
TclUDP	O
The	O
TclUDP	O
extension	O
provides	O
a	O
simple	O
library	B-Library
to	O
support	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
sockets	B-Protocol
in	O
Tcl	B-Operating_System
.	O
</s>
<s>
Databases	O
Tcl	B-Operating_System
Database	O
Connectivity	O
(	O
TDBC	O
)	O
,	O
part	O
of	O
Tcl	B-Operating_System
8.6	O
,	O
is	O
a	O
common	O
database	O
access	O
interface	O
for	O
Tcl	B-Operating_System
scripts	O
.	O
</s>
<s>
It	O
currently	O
supports	O
drivers	O
for	O
accessing	O
MySQL	B-Application
,	O
ODBC	B-Application
,	O
PostgreSQL	B-Application
and	O
SQLite	B-Language
databases	O
.	O
</s>
