<s>
Icon	B-Language
is	O
a	O
very	B-Language
high-level	I-Language
programming	I-Language
language	I-Language
based	O
on	O
the	O
concept	O
of	O
"	O
goal-directed	O
execution	O
"	O
in	O
which	O
code	O
returns	O
a	O
"	O
success	O
"	O
along	O
with	O
valid	O
values	O
,	O
or	O
a	O
"	O
failure	O
"	O
,	O
indicating	O
that	O
there	O
is	O
no	O
valid	O
data	O
to	O
return	O
.	O
</s>
<s>
Because	O
the	O
logic	O
for	O
basic	O
control	O
structures	O
is	O
often	O
implicit	O
in	O
Icon	B-Language
,	O
common	O
tasks	O
can	O
be	O
completed	O
with	O
less	O
explicit	O
code	O
.	O
</s>
<s>
Icon	B-Language
was	O
designed	O
by	O
Ralph	O
Griswold	O
after	O
leaving	O
Bell	O
Labs	O
where	O
he	O
was	O
a	O
major	O
contributor	O
to	O
the	O
SNOBOL	B-Application
language	O
.	O
</s>
<s>
SNOBOL	B-Application
was	O
a	O
string-processing	O
language	O
with	O
what	O
would	O
be	O
considered	O
dated	O
syntax	O
by	O
the	O
standards	O
of	O
the	O
early	O
1970s	O
.	O
</s>
<s>
After	O
moving	O
to	O
the	O
University	O
of	O
Arizona	O
,	O
he	O
further	O
developed	O
the	O
underlying	O
SNOBOL	B-Application
concepts	O
in	O
SL5	O
,	O
but	O
considered	O
the	O
result	O
to	O
be	O
a	O
failure	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
significantly	O
updated	O
Icon	B-Language
,	O
which	O
blends	O
the	O
short	O
but	O
conceptually	O
dense	O
code	O
of	O
SNOBOL-like	O
languages	O
with	O
the	O
more	O
familiar	O
syntax	O
of	O
ALGOL-inspired	O
languages	O
like	O
C	B-Language
or	O
Pascal	B-Application
.	O
</s>
<s>
Like	O
the	O
languages	O
that	O
inspired	O
it	O
,	O
the	O
primary	O
area	O
of	O
use	O
of	O
Icon	B-Language
is	O
managing	O
strings	O
and	O
textual	O
patterns	O
.	O
</s>
<s>
In	O
Icon	B-Language
most	O
of	O
these	O
sorts	O
of	O
tests	O
are	O
simply	O
not	O
required	O
,	O
reducing	O
the	O
amount	O
of	O
code	O
written	O
by	O
the	O
programmer	O
.	O
</s>
<s>
Complex	O
pattern	O
handling	O
can	O
be	O
accomplished	O
in	O
a	O
few	O
lines	O
of	O
terse	O
code	O
,	O
similar	O
to	O
more	O
dedicated	O
languages	O
like	O
Perl	B-Language
but	O
retaining	O
a	O
more	O
function-oriented	O
syntax	O
familiar	O
to	O
users	O
of	O
other	O
ALGOL-like	B-Language
languages	O
.	O
</s>
<s>
Icon	B-Language
is	O
not	O
object-oriented	B-Language
,	O
but	O
an	O
object-oriented	B-Language
extension	O
called	O
Idol	O
was	O
developed	O
in	O
1996	O
which	O
eventually	O
became	O
Unicon	B-Language
.	O
</s>
<s>
It	O
also	O
inspired	O
other	O
languages	O
,	O
with	O
its	O
simple	O
generators	O
being	O
especially	O
influential	O
;	O
Icon	B-Language
's	O
generators	O
were	O
a	O
major	O
inspiration	O
for	O
the	O
Python	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
The	O
original	O
SNOBOL	B-Application
effort	O
,	O
retroactively	O
known	O
as	O
SNOBOL1	O
,	O
launched	O
in	O
the	O
fall	O
of	O
1962	O
at	O
the	O
Bell	O
Labs	O
Programming	O
Research	O
Studies	O
Department	O
.	O
</s>
<s>
The	O
first	O
versions	O
were	O
running	O
on	O
the	O
IBM	B-Device
7090	I-Device
in	O
early	O
1963	O
,	O
and	O
by	O
the	O
summer	O
had	O
been	O
built	O
out	O
and	O
was	O
being	O
used	O
across	O
Bell	O
.	O
</s>
<s>
This	O
led	O
almost	O
immediately	O
to	O
SNOBOL2	O
,	O
which	O
added	O
a	O
number	O
of	O
built-in	O
functions	O
,	O
and	O
the	O
ability	O
to	O
link	O
to	O
external	O
assembly	B-Language
language	I-Language
code	O
.	O
</s>
<s>
SNOBOL3	O
's	O
introduction	O
corresponded	O
with	O
major	O
changes	O
within	O
the	O
Bell	O
Labs	O
computing	O
department	O
,	O
including	O
the	O
addition	O
of	O
the	O
new	O
GE	B-Device
645	I-Device
mainframe	O
which	O
would	O
require	O
a	O
rewrite	O
of	O
SNOBOL	B-Application
.	O
</s>
<s>
Instead	O
,	O
the	O
team	O
suggested	O
writing	O
a	O
new	O
version	O
that	O
would	O
run	O
on	O
a	O
virtual	B-Architecture
machine	I-Architecture
,	O
named	O
SIL	O
for	O
SNOBOL	B-Application
Intermediate	O
Language	O
,	O
allowing	O
it	O
to	O
be	O
easily	O
ported	O
to	O
any	O
sufficiently	O
powerful	O
platform	O
.	O
</s>
<s>
This	O
proposal	O
was	O
accepted	O
as	O
SNOBOL4	B-Application
in	O
September	O
1965	O
.	O
</s>
<s>
Further	O
work	O
on	O
the	O
language	O
continued	O
throughout	O
the	O
rest	O
of	O
the	O
1960s	O
,	O
notably	O
adding	O
the	O
associative	B-Application
array	I-Application
type	O
in	O
later	O
version	O
,	O
which	O
they	O
referred	O
to	O
as	O
a	O
table	O
.	O
</s>
<s>
He	O
introduced	O
SNOBOL4	B-Application
as	O
a	O
research	O
tool	O
at	O
that	O
time	O
.	O
</s>
<s>
As	O
a	O
language	O
originally	O
developed	O
in	O
the	O
early	O
1960s	O
,	O
SNOBOL	B-Application
's	O
syntax	O
bears	O
the	O
marks	O
of	O
other	O
early	O
programming	O
languages	O
like	O
FORTRAN	B-Application
and	O
COBOL	B-Application
.	O
</s>
<s>
In	O
particular	O
,	O
the	O
language	O
is	O
column-dependant	O
,	O
as	O
many	O
of	O
these	O
languages	O
were	O
entered	O
on	O
punch	B-Architecture
cards	I-Architecture
where	O
column	O
layout	O
is	O
natural	O
.	O
</s>
<s>
Additionally	O
,	O
control	O
structures	O
were	O
almost	O
entirely	O
based	O
on	O
branching	O
around	O
code	O
rather	O
than	O
the	O
use	O
of	O
blocks	O
,	O
which	O
were	O
becoming	O
a	O
must-have	O
feature	O
after	O
the	O
introduction	O
of	O
ALGOL	B-Language
60	I-Language
.	O
</s>
<s>
By	O
the	O
time	O
he	O
moved	O
to	O
Arizona	O
,	O
the	O
syntax	O
of	O
SNOBOL4	B-Application
was	O
hopelessly	O
outdated	O
.	O
</s>
<s>
Griswold	O
began	O
the	O
effort	O
of	O
implementing	O
SNOBOL	B-Application
's	O
underlying	O
success/failure	O
concept	O
with	O
traditional	O
flow	O
control	O
structures	O
like	O
if/then	O
.	O
</s>
<s>
This	O
became	O
SL5	O
,	O
short	O
for	O
"	O
SNOBOL	B-Application
Language	O
5	O
"	O
,	O
but	O
the	O
result	O
was	O
unsatisfying	O
.	O
</s>
<s>
He	O
abandoned	O
the	O
very	O
powerful	O
function	O
system	O
introduced	O
in	O
SL5	O
with	O
a	O
simpler	O
concept	O
of	O
suspend/resume	O
and	O
developed	O
a	O
new	O
concept	O
for	O
the	O
natural	O
successor	O
to	O
SNOBOL4	B-Application
with	O
the	O
following	O
principles	O
;	O
</s>
<s>
The	O
new	O
language	O
was	O
initially	O
known	O
as	O
SNOBOL5	O
,	O
but	O
as	O
it	O
was	O
significantly	O
different	O
from	O
SNOBOL	B-Application
in	O
all	O
but	O
the	O
underlying	O
concept	O
,	O
a	O
new	O
name	O
was	O
ultimately	O
desired	O
.	O
</s>
<s>
After	O
considering	O
"	O
s	O
"	O
as	O
a	O
sort	O
of	O
homage	O
to	O
"	O
C	B-Language
"	O
,	O
but	O
this	O
was	O
ultimately	O
abandoned	O
due	O
to	O
the	O
problems	O
with	O
typesetting	O
documents	O
using	O
that	O
name	O
.	O
</s>
<s>
It	O
was	O
at	O
this	O
time	O
that	O
Xerox	O
PARC	O
began	O
publishing	O
about	O
their	O
work	O
on	O
graphical	B-Application
user	I-Application
interfaces	I-Application
and	O
the	O
term	O
"	O
icon	B-Language
"	O
began	O
to	O
enter	O
the	O
computer	O
lexicon	O
.	O
</s>
<s>
The	O
decision	O
was	O
made	O
to	O
change	O
the	O
name	O
initially	O
to	O
"	O
icon	B-Language
"	O
before	O
finally	O
choosing	O
"	O
Icon	B-Language
"	O
.	O
</s>
<s>
The	O
Icon	B-Language
language	I-Language
is	O
derived	O
from	O
the	O
ALGOL-class	O
of	O
structured	B-Language
programming	I-Language
languages	O
,	O
and	O
thus	O
has	O
syntax	O
similar	O
to	O
C	B-Language
or	O
Pascal	B-Application
.	O
</s>
<s>
Icon	B-Language
is	O
most	O
similar	O
to	O
Pascal	B-Application
,	O
using	O
syntax	O
for	O
assignments	O
,	O
the	O
keyword	O
and	O
similar	O
syntax	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
Icon	B-Language
uses	O
C-style	O
braces	O
for	O
structuring	O
execution	O
groups	O
,	O
and	O
programs	O
start	O
by	O
running	O
a	O
procedure	O
called	O
.	O
</s>
<s>
In	O
many	O
ways	O
Icon	B-Language
also	O
shares	O
features	O
with	O
most	O
scripting	B-Language
languages	I-Language
(	O
as	O
well	O
as	O
SNOBOL	B-Application
and	O
SL5	O
,	O
from	O
which	O
they	O
were	O
taken	O
)	O
:	O
variables	O
do	O
not	O
have	O
to	O
be	O
declared	O
,	O
types	O
are	O
cast	O
automatically	O
,	O
and	O
numbers	O
can	O
be	O
converted	O
to	O
strings	O
and	O
back	O
automatically	O
.	O
</s>
<s>
Another	O
feature	O
common	O
to	O
many	O
scripting	B-Language
languages	I-Language
,	O
but	O
not	O
all	O
,	O
is	O
the	O
lack	O
of	O
a	O
line-ending	O
character	O
;	O
in	O
Icon	B-Language
,	O
lines	O
that	O
do	O
not	O
end	O
with	O
a	O
semicolon	O
get	O
ended	O
by	O
an	O
implied	O
semicolon	O
if	O
it	O
makes	O
sense	O
.	O
</s>
<s>
Procedures	O
are	O
the	O
basic	O
building	O
blocks	O
of	O
Icon	B-Language
programs	O
.	O
</s>
<s>
Although	O
they	O
use	O
Pascal	B-Application
naming	O
,	O
they	O
work	O
more	O
like	O
C	B-Language
functions	O
and	O
can	O
return	O
values	O
;	O
there	O
is	O
no	O
keyword	O
in	O
Icon	B-Language
.	O
</s>
<s>
One	O
of	O
the	O
key	O
concepts	O
in	O
SNOBOL	B-Application
was	O
that	O
its	O
functions	O
returned	O
the	O
"	O
success	O
"	O
or	O
"	O
failure	O
"	O
as	O
primitives	O
of	O
the	O
language	O
rather	O
than	O
using	O
magic	O
numbers	O
or	O
other	O
techniques	O
.	O
</s>
<s>
For	O
instance	O
,	O
a	O
function	O
that	O
returns	O
the	O
position	O
of	O
a	O
substring	O
within	O
another	O
string	O
is	O
a	O
common	O
routine	O
found	O
in	O
most	O
language	O
runtime	B-Device
systems	I-Device
;	O
in	O
JavaScript	B-Language
one	O
might	O
want	O
to	O
find	O
the	O
position	O
of	O
the	O
word	O
"	O
World	O
"	O
within	O
"	O
Hello	O
,	O
World	O
!	O
</s>
<s>
In	O
JavaScript	B-Language
,	O
as	O
in	O
most	O
languages	O
,	O
this	O
will	O
be	O
indicated	O
by	O
returning	O
a	O
magic	O
number	O
,	O
in	O
this	O
case	O
-1	O
.	O
</s>
<s>
In	O
SNOBOL	B-Application
a	O
failure	O
of	O
this	O
sort	O
returns	O
a	O
special	O
value	O
,	O
.	O
</s>
<s>
SNOBOL	B-Application
's	O
syntax	O
operates	O
directly	O
on	O
the	O
success	O
or	O
failure	O
of	O
the	O
operation	O
,	O
jumping	O
to	O
labelled	O
sections	O
of	O
the	O
code	O
without	O
having	O
to	O
write	O
a	O
separate	O
test	O
.	O
</s>
<s>
Icon	B-Language
retained	O
the	O
concept	O
of	O
flow	O
control	O
based	O
on	O
success	O
or	O
failure	O
but	O
developed	O
the	O
language	O
further	O
.	O
</s>
<s>
One	O
change	O
was	O
the	O
replacement	O
of	O
the	O
labelled	O
-like	O
branching	O
with	O
block-oriented	O
structures	O
in	O
keeping	O
with	O
the	O
structured	B-Language
programming	I-Language
style	O
that	O
was	O
sweeping	O
the	O
computer	O
industry	O
in	O
the	O
late	O
1960s	O
.	O
</s>
<s>
This	O
is	O
a	O
key	O
concept	O
of	O
the	O
Icon	B-Language
language	I-Language
.	O
</s>
<s>
Whereas	O
in	O
traditional	O
languages	O
one	O
would	O
have	O
to	O
include	O
code	O
to	O
test	O
the	O
success	O
or	O
failure	O
based	O
on	O
boolean	O
logic	O
and	O
then	O
branch	O
based	O
on	O
the	O
outcome	O
,	O
such	O
tests	O
and	O
branches	O
are	O
inherent	O
to	O
Icon	B-Language
code	O
and	O
do	O
not	O
have	O
to	O
be	O
explicitly	O
written	O
.	O
</s>
<s>
For	O
instance	O
,	O
consider	O
this	O
bit	O
of	O
code	O
written	O
in	O
the	O
Java	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
will	O
eventually	O
run	O
out	O
of	O
characters	O
to	O
read	O
from	O
the	O
file	O
,	O
potentially	O
on	O
its	O
very	O
first	O
call	O
,	O
which	O
would	O
leave	O
in	O
an	O
undetermined	O
state	O
and	O
potentially	O
cause	O
to	O
cause	O
a	O
null	O
pointer	O
exception	B-General_Concept
.	O
</s>
<s>
In	O
contrast	O
,	O
in	O
Icon	B-Language
the	O
function	O
returns	O
a	O
line	O
of	O
text	O
or	O
.	O
</s>
<s>
The	O
equivalent	O
code	O
in	O
Icon	B-Language
is	O
:	O
</s>
<s>
There	O
is	O
no	O
need	O
to	O
specify	O
a	O
test	O
against	O
the	O
magic	O
number	O
as	O
in	O
the	O
Java	B-Language
example	O
,	O
this	O
is	O
implicit	O
,	O
and	O
the	O
resulting	O
code	O
is	O
simplified	O
.	O
</s>
<s>
Icon	B-Language
's	O
branching	O
and	O
looping	O
constructs	O
are	O
all	O
based	O
on	O
the	O
success	O
or	O
failure	O
of	O
the	O
code	O
inside	O
them	O
,	O
not	O
on	O
an	O
arbitrary	O
boolean	O
test	O
provided	O
by	O
the	O
programmer	O
.	O
</s>
<s>
Icon	B-Language
refers	O
to	O
this	O
concept	O
as	O
goal-directed	O
execution	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
contrast	O
the	O
concept	O
of	O
success	O
and	O
failure	O
with	O
the	O
concept	O
of	O
an	O
exception	B-General_Concept
;	O
exceptions	O
are	O
unusual	O
situations	O
,	O
not	O
expected	O
outcomes	O
.	O
</s>
<s>
Fails	O
in	O
Icon	B-Language
are	O
expected	O
outcomes	O
;	O
reaching	O
the	O
end	O
of	O
a	O
file	O
is	O
an	O
expected	O
situation	O
and	O
not	O
an	O
exception	B-General_Concept
.	O
</s>
<s>
Icon	B-Language
does	O
not	O
have	O
exception	B-General_Concept
handling	I-General_Concept
in	O
the	O
traditional	O
sense	O
,	O
although	O
fail	O
is	O
often	O
used	O
in	O
exception-like	O
situations	O
.	O
</s>
<s>
In	O
traditional	O
language	O
,	O
these	O
"	O
other	O
conditions	O
"	O
have	O
no	O
natural	O
way	O
of	O
being	O
indicated	O
;	O
additional	O
magic	O
numbers	O
may	O
be	O
used	O
,	O
but	O
more	O
typically	O
exception	B-General_Concept
handling	I-General_Concept
is	O
used	O
to	O
"	O
throw	O
"	O
a	O
value	O
.	O
</s>
<s>
For	O
instance	O
,	O
to	O
handle	O
a	O
missing	O
file	O
in	O
the	O
Java	B-Language
code	I-Language
,	O
one	O
might	O
see	O
:	O
</s>
<s>
Since	O
Java	B-Language
does	O
not	O
allow	O
exceptions	O
to	O
be	O
compared	O
as	O
logic	O
elements	O
,	O
as	O
under	O
Icon	B-Language
,	O
the	O
lengthy	O
syntax	O
must	O
be	O
used	O
instead	O
.	O
</s>
<s>
Try	B-General_Concept
blocks	I-General_Concept
also	O
impose	O
a	O
performance	O
penalty	O
even	O
if	O
no	O
exception	B-General_Concept
is	O
thrown	O
,	O
a	O
distributed	O
cost	O
that	O
Icon	B-Language
normally	O
avoids	O
.	O
</s>
<s>
Icon	B-Language
uses	O
this	O
same	O
goal-directed	O
mechanism	O
to	O
perform	O
traditional	O
boolean	O
tests	O
,	O
although	O
with	O
subtle	O
differences	O
.	O
</s>
<s>
For	O
this	O
task	O
,	O
Icon	B-Language
has	O
the	O
reversible	O
assignment	O
operator	O
,	O
,	O
and	O
the	O
reversible	O
exchange	O
,	O
.	O
</s>
<s>
This	O
provides	O
an	O
analog	O
of	O
atomicity	B-Operating_System
in	O
the	O
execution	O
.	O
</s>
<s>
Expressions	O
in	O
Icon	B-Language
may	O
return	O
a	O
single	O
value	O
,	O
for	O
instance	O
,	O
will	O
evaluate	O
and	O
return	O
x	O
if	O
the	O
value	O
of	O
x	O
is	O
less	O
than	O
5	O
,	O
or	O
else	O
fail	O
.	O
</s>
<s>
However	O
,	O
Icon	B-Language
also	O
includes	O
the	O
concept	O
of	O
procedures	O
that	O
do	O
not	O
immediately	O
return	O
success	O
or	O
failure	O
,	O
and	O
instead	O
return	O
new	O
values	O
every	O
time	O
they	O
are	O
called	O
.	O
</s>
<s>
These	O
are	O
known	O
as	O
generators	O
,	O
and	O
are	O
a	O
key	O
part	O
of	O
the	O
Icon	B-Language
language	I-Language
.	O
</s>
<s>
Within	O
the	O
parlance	O
of	O
Icon	B-Language
,	O
the	O
evaluation	O
of	O
an	O
expression	O
or	O
function	O
produces	O
a	O
result	O
sequence	O
.	O
</s>
<s>
Icon	B-Language
allows	O
any	O
procedure	O
to	O
return	O
a	O
single	O
value	O
or	O
multiple	O
values	O
,	O
controlled	O
using	O
the	O
,	O
and	O
keywords	O
.	O
</s>
<s>
In	O
this	O
respect	O
it	O
is	O
something	O
like	O
a	O
combination	O
of	O
the	O
concept	O
in	O
C	B-Language
and	O
.	O
</s>
<s>
As	O
lists	O
of	O
integers	O
are	O
commonly	O
found	O
in	O
many	O
programming	O
contexts	O
,	O
Icon	B-Language
also	O
includes	O
the	O
keyword	O
to	O
construct	O
ad	O
hoc	O
integer	O
generators	O
:	O
</s>
<s>
Icon	B-Language
is	O
not	O
strongly	O
typed	O
,	O
so	O
the	O
alternator	O
lists	O
can	O
contain	O
different	O
types	O
of	O
items	O
:	O
</s>
<s>
The	O
concept	O
of	O
generators	O
is	O
particularly	O
useful	O
and	O
powerful	O
when	O
used	O
with	O
string	O
operations	O
,	O
and	O
is	O
a	O
major	O
underlying	O
basis	O
for	O
Icon	B-Language
's	O
overall	O
design	O
.	O
</s>
<s>
In	O
Icon	B-Language
,	O
the	O
equivalent	O
is	O
a	O
generator	O
,	O
so	O
the	O
same	O
results	O
can	O
be	O
created	O
with	O
a	O
single	O
line	O
:	O
</s>
<s>
actually	O
injects	O
values	O
into	O
the	O
function	O
in	O
a	O
fashion	O
similar	O
to	O
blocks	O
under	O
Smalltalk	B-Application
.	O
</s>
<s>
Icon	B-Language
includes	O
several	O
collection	B-Application
types	I-Application
including	O
lists	O
that	O
can	O
also	O
be	O
used	O
as	O
stacks	B-Application
and	O
queues	B-Application
,	O
tables	B-Application
(	O
also	O
known	O
as	O
maps	O
or	O
dictionaries	B-Application
in	O
other	O
languages	O
)	O
,	O
sets	O
and	O
others	O
.	O
</s>
<s>
Icon	B-Language
refers	O
to	O
these	O
as	O
structures	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
bang	O
in	O
causes	O
Icon	B-Language
to	O
return	O
a	O
line	O
of	O
text	O
one	O
by	O
one	O
from	O
the	O
array	O
and	O
finally	O
fail	O
at	O
the	O
end	O
.	O
</s>
<s>
As	O
Icon	B-Language
is	O
typeless	O
,	O
lists	O
can	O
contain	O
any	O
different	O
types	O
of	O
values	O
:	O
</s>
<s>
To	O
build	O
larger	O
lists	O
,	O
Icon	B-Language
includes	O
the	O
generator	O
;	O
generates	O
a	O
list	O
containing	O
10	O
copies	O
of	O
"	O
word	O
"	O
.	O
</s>
<s>
Like	O
arrays	O
in	O
other	O
languages	O
,	O
Icon	B-Language
allows	O
items	O
to	O
be	O
looked	O
up	O
by	O
position	O
,	O
e.g.	O
,	O
.	O
</s>
<s>
Tables	B-Application
are	O
essentially	O
lists	O
with	O
arbitrary	O
index	O
keys	O
rather	O
than	O
integers	O
:	O
</s>
<s>
Icon	B-Language
includes	O
the	O
to	O
produce	O
the	O
union	O
of	O
two	O
sets	O
,	O
the	O
intersection	O
,	O
and	O
the	O
difference	O
.	O
</s>
<s>
Icon	B-Language
includes	O
a	O
number	O
of	O
pre-defined	O
"	O
Cset	O
"	O
s	O
,	O
a	O
set	O
containing	O
various	O
characters	O
.	O
</s>
<s>
There	O
are	O
four	O
standard	O
Csets	O
in	O
Icon	B-Language
,	O
,	O
,	O
,	O
and	O
.	O
</s>
<s>
In	O
Icon	B-Language
,	O
strings	O
are	O
lists	O
of	O
characters	O
.	O
</s>
<s>
Icon	B-Language
refers	O
to	O
the	O
left-hand-side	O
of	O
the	O
as	O
the	O
subject	O
,	O
and	O
passes	O
it	O
into	O
string	O
functions	O
.	O
</s>
<s>
Icon	B-Language
function	O
signatures	O
identify	O
the	O
subject	O
parameter	O
in	O
their	O
definitions	O
so	O
the	O
parameter	O
can	O
be	O
hoisted	O
in	O
this	O
fashion	O
.	O
</s>
<s>
Icon	B-Language
's	O
various	O
string	O
manipulation	O
procedures	O
use	O
these	O
two	O
variables	O
so	O
they	O
do	O
not	O
have	O
to	O
be	O
explicitly	O
supplied	O
by	O
the	O
programmer	O
.	O
</s>
<s>
Thus	O
provides	O
a	O
lightweight	O
wrapper	O
on	O
that	O
allows	O
Icon	B-Language
's	O
goal-directed	O
flow	O
control	O
to	O
be	O
easily	O
used	O
without	O
having	O
to	O
provide	O
hand-written	O
boolean	O
tests	O
against	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
test	O
is	O
"	O
is	O
&	O
pos	O
zero	O
"	O
,	O
which	O
,	O
in	O
the	O
odd	O
numbering	O
of	O
Icon	B-Language
's	O
string	O
locations	O
,	O
is	O
the	O
end	O
of	O
the	O
line	O
.	O
</s>
<s>
Laurence	O
Tratt	O
wrote	O
a	O
paper	O
on	O
Icon	B-Language
examining	O
its	O
real-world	O
applications	O
and	O
pointing	O
out	O
a	O
number	O
of	O
areas	O
of	O
concern	O
.	O
</s>
<s>
This	O
could	O
be	O
dismissed	O
as	O
one	O
of	O
those	O
"	O
gotchas	O
"	O
that	O
programmers	O
have	O
to	O
be	O
aware	O
of	O
in	O
any	O
language	O
,	O
but	O
Tratt	O
examined	O
a	O
variety	O
of	O
Icon	B-Language
programs	O
and	O
found	O
that	O
the	O
vast	O
majority	O
of	O
procedures	O
are	O
not	O
generators	O
.	O
</s>
<s>
This	O
means	O
that	O
Icon	B-Language
's	O
default	O
behaviour	O
is	O
only	O
used	O
by	O
a	O
tiny	O
minority	O
of	O
its	O
constructs	O
,	O
yet	O
represents	O
a	O
major	O
source	O
of	O
potential	O
errors	O
in	O
all	O
the	O
others	O
.	O
</s>
<s>
Tratt	O
supposed	O
that	O
it	O
detracts	O
from	O
the	O
self-documenting	O
code	O
,	O
having	O
supposed	O
erroneously	O
that	O
it	O
is	O
testing	O
"	O
is	O
c	B-Language
zero	O
"	O
or	O
"	O
does	O
c	B-Language
exist	O
"	O
.	O
</s>
