<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
a	O
one-pass	B-Language
compiler	I-Language
is	O
a	O
compiler	B-Language
that	O
passes	O
through	O
the	O
parts	O
of	O
each	O
compilation	B-Language
unit	I-Language
only	O
once	O
,	O
immediately	O
translating	O
each	O
part	O
into	O
its	O
final	O
machine	O
code	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
a	O
multi-pass	B-Application
compiler	I-Application
which	O
converts	O
the	O
program	O
into	O
one	O
or	O
more	O
intermediate	O
representations	O
in	O
steps	O
between	O
source	O
code	O
and	O
machine	O
code	O
,	O
and	O
which	O
reprocesses	O
the	O
entire	O
compilation	B-Language
unit	I-Language
in	O
each	O
sequential	O
pass	O
.	O
</s>
<s>
This	O
refers	O
to	O
the	O
logical	O
functioning	O
of	O
the	O
compiler	B-Language
,	O
not	O
to	O
the	O
actual	O
reading	O
of	O
the	O
source	O
file	O
once	O
only	O
.	O
</s>
<s>
The	O
IBM	B-Device
1130	I-Device
Fortran	B-Application
compiler	B-Language
stored	O
the	O
source	O
in	O
memory	O
and	O
used	O
many	O
passes	O
;	O
by	O
contrast	O
the	O
assembler	O
,	O
on	O
systems	O
lacking	O
a	O
disc	O
storage	O
unit	O
,	O
required	O
that	O
the	O
source	O
deck	O
of	O
cards	O
be	O
presented	O
twice	O
to	O
the	O
card	O
reader/punch	O
.	O
</s>
<s>
One-pass	B-Language
compilers	I-Language
are	O
smaller	O
and	O
faster	O
than	O
multi-pass	B-Application
compilers	I-Application
.	O
</s>
<s>
One-pass	B-Language
compilers	I-Language
are	O
unable	O
to	O
generate	O
as	O
efficient	O
programs	O
as	O
multi-pass	B-Application
compilers	I-Application
due	O
to	O
the	O
limited	O
scope	O
of	O
available	O
information	O
.	O
</s>
<s>
Many	O
effective	O
compiler	B-Application
optimizations	I-Application
require	O
multiple	O
passes	O
over	O
a	O
basic	B-Application
block	I-Application
,	O
loop	O
(	O
especially	O
nested	O
loops	O
)	O
,	O
subroutine	O
,	O
or	O
entire	O
module	O
.	O
</s>
<s>
Some	O
programming	O
languages	O
simply	O
cannot	O
be	O
compiled	B-Language
in	O
a	O
single	O
pass	O
,	O
as	O
a	O
result	O
of	O
their	O
design	O
.	O
</s>
<s>
The	O
language	O
definition	O
also	O
includes	O
pre-processor	O
statements	O
that	O
generate	O
source	O
code	O
to	O
be	O
compiled	B-Language
:	O
multiple	O
passes	O
are	O
certain	O
.	O
</s>
<s>
In	O
contrast	O
,	O
many	O
programming	O
languages	O
have	O
been	O
designed	O
specifically	O
to	O
be	O
compiled	B-Language
with	O
one-pass	B-Language
compilers	I-Language
,	O
and	O
include	O
special	O
constructs	O
to	O
allow	O
one-pass	O
compilation	B-Language
.	O
</s>
<s>
Notably	O
,	O
COBOL	O
compilers	B-Language
devote	O
a	O
pass	O
to	O
distinguishing	O
between	O
full	O
stops	O
appearing	O
in	O
decimal	O
constants	O
and	O
the	O
full	O
stops	O
that	O
appear	O
at	O
the	O
end	O
of	O
statements	O
.	O
</s>
<s>
Such	O
a	O
scheme	O
is	O
unavailable	O
to	O
a	O
single-pass	B-Language
compiler	I-Language
.	O
</s>
<s>
It	O
is	O
usual	O
to	O
allow	O
an	O
arbitrary	O
number	O
of	O
space	O
characters	O
between	O
tokens	O
,	O
and	O
Fortran	B-Application
is	O
unusual	O
in	O
allowing	O
(	O
and	O
ignoring	O
)	O
spaces	O
within	O
apparent	O
tokens	O
also	O
so	O
that	O
"	O
GO	O
TO	O
"	O
and	O
"	O
GOTO	O
"	O
are	O
equivalent	O
as	O
are	O
"	O
<=	O
"	O
and	O
"	O
<	O
=	O
"	O
.	O
</s>
<s>
An	O
optimising	B-Application
compiler	I-Application
may	O
analyse	O
the	O
form	O
of	O
an	O
arithmetic	O
expression	O
,	O
to	O
identify	O
and	O
remove	O
repetition	O
or	O
make	O
other	O
potential	O
improvements	O
.	O
</s>
<s>
Instead	O
,	O
the	O
compiler	B-Language
could	O
represent	O
the	O
entire	O
expression	O
's	O
form	O
(	O
typically	O
using	O
a	O
tree	O
structure	O
)	O
,	O
analyse	O
and	O
modify	O
that	O
structure	O
,	O
and	O
then	O
emit	O
the	O
code	O
for	O
the	O
improved	O
form	O
.	O
</s>
<s>
Consider	O
the	O
following	O
statements	O
in	O
fortran	B-Application
pseudo	O
code	O
:	O
</s>
<s>
Fortran	B-Application
is	O
unusual	O
in	O
that	O
it	O
contains	O
no	O
reserved	O
words	O
,	O
so	O
a	O
token	O
"	O
write	O
"	O
does	O
not	O
necessarily	O
mean	O
that	O
there	O
is	O
a	O
write-statement	O
in	O
progress	O
.	O
</s>
<s>
The	O
other	O
statements	O
are	O
indeed	O
if-statements	O
-	O
the	O
second	O
is	O
an	O
arithmetic-if	O
that	O
examines	O
the	O
sign	O
of	O
the	O
result	O
of	O
the	O
expression	O
and	O
based	O
on	O
it	O
being	O
negative	O
,	O
zero	O
,	O
or	O
positive	O
jumps	O
to	O
label	O
1	O
,	O
2	O
,	O
or	O
3	O
;	O
the	O
third	O
is	O
a	O
logical-if	O
,	O
and	O
requires	O
that	O
the	O
result	O
of	O
its	O
expression	O
be	O
boolean	O
-	O
thus	O
,	O
the	O
correct	O
interpretation	O
of	O
the	O
token	O
"	O
if	O
"	O
emerging	O
from	O
the	O
lexical	O
analyser	O
cannot	O
be	O
made	O
until	O
after	O
the	O
expression	O
has	O
been	O
scanned	O
and	O
following	O
the	O
closing	O
bracket	O
there	O
appears	O
either	O
an	O
equals	O
sign	O
,	O
a	O
digit	O
(	O
being	O
the	O
text	O
of	O
label1	O
:	O
fortran	B-Application
uses	O
only	O
integers	O
as	O
labels	O
though	O
if	O
letters	O
were	O
allowed	O
the	O
scan	O
would	O
have	O
to	O
rely	O
on	O
finding	O
the	O
commas	O
)	O
or	O
something	O
starting	O
with	O
a	O
letter	O
(	O
that	O
must	O
be	O
"	O
then	O
"	O
)	O
,	O
and	O
so	O
now	O
,	O
the	O
context	O
spans	O
an	O
arbitrary	O
amount	O
of	O
source	O
text	O
because	O
the	O
expression	O
is	O
arbitrary	O
.	O
</s>
<s>
However	O
,	O
in	O
all	O
three	O
cases	O
the	O
compiler	B-Language
can	O
generate	O
the	O
code	O
for	O
evaluating	O
the	O
expression	O
as	O
its	O
scan	O
advances	O
.	O
</s>
<s>
The	O
B6700	O
Algol	O
compiler	B-Language
for	O
example	O
was	O
notorious	O
for	O
error	O
messages	O
such	O
as	O
"	O
semicolon	O
expected	O
"	O
along	O
with	O
a	O
listing	O
of	O
the	O
source	O
line	O
plus	O
a	O
marker	O
showing	O
the	O
location	O
of	O
trouble	O
-	O
often	O
marking	O
a	O
semicolon	O
.	O
</s>
<s>
Often	O
,	O
only	O
the	O
first	O
error	O
message	O
from	O
a	O
compiler	B-Language
will	O
be	O
worth	O
attending	O
to	O
,	O
because	O
subsequent	O
messages	O
went	O
awry	O
.	O
</s>
<s>
With	O
Pascal	B-Application
,	O
if	O
a	O
or	O
b	O
then	O
etc	O
.	O
</s>
<s>
Source	O
file	O
listings	O
produced	O
by	O
the	O
compiler	B-Language
can	O
be	O
made	O
easier	O
to	O
read	O
by	O
having	O
the	O
reserved	O
words	O
it	O
identifies	O
presented	O
underlined	O
or	O
in	O
bold	O
or	O
italic	O
,	O
but	O
there	O
has	O
been	O
criticism	O
:	O
"	O
Algol	O
is	O
the	O
only	O
language	O
that	O
distinguishes	O
between	O
an	O
italic	O
and	O
normal	O
full	O
stop	O
"	O
.	O
</s>
<s>
In	O
fortran	B-Application
,	O
a	O
do-statement	O
'	O
s	O
start	O
such	O
as	O
DO	O
12	O
I	O
=	O
1	O
,	O
15	O
is	O
distinguished	O
from	O
DO	O
12	O
I	O
=	O
1.15	O
(	O
an	O
assignment	O
of	O
the	O
value	O
1.15	O
to	O
a	O
variable	O
called	O
DO12I	O
;	O
recall	O
that	O
spaces	O
are	O
irrelevant	O
)	O
only	O
by	O
the	O
difference	O
between	O
a	O
comma	O
and	O
a	O
full	O
stop	O
,	O
and	O
a	O
printed	O
listing	O
's	O
glyphs	O
may	O
not	O
be	O
well-formed	O
.	O
</s>
<s>
Careful	O
attention	O
to	O
the	O
design	O
of	O
a	O
language	O
can	O
promote	O
clarity	O
and	O
simplicity	O
of	O
expression	O
with	O
a	O
view	O
to	O
creating	O
a	O
reliable	O
compiler	B-Language
whose	O
behaviour	O
is	O
easily	O
understandable	O
.	O
</s>
<s>
It	O
is	O
at	O
this	O
stage	O
that	O
pre-processor	O
options	O
are	O
exercised	O
,	O
so	O
called	O
because	O
they	O
are	O
exercised	O
previous	O
to	O
the	O
compiler	B-Language
proper	O
processing	O
the	O
incoming	O
source	O
.	O
</s>
<s>
Some	O
systems	O
allow	O
the	O
definition	O
of	O
pre-processor	O
procedures	O
whose	O
output	O
is	O
source	O
text	O
to	O
be	O
compiled	B-Language
,	O
and	O
may	O
even	O
allow	O
such	O
source	O
to	O
define	O
still	O
further	O
pre-processor	O
items	O
.	O
</s>
<s>
and	O
the	O
temptation	O
would	O
be	O
to	O
devise	O
a	O
pre-processor	O
programme	O
which	O
would	O
accept	O
the	O
source	O
file	O
,	O
perform	O
the	O
pre-processor	O
actions	O
and	O
output	O
the	O
result	O
ready	O
for	O
the	O
next	O
stage	O
,	O
the	O
compilation	B-Language
.	O
</s>
<s>
But	O
this	O
clearly	O
constitutes	O
at	O
least	O
one	O
extra	O
pass	O
through	O
the	O
source	O
and	O
so	O
such	O
a	O
solution	O
would	O
be	O
unavailable	O
to	O
a	O
single-pass	B-Language
compiler	I-Language
.	O
</s>
<s>
Code	B-Application
generation	I-Application
by	O
the	O
compiler	B-Language
also	O
faces	O
the	O
problem	O
of	O
forwards	O
reference	O
,	O
most	O
directly	O
in	O
the	O
likes	O
of	O
Go	O
to	O
label	O
where	O
the	O
destination	O
label	O
is	O
an	O
unknown	O
distance	O
further	O
ahead	O
in	O
the	O
source	O
file	O
,	O
and	O
thus	O
the	O
jump	O
instruction	O
to	O
reach	O
that	O
label	O
's	O
location	O
involves	O
an	O
unknown	O
distance	O
across	O
yet-to-be-generated	O
code	O
.	O
</s>
<s>
A	O
recursive-descent	O
compiler	B-Language
would	O
activate	O
a	O
procedure	O
for	O
each	O
type	O
of	O
statement	O
,	O
such	O
as	O
an	O
if-statement	O
,	O
in	O
turn	O
invoking	O
the	O
appropriate	O
procedures	O
to	O
generate	O
the	O
code	O
for	O
the	O
statements	O
of	O
the	O
code	O
true	O
and	O
code	O
false	O
parts	O
of	O
its	O
statement	O
and	O
similarly	O
for	O
the	O
other	O
statements	O
according	O
to	O
their	O
syntax	O
.	O
</s>
<s>
All	O
unknown-destination	O
jumps	O
could	O
be	O
made	O
via	O
an	O
entry	O
in	O
a	O
jump	O
table	O
(	O
whose	O
addresses	O
are	O
later	O
filled	O
in	O
as	O
the	O
destinations	O
are	O
encountered	O
)	O
,	O
however	O
the	O
necessary	O
size	O
of	O
this	O
table	O
is	O
unknown	O
until	O
the	O
end	O
of	O
the	O
compilation	B-Language
.	O
</s>
<s>
One	O
solution	O
to	O
this	O
is	O
for	O
the	O
compiler	B-Language
to	O
emit	O
assembler	O
source	O
(	O
with	O
compiler-generated	O
labels	O
as	O
the	O
destinations	O
for	O
jumps	O
,	O
etc	O
.	O
</s>
<s>
But	O
this	O
clearly	O
requires	O
an	O
additional	O
pass	O
through	O
(	O
a	O
version	O
of	O
)	O
the	O
source	O
file	O
and	O
so	O
is	O
disallowed	O
for	O
single-pass	B-Language
compilers	I-Language
.	O
</s>
<s>
In	O
the	O
limit	O
this	O
means	O
the	O
compiler	B-Language
would	O
generate	O
a	O
data	O
structure	O
representing	O
the	O
entire	O
program	O
in	O
an	O
internal	O
form	O
,	O
but	O
a	O
straw	O
could	O
be	O
clutched	O
and	O
the	O
claim	O
made	O
that	O
there	O
is	O
no	O
actual	O
second	O
pass	O
of	O
the	O
source	O
file	O
from	O
start	O
to	O
end	O
.	O
</s>
<s>
Possibly	O
in	O
the	O
PR	O
document	O
advertising	O
the	O
compiler	B-Language
.	O
</s>
<s>
Notably	O
therefore	O
,	O
a	O
compiler	B-Language
cannot	O
generate	O
its	O
code	O
in	O
a	O
single	O
relentlessly-forwards	O
sequence	O
,	O
still	O
less	O
immediately	O
as	O
each	O
part	O
of	O
the	O
source	O
is	O
read	O
.	O
</s>
<s>
When	O
generating	O
code	O
for	O
the	O
various	O
expressions	O
,	O
the	O
compiler	B-Language
needs	O
to	O
know	O
the	O
nature	O
of	O
the	O
operands	O
.	O
</s>
<s>
Should	O
a	O
fortran	B-Application
compiler	B-Language
encounter	O
a	O
previously-unknown	O
name	O
whose	O
first	O
letter	O
is	O
one	O
of	O
I	O
,	O
J	O
,...,	O
N	O
,	O
then	O
the	O
variable	O
will	O
be	O
an	O
integer	O
,	O
otherwise	O
a	O
floating-point	O
variable	O
.	O
</s>
<s>
This	O
is	O
a	O
convenience	O
,	O
but	O
after	O
a	O
few	O
experiences	O
with	O
mistyped	O
names	O
,	O
most	O
programmers	O
agree	O
that	O
the	O
compiler	B-Language
option	O
"	O
implicit	O
none	O
"	O
should	O
be	O
used	O
.	O
</s>
<s>
Should	O
there	O
be	O
a	O
compiler	B-Language
for	O
such	O
a	O
language	O
,	O
it	O
would	O
have	O
to	O
create	O
a	O
complex	O
entity	O
to	O
represent	O
the	O
variable	O
pi	O
,	O
containing	O
an	O
indication	O
as	O
to	O
just	O
what	O
its	O
current	O
type	O
is	O
and	O
associated	O
storage	O
to	O
represent	O
such	O
a	O
type	O
.	O
</s>
<s>
As	O
with	O
ALGOL	O
,	O
Pascal	B-Application
,	O
PL/I	O
and	O
many	O
others	O
,	O
MATLAB	O
and	O
(	O
since	O
1995	O
)	O
Fortran	B-Application
allow	O
a	O
function	O
(	O
or	O
procedure	O
)	O
to	O
contain	O
the	O
definition	O
of	O
another	O
function	O
(	O
or	O
procedure	O
)	O
,	O
visible	O
only	O
within	O
the	O
containing	O
function	O
,	O
but	O
these	O
systems	O
require	O
that	O
they	O
be	O
defined	O
after	O
the	O
end	O
of	O
the	O
containing	O
procedure	O
.	O
</s>
<s>
This	O
need	O
not	O
matter	O
if	O
,	O
as	O
on	O
the	O
encounter	O
with	O
an	O
unknown	O
variable	O
,	O
sufficient	O
can	O
be	O
deduced	O
from	O
the	O
encounter	O
that	O
the	O
compiler	B-Language
could	O
generate	O
suitable	O
code	O
for	O
the	O
invocation	O
of	O
the	O
unknown	O
procedure	O
,	O
with	O
of	O
course	O
the	O
"	O
fixup	O
"	O
apparatus	O
in	O
place	O
to	O
come	O
back	O
and	O
fill	O
in	O
the	O
correct	O
address	O
for	O
the	O
destination	O
when	O
the	O
procedure	O
's	O
definition	O
is	O
encountered	O
.	O
</s>
<s>
Pascal	B-Application
solves	O
this	O
problem	O
by	O
requiring	O
"	O
predeclaration.	O
"	O
</s>
<s>
Fortran	B-Application
for	O
example	O
passes	O
all	O
parameters	O
by	O
reference	O
(	O
i.e.	O
</s>
<s>
by	O
address	O
)	O
so	O
there	O
is	O
no	O
immediate	O
difficulty	O
with	O
generating	O
the	O
code	O
(	O
as	O
always	O
,	O
with	O
actual	O
addresses	O
to	O
be	O
fixed	O
up	O
later	O
)	O
,	O
but	O
Pascal	B-Application
and	O
other	O
languages	O
allow	O
parameters	O
to	O
be	O
passed	O
by	O
different	O
methods	O
at	O
the	O
programmer	O
's	O
choice	O
(	O
by	O
reference	O
,	O
or	O
by	O
value	O
,	O
or	O
even	O
perhaps	O
by	O
"	O
name	O
"	O
)	O
and	O
this	O
is	O
signified	O
only	O
in	O
the	O
definition	O
of	O
the	O
procedure	O
,	O
which	O
is	O
unknown	O
before	O
the	O
definition	O
has	O
been	O
encountered	O
.	O
</s>
<s>
Specifically	O
for	O
Pascal	B-Application
,	O
in	O
the	O
specification	O
of	O
parameters	O
a	O
prefix	O
"	O
Var	O
"	O
signifies	O
that	O
it	O
must	O
be	O
received	O
by	O
reference	O
,	O
its	O
absence	O
signifies	O
by	O
value	O
.	O
</s>
<s>
In	O
the	O
first	O
case	O
the	O
compiler	B-Language
must	O
generate	O
code	O
that	O
passes	O
the	O
address	O
of	O
the	O
parameter	O
,	O
while	O
in	O
the	O
second	O
it	O
must	O
generate	O
different	O
code	O
that	O
passes	O
a	O
copy	O
of	O
the	O
value	O
,	O
usually	O
via	O
a	O
stack	O
.	O
</s>
<s>
Multi-pass	B-Application
compilers	I-Application
can	O
of	O
course	O
collate	O
all	O
the	O
required	O
information	O
as	O
they	O
shuttle	O
back	O
and	O
forth	O
,	O
but	O
single-pass	B-Language
compilers	I-Language
cannot	O
.	O
</s>
<s>
Code	B-Application
generation	I-Application
could	O
be	O
paused	O
while	O
the	O
scan	O
advances	O
(	O
and	O
its	O
results	O
be	O
held	O
in	O
internal	O
storage	O
)	O
until	O
such	O
time	O
as	O
the	O
needed	O
entity	O
is	O
encountered	O
,	O
and	O
this	O
might	O
not	O
be	O
regarded	O
as	O
resulting	O
in	O
a	O
second	O
pass	O
through	O
the	O
source	O
because	O
the	O
code	B-Application
generation	I-Application
stage	O
will	O
soon	O
catch	O
up	O
,	O
it	O
was	O
merely	O
halting	O
for	O
a	O
while	O
.	O
</s>
<s>
Instead	O
a	O
special	O
construction	O
is	O
introduced	O
,	O
whereby	O
the	O
procedure	O
's	O
definition	O
of	O
parameter	O
usage	O
is	O
declared	O
"	O
forward	O
"	O
of	O
its	O
later	O
full	O
definition	O
so	O
that	O
the	O
compiler	B-Language
may	O
know	O
it	O
before	O
use	O
,	O
as	O
it	O
requires	O
.	O
</s>
<s>
From	O
First	O
Fortran	B-Application
(	O
1957	O
)	O
onwards	O
,	O
separate	O
compilation	B-Language
of	O
portions	O
of	O
a	O
program	O
has	O
been	O
possible	O
,	O
supporting	O
the	O
creation	O
of	O
libraries	O
of	O
procedures	O
and	O
functions	O
.	O
</s>
<s>
A	O
procedure	O
in	O
the	O
source	O
file	O
being	O
compiled	B-Language
that	O
invokes	O
a	O
function	O
from	O
such	O
an	O
outside	O
collection	O
must	O
know	O
the	O
type	O
of	O
result	O
returned	O
by	O
the	O
unknown	O
function	O
,	O
if	O
only	O
to	O
generate	O
code	O
that	O
looks	O
in	O
the	O
right	O
place	O
to	O
find	O
the	O
result	O
.	O
</s>
<s>
The	O
requirement	O
to	O
be	O
met	O
is	O
that	O
at	O
the	O
current	O
point	O
in	O
a	O
single-pass	B-Language
compilation	I-Language
,	O
information	O
on	O
an	O
entity	O
is	O
needed	O
so	O
that	O
the	O
correct	O
code	O
for	O
it	O
can	O
be	O
produced	O
now	O
,	O
if	O
with	O
address	O
fixups	O
later	O
.	O
</s>
<s>
Whether	O
the	O
required	O
information	O
will	O
be	O
encountered	O
later	O
on	O
in	O
the	O
source	O
file	O
or	O
is	O
to	O
be	O
found	O
in	O
some	O
separately-compiled	O
code	O
file	O
,	O
the	O
information	O
is	O
provided	O
by	O
some	O
protocol	O
here	O
.	O
</s>
<s>
Systems	O
that	O
allow	O
separate	O
compilation	B-Language
of	O
parts	O
of	O
a	O
complete	O
programme	O
that	O
later	O
are	O
"	O
linked	O
"	O
together	O
should	O
also	O
check	O
for	O
the	O
correct	O
type	O
and	O
number	O
of	O
parameters	O
and	O
results	O
as	O
mistakes	O
are	O
even	O
easier	O
to	O
make	O
,	O
but	O
often	O
do	O
not	O
.	O
</s>
<s>
Some	O
languages	O
(	O
such	O
as	O
Algol	O
)	O
have	O
a	O
formal	O
notion	O
of	O
"	O
upgrading	O
"	O
or	O
"	O
widening	O
"	O
or	O
"	O
promotion	O
"	O
,	O
whereby	O
a	O
procedure	O
that	O
expects	O
say	O
a	O
double-precision	O
parameter	O
may	O
be	O
invoked	O
with	O
it	O
as	O
a	O
single	O
precision	O
variable	O
,	O
and	O
in	O
this	O
case	O
the	O
compiler	B-Language
generates	O
code	O
that	O
stores	O
the	O
single	O
precision	O
variable	O
into	O
a	O
temporary	O
double-precision	O
variable	O
which	O
becomes	O
the	O
actual	O
parameter	O
.	O
</s>
<s>
An	O
example	O
of	O
such	O
a	O
construct	O
is	O
the	O
forward	O
declaration	O
in	O
Pascal	B-Application
.	O
</s>
<s>
Pascal	B-Application
requires	O
that	O
procedures	O
be	O
declared	O
or	O
fully	O
defined	O
before	O
use	O
.	O
</s>
<s>
This	O
helps	O
a	O
one-pass	B-Language
compiler	I-Language
with	O
its	O
type	O
checking	O
:	O
calling	O
a	O
procedure	O
that	O
has	O
n't	O
been	O
declared	O
anywhere	O
is	O
a	O
clear	O
error	O
.	O
</s>
<s>
Forward	O
declarations	O
help	O
mutually	B-Algorithm
recursive	I-Algorithm
procedures	O
call	O
each	O
other	O
directly	O
,	O
despite	O
the	O
declare-before-use	O
rule	O
:	O
</s>
<s>
By	O
adding	O
a	O
forward	O
declaration	O
for	O
the	O
function	O
even	O
before	O
the	O
function	O
odd	O
,	O
the	O
one-pass	B-Language
compiler	I-Language
is	O
told	O
that	O
there	O
will	O
be	O
a	O
definition	O
of	O
even	O
later	O
on	O
in	O
the	O
program	O
.	O
</s>
<s>
Anyone	O
who	O
has	O
attempted	O
to	O
maintain	O
coherence	O
amongst	O
the	O
declarations	O
and	O
usages	O
of	O
procedures	O
in	O
a	O
large	O
program	O
and	O
its	O
usage	O
of	O
libraries	O
of	O
routines	O
,	O
especially	O
one	O
undergoing	O
changes	O
,	O
will	O
have	O
struggled	O
over	O
the	O
usage	O
of	O
forward	O
or	O
similar	O
added	O
declarations	O
for	O
procedures	O
invoked	O
but	O
not	O
defined	O
in	O
the	O
current	O
compilation	B-Language
.	O
</s>
<s>
The	O
gain	O
of	O
supposedly	O
swifter	O
compilation	B-Language
may	O
seem	O
insufficient	O
when	O
simply	O
abandoning	O
the	O
goal	O
of	O
one-pass	O
compilation	B-Language
would	O
remove	O
this	O
imposition	O
.	O
</s>
