<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
parsing	B-Language
reveals	O
the	O
grammatical	O
structure	O
of	O
linear	O
input	O
text	O
,	O
as	O
a	O
first	O
step	O
in	O
working	O
out	O
its	O
meaning	O
.	O
</s>
<s>
Bottom-up	B-Application
parsing	I-Application
recognizes	O
the	O
text	O
's	O
lowest-level	O
small	O
details	O
first	O
,	O
before	O
its	O
mid-level	O
structures	O
,	O
and	O
leaving	O
the	O
highest-level	O
overall	O
structure	O
to	O
last	O
.	O
</s>
<s>
The	O
bottom-up	O
name	O
comes	O
from	O
the	O
concept	O
of	O
a	O
parse	B-Language
tree	O
,	O
in	O
which	O
the	O
most	O
detailed	O
parts	O
are	O
at	O
the	O
bottom	O
of	O
the	O
upside-down	O
tree	O
,	O
and	O
larger	O
structures	O
composed	O
from	O
them	O
are	O
in	O
successively	O
higher	O
layers	O
,	O
until	O
at	O
the	O
top	O
or	O
"	O
root	O
"	O
of	O
the	O
tree	O
a	O
single	O
unit	O
describes	O
the	O
entire	O
input	O
stream	O
.	O
</s>
<s>
A	O
bottom-up	O
parse	B-Language
discovers	O
and	O
processes	O
that	O
tree	O
starting	O
from	O
the	O
bottom	O
left	O
end	O
,	O
and	O
incrementally	O
works	O
its	O
way	O
upwards	O
and	O
rightwards	O
.	O
</s>
<s>
Bottom-up	B-Application
parsing	I-Application
patiently	O
waits	O
until	O
it	O
has	O
scanned	O
and	O
parsed	B-Language
all	O
parts	O
of	O
some	O
construct	O
before	O
committing	O
to	O
what	O
the	O
combined	O
construct	O
is	O
.	O
</s>
<s>
The	O
opposite	O
of	O
this	O
is	O
top-down	B-Application
parsing	I-Application
,	O
in	O
which	O
the	O
input	O
's	O
overall	O
structure	O
is	O
decided	O
(	O
or	O
guessed	O
at	O
)	O
first	O
,	O
before	O
dealing	O
with	O
mid-level	O
parts	O
,	O
leaving	O
completion	O
of	O
all	O
lowest-level	O
details	O
to	O
last	O
.	O
</s>
<s>
A	O
top-down	B-Application
parser	I-Application
discovers	O
and	O
processes	O
the	O
hierarchical	O
tree	O
starting	O
from	O
the	O
top	O
,	O
and	O
incrementally	O
works	O
its	O
way	O
first	O
downwards	O
and	O
then	O
rightwards	O
.	O
</s>
<s>
Top-down	B-Application
parsing	I-Application
eagerly	O
decides	O
what	O
a	O
construct	O
is	O
much	O
earlier	O
,	O
when	O
it	O
has	O
only	O
scanned	O
the	O
leftmost	O
symbol	O
of	O
that	O
construct	O
and	O
has	O
not	O
yet	O
parsed	B-Language
any	O
of	O
its	O
parts	O
.	O
</s>
<s>
Left	B-Application
corner	I-Application
parsing	B-Language
is	O
a	O
hybrid	O
method	O
that	O
works	O
bottom-up	O
along	O
the	O
left	O
edges	O
of	O
each	O
subtree	O
,	O
and	O
top-down	O
on	O
the	O
rest	O
of	O
the	O
parse	B-Language
tree	O
.	O
</s>
<s>
If	O
a	O
language	O
grammar	O
has	O
multiple	O
rules	O
that	O
may	O
start	O
with	O
the	O
same	O
leftmost	O
symbols	O
but	O
have	O
different	O
endings	O
,	O
then	O
that	O
grammar	O
can	O
be	O
efficiently	O
handled	O
by	O
a	O
deterministic	O
bottom-up	O
parse	B-Language
but	O
cannot	O
be	O
handled	O
top-down	O
without	O
guesswork	O
and	O
backtracking	B-Algorithm
.	O
</s>
<s>
So	O
bottom-up	B-Application
parsers	I-Application
in	O
practice	O
handle	O
a	O
somewhat	O
larger	O
range	O
of	O
computer	O
language	O
grammars	O
than	O
deterministic	O
top-down	B-Application
parsers	I-Application
do	O
.	O
</s>
<s>
Bottom-up	B-Application
parsing	I-Application
is	O
sometimes	O
done	O
by	O
backtracking	B-Algorithm
.	O
</s>
<s>
But	O
much	O
more	O
commonly	O
,	O
bottom-up	B-Application
parsing	I-Application
is	O
done	O
by	O
a	O
shift-reduce	B-Application
parser	I-Application
such	O
as	O
a	O
LALR	B-Application
parser	I-Application
.	O
</s>
<s>
Some	O
of	O
the	O
parsers	B-Language
that	O
use	O
bottom-up	B-Application
parsing	I-Application
include	O
:	O
</s>
