<s>
In	O
a	O
programming	O
language	O
,	O
an	O
evaluation	B-Language
strategy	I-Language
is	O
a	O
set	O
of	O
rules	O
for	O
evaluating	O
expressions	O
.	O
</s>
<s>
The	O
notion	O
of	O
reduction	B-Application
strategy	I-Application
is	O
distinct	O
,	O
although	O
some	O
authors	O
conflate	O
the	O
two	O
terms	O
and	O
the	O
definition	O
of	O
each	O
term	O
is	O
not	O
widely	O
agreed	O
upon	O
.	O
</s>
<s>
This	O
is	O
the	O
call-by-reference	O
evaluation	B-Language
strategy	I-Language
.	O
</s>
<s>
Evaluation	B-Language
strategy	I-Language
is	O
part	O
of	O
the	O
semantics	O
of	O
the	O
programming	O
language	O
definition	O
.	O
</s>
<s>
Some	O
languages	O
,	O
such	O
as	O
PureScript	B-Language
,	O
have	O
variants	O
with	O
different	O
evaluation	B-Language
strategies	I-Language
.	O
</s>
<s>
Some	O
declarative	B-Language
languages	I-Language
,	O
such	O
as	O
Datalog	B-Language
,	O
support	O
multiple	O
evaluation	B-Language
strategies	I-Language
.	O
</s>
<s>
This	O
is	O
a	O
table	O
of	O
evaluation	B-Language
strategies	I-Language
and	O
representative	O
languages	O
by	O
year	O
introduced	O
.	O
</s>
<s>
While	O
the	O
order	O
of	O
operations	O
defines	O
the	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
of	O
the	O
expression	O
,	O
the	O
evaluation	O
order	O
defines	O
the	O
order	O
in	O
which	O
expressions	O
are	O
evaluated	O
.	O
</s>
<s>
outputs	O
1	O
2	O
due	O
to	O
Python	B-Language
's	O
left-to-right	O
evaluation	O
order	O
,	O
but	O
a	O
similar	O
program	O
in	O
OCaml	B-Language
:	O
</s>
<s>
outputs	O
2	O
1	O
due	O
to	O
OCaml	B-Language
's	O
right-to-left	O
evaluation	O
order	O
.	O
</s>
<s>
For	O
this	O
reason	O
language	O
standards	O
such	O
as	O
C++	B-Language
traditionally	O
left	O
the	O
order	O
unspecified	O
,	O
although	O
languages	O
such	O
as	O
Java	B-Language
and	O
C#	B-Application
define	O
the	O
evaluation	O
order	O
as	O
left-to-right	O
and	O
the	O
C++17	B-Language
standard	O
has	O
added	O
constraints	O
on	O
the	O
evaluation	O
order	O
.	O
</s>
<s>
This	O
has	O
the	O
effect	O
of	O
making	O
the	O
function	O
strict	O
,	O
i.e.	O
</s>
<s>
the	O
function	O
's	O
result	O
is	O
undefined	B-Language
if	O
any	O
of	O
the	O
arguments	O
are	O
undefined	B-Language
,	O
so	O
applicative	O
order	O
evaluation	O
is	O
more	O
commonly	O
called	O
strict	O
evaluation	O
.	O
</s>
<s>
Common	O
Lisp	O
,	O
Eiffel	B-Language
and	O
Java	B-Language
evaluate	O
function	O
arguments	O
left-to-right	O
.	O
</s>
<s>
C	B-Language
leaves	O
the	O
order	O
undefined	B-Language
.	O
</s>
<s>
Scheme	B-Language
requires	O
the	O
execution	O
order	O
to	O
be	O
the	O
sequential	O
execution	O
of	O
an	O
unspecified	O
permutation	O
of	O
the	O
arguments	O
.	O
</s>
<s>
OCaml	B-Language
similarly	O
leaves	O
the	O
order	O
unspecified	O
,	O
but	O
in	O
practice	O
evaluates	O
arguments	O
right-to-left	O
due	O
to	O
the	O
design	O
of	O
its	O
abstract	B-Application
machine	I-Application
.	O
</s>
<s>
Normal	O
order	O
evaluation	O
uses	O
complex	O
structures	O
such	O
as	O
thunks	B-Application
for	O
unevaluated	O
expressions	O
,	O
compared	O
to	O
the	O
call	B-General_Concept
stack	I-General_Concept
used	O
in	O
applicative	O
order	O
evaluation	O
.	O
</s>
<s>
If	O
the	O
function	O
or	O
procedure	O
is	O
able	O
to	O
assign	O
values	O
to	O
its	O
parameters	O
,	O
only	O
its	O
local	O
variable	O
is	O
assigned	O
—	O
that	O
is	O
,	O
anything	O
passed	O
into	O
a	O
function	O
call	O
is	O
unchanged	O
in	O
the	O
caller	O
's	O
scope	B-Language
when	O
the	O
function	O
returns	O
.	O
</s>
<s>
The	O
reason	O
for	O
passing	O
a	O
reference	O
is	O
often	O
that	O
the	O
language	O
technically	O
does	O
not	O
provide	O
a	O
value	O
representation	O
of	O
complicated	O
data	O
,	O
but	O
instead	O
represents	O
them	O
as	O
a	O
data	B-General_Concept
structure	I-General_Concept
while	O
preserving	O
some	O
semblance	O
of	O
value	O
appearance	O
in	O
the	O
source	O
code	O
.	O
</s>
<s>
Exactly	O
where	O
the	O
boundary	O
is	O
drawn	O
between	O
proper	O
values	O
and	O
data	B-General_Concept
structures	I-General_Concept
masquerading	O
as	O
such	O
is	O
often	O
hard	O
to	O
predict	O
.	O
</s>
<s>
In	O
C	B-Language
,	O
an	O
array	O
(	O
of	O
which	O
strings	O
are	O
special	O
cases	O
)	O
is	O
a	O
data	B-General_Concept
structure	I-General_Concept
but	O
the	O
name	O
of	O
an	O
array	O
is	O
treated	O
as	O
(	O
has	O
as	O
value	O
)	O
the	O
reference	O
to	O
the	O
first	O
element	O
of	O
the	O
array	O
,	O
while	O
a	O
struct	B-Language
variable	O
's	O
name	O
refers	O
to	O
a	O
value	O
even	O
if	O
it	O
has	O
fields	O
that	O
are	O
vectors	O
.	O
</s>
<s>
In	O
Maple	B-Language
,	O
a	O
vector	O
is	O
a	O
special	O
case	O
of	O
a	O
table	O
and	O
therefore	O
a	O
data	B-General_Concept
structure	I-General_Concept
,	O
but	O
a	O
list	O
(	O
which	O
gets	O
rendered	O
and	O
can	O
be	O
indexed	O
in	O
exactly	O
the	O
same	O
way	O
)	O
is	O
a	O
value	O
.	O
</s>
<s>
In	O
Tcl	B-Operating_System
,	O
values	O
are	O
"	O
dual-ported	O
"	O
such	O
that	O
the	O
value	O
representation	O
is	O
used	O
at	O
the	O
script	O
level	O
,	O
and	O
the	O
language	O
itself	O
manages	O
the	O
corresponding	O
data	B-General_Concept
structure	I-General_Concept
,	O
if	O
one	O
is	O
required	O
.	O
</s>
<s>
Modifications	O
made	O
via	O
the	O
data	B-General_Concept
structure	I-General_Concept
are	O
reflected	O
back	O
to	O
the	O
value	O
representation	O
and	O
vice	O
versa	O
.	O
</s>
<s>
Thus	O
the	O
behaviour	O
of	O
call	O
by	O
value	O
Java	B-Language
or	O
Visual	B-Language
Basic	I-Language
and	O
call	O
by	O
value	O
C	B-Language
or	O
Pascal	B-Application
are	O
significantly	O
different	O
:	O
in	O
C	B-Language
or	O
Pascal	B-Application
,	O
calling	O
a	O
function	O
with	O
a	O
large	O
structure	O
as	O
an	O
argument	O
will	O
cause	O
the	O
entire	O
structure	O
to	O
be	O
copied	O
(	O
except	O
if	O
it	O
's	O
actually	O
a	O
reference	O
to	O
a	O
structure	O
)	O
,	O
potentially	O
causing	O
serious	O
performance	O
degradation	O
,	O
and	O
mutations	O
to	O
the	O
structure	O
are	O
invisible	O
to	O
the	O
caller	O
.	O
</s>
<s>
However	O
,	O
in	O
Java	B-Language
or	O
Visual	B-Language
Basic	I-Language
only	O
the	O
reference	O
to	O
the	O
structure	O
is	O
copied	O
,	O
which	O
is	O
fast	O
,	O
and	O
mutations	O
to	O
the	O
structure	O
are	O
visible	O
to	O
the	O
caller	O
.	O
</s>
<s>
Call	O
by	O
reference	O
(	O
or	O
pass	O
by	O
reference	O
)	O
is	O
an	O
evaluation	B-Language
strategy	I-Language
where	O
a	O
parameter	O
is	O
bound	O
to	O
an	O
implicit	O
reference	O
to	O
the	O
variable	O
used	O
as	O
argument	O
,	O
rather	O
than	O
a	O
copy	O
of	O
its	O
value	O
.	O
</s>
<s>
This	O
typically	O
means	O
that	O
the	O
function	O
can	O
modify	O
(	O
i.e.	O
,	O
assign	O
to	O
)	O
the	O
variable	O
used	O
as	O
argument	O
—	O
something	O
that	O
will	O
be	O
seen	O
by	O
its	O
caller	O
.	O
</s>
<s>
Languages	O
such	O
as	O
C	B-Language
,	O
ML	B-Language
and	O
Rust	B-Application
use	O
this	O
technique	O
.	O
</s>
<s>
It	O
is	O
not	O
a	O
separate	O
evaluation	B-Language
strategy	I-Language
—	O
the	O
language	O
calls	O
by	O
value	O
—	O
but	O
sometimes	O
it	O
is	O
referred	O
to	O
as	O
"	O
call	O
by	O
address	O
"	O
or	O
"	O
pass	O
by	O
address	O
"	O
.	O
</s>
<s>
In	O
ML	B-Language
,	O
references	O
are	O
type	B-Language
-	I-Language
and	O
memory-safe	B-Application
,	O
similar	O
to	O
Rust	B-Application
.	O
</s>
<s>
In	O
purely	O
functional	O
languages	O
there	O
is	O
typically	O
no	O
semantic	O
difference	O
between	O
the	O
two	O
strategies	O
(	O
since	O
their	O
data	B-General_Concept
structures	I-General_Concept
are	O
immutable	B-Application
,	O
so	O
there	O
is	O
no	O
possibility	O
for	O
a	O
function	O
to	O
modify	O
any	O
of	O
its	O
arguments	O
)	O
,	O
so	O
they	O
are	O
typically	O
described	O
as	O
call	O
by	O
value	O
even	O
though	O
implementations	O
frequently	O
use	O
call	O
by	O
reference	O
internally	O
for	O
the	O
efficiency	O
benefits	O
.	O
</s>
<s>
Following	O
is	O
an	O
example	O
that	O
demonstrates	O
call	O
by	O
reference	O
in	O
the	O
E	B-Language
programming	I-Language
language	I-Language
:	O
</s>
<s>
Following	O
is	O
an	O
example	O
of	O
call	O
by	O
address	O
that	O
simulates	O
call	O
by	O
reference	O
in	O
C	B-Language
:	O
</s>
<s>
Call	O
by	O
sharing	O
(	O
also	O
known	O
as	O
"	O
call	O
by	O
object	O
"	O
or	O
"	O
call	O
by	O
object-sharing	O
"	O
)	O
is	O
an	O
evaluation	B-Language
strategy	I-Language
first	O
noted	O
by	O
Barbara	O
Liskov	O
in	O
1974	O
for	O
the	O
CLU	B-Language
language	O
.	O
</s>
<s>
It	O
is	O
used	O
by	O
languages	O
such	O
as	O
Python	B-Language
,	O
Java	B-Language
(	O
for	O
object	O
references	O
)	O
,	O
Ruby	B-Language
,	O
JavaScript	B-Language
,	O
Scheme	B-Language
,	O
OCaml	B-Language
,	O
AppleScript	B-Application
,	O
and	O
many	O
others	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
Java	B-Language
community	O
,	O
they	O
say	O
that	O
Java	B-Language
is	O
call	O
by	O
value	O
.	O
</s>
<s>
Call	O
by	O
sharing	O
implies	O
that	O
values	O
in	O
the	O
language	O
are	O
based	O
on	O
objects	O
rather	O
than	O
primitive	O
types	O
,	O
i.e.	O
,	O
that	O
all	O
values	O
are	O
"	O
boxed	O
"	O
.	O
</s>
<s>
So	O
,	O
for	O
example	O
,	O
if	O
a	O
variable	O
was	O
passed	O
,	O
it	O
is	O
not	O
possible	O
to	O
simulate	O
an	O
assignment	O
on	O
that	O
variable	O
in	O
the	O
caller	O
's	O
scope	B-Language
.	O
</s>
<s>
However	O
,	O
since	O
the	O
function	O
has	O
access	O
to	O
the	O
same	O
object	O
as	O
the	O
caller	O
(	O
no	O
copy	O
is	O
made	O
)	O
,	O
mutations	O
to	O
those	O
objects	O
,	O
if	O
the	O
objects	O
are	O
mutable	B-Application
,	O
within	O
the	O
function	O
are	O
visible	O
to	O
the	O
caller	O
,	O
which	O
may	O
appear	O
to	O
differ	O
from	O
call	O
by	O
value	O
semantics	O
.	O
</s>
<s>
Mutations	O
of	O
a	O
mutable	B-Application
object	I-Application
within	O
the	O
function	O
are	O
visible	O
to	O
the	O
caller	O
because	O
the	O
object	O
is	O
not	O
copied	O
or	O
clonedit	O
is	O
shared	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
Python	B-Language
,	O
lists	O
are	O
mutable	B-Application
,	O
so	O
:	O
</s>
<s>
Since	O
the	O
rebound	O
variable	O
only	O
exists	O
within	O
the	O
scope	B-Language
of	O
the	O
function	O
,	O
the	O
counterpart	O
in	O
the	O
caller	O
retains	O
its	O
original	O
binding	O
.	O
</s>
<s>
Compare	O
the	O
Python	B-Language
mutation	B-Application
above	O
with	O
the	O
code	O
below	O
,	O
which	O
binds	O
the	O
formal	O
argument	O
to	O
a	O
new	O
object	O
:	O
</s>
<s>
For	O
immutable	B-Application
objects	I-Application
,	O
there	O
is	O
no	O
real	O
difference	O
between	O
call	O
by	O
sharing	O
and	O
call	O
by	O
value	O
,	O
except	O
if	O
object	O
identity	O
is	O
visible	O
in	O
the	O
language	O
.	O
</s>
<s>
The	O
use	O
of	O
call	O
by	O
sharing	O
with	O
mutable	B-Application
objects	I-Application
is	O
an	O
alternative	O
to	O
input/output	O
parameters	O
:	O
the	O
parameter	O
is	O
not	O
assigned	O
to	O
(	O
the	O
argument	O
is	O
not	O
overwritten	O
and	O
object	O
identity	O
is	O
not	O
changed	O
)	O
,	O
but	O
the	O
object	O
(	O
argument	O
)	O
is	O
mutated	O
.	O
</s>
<s>
Call	O
by	O
copy-restore	O
—	O
also	O
known	O
as	O
"	O
copy-in	O
copy-out	O
"	O
,	O
"	O
call	O
by	O
value	O
result	O
"	O
,	O
"	O
call	O
by	O
value	O
return	O
"	O
(	O
as	O
termed	O
in	O
the	O
Fortran	B-Application
community	O
)	O
—	O
is	O
a	O
special	O
case	O
of	O
call	O
by	O
reference	O
where	O
the	O
provided	O
reference	O
is	O
unique	O
to	O
the	O
caller	O
.	O
</s>
<s>
This	O
variant	O
has	O
gained	O
attention	O
in	O
multiprocessing	B-Operating_System
contexts	O
and	O
Remote	B-Operating_System
procedure	I-Operating_System
call	I-Operating_System
:	O
if	O
a	O
parameter	O
to	O
a	O
function	O
call	O
is	O
a	O
reference	O
that	O
might	O
be	O
accessible	O
by	O
another	O
thread	O
of	O
execution	O
,	O
its	O
contents	O
may	O
be	O
copied	O
to	O
a	O
new	O
reference	O
that	O
is	O
not	O
;	O
when	O
the	O
function	O
call	O
returns	O
,	O
the	O
updated	O
contents	O
of	O
this	O
new	O
reference	O
are	O
copied	O
back	O
to	O
the	O
original	O
reference	O
(	O
"	O
restored	O
"	O
)	O
.	O
</s>
<s>
The	O
semantics	O
of	O
call	O
by	O
copy-restore	O
also	O
differ	O
from	O
those	O
of	O
call	O
by	O
reference	O
,	O
where	O
two	O
or	O
more	O
function	O
arguments	O
alias	B-Application
one	O
another	O
(	O
i.e.	O
,	O
point	O
to	O
the	O
same	O
variable	O
in	O
the	O
caller	O
's	O
environment	O
)	O
.	O
</s>
<s>
Under	O
call	O
by	O
reference	O
,	O
writing	O
to	O
one	O
will	O
affect	O
the	O
other	O
;	O
call	O
by	O
copy-restore	O
avoids	O
this	O
by	O
giving	O
the	O
function	O
distinct	O
copies	O
,	O
but	O
leaves	O
the	O
result	O
in	O
the	O
caller	O
's	O
environment	O
undefined	B-Language
depending	O
on	O
which	O
of	O
the	O
aliased	O
arguments	O
is	O
copied	O
back	O
first	O
—	O
will	O
the	O
copies	O
be	O
made	O
in	O
left-to-right	O
order	O
both	O
on	O
entry	O
and	O
on	O
return	O
?	O
</s>
<s>
When	O
the	O
reference	O
is	O
passed	O
to	O
the	O
caller	O
uninitialized	O
,	O
this	O
evaluation	B-Language
strategy	I-Language
may	O
be	O
called	O
"	O
call	O
by	O
result	O
"	O
.	O
</s>
<s>
Call	O
by	O
name	O
is	O
an	O
evaluation	B-Language
strategy	I-Language
where	O
the	O
arguments	O
to	O
a	O
function	O
are	O
not	O
evaluated	O
before	O
the	O
function	O
is	O
called	O
—	O
rather	O
,	O
they	O
are	O
substituted	O
directly	O
into	O
the	O
function	O
body	O
(	O
using	O
capture-avoiding	O
substitution	O
)	O
and	O
then	O
left	O
to	O
be	O
evaluated	O
whenever	O
they	O
appear	O
in	O
the	O
function	O
.	O
</s>
<s>
However	O
,	O
when	O
the	O
function	O
argument	O
is	O
used	O
,	O
call	O
by	O
name	O
is	O
often	O
slower	O
,	O
requiring	O
a	O
mechanism	O
such	O
as	O
a	O
thunk	B-Application
.	O
</s>
<s>
Today	O
's	O
.NET	B-Language
languages	I-Language
can	O
simulate	O
call	O
by	O
name	O
using	O
delegates	O
or	O
Expressionxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1	O
parameters	O
.	O
</s>
<s>
The	O
latter	O
results	O
in	O
an	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
being	O
given	O
to	O
the	O
function	O
.	O
</s>
<s>
Eiffel	B-Language
provides	O
agents	O
,	O
which	O
represent	O
an	O
operation	O
to	O
be	O
evaluated	O
when	O
needed	O
.	O
</s>
<s>
Seed7	B-Language
provides	O
call	O
by	O
name	O
with	O
function	O
parameters	O
.	O
</s>
<s>
Java	B-Language
programs	O
can	O
accomplish	O
similar	O
lazy	O
evaluation	O
using	O
lambda	B-General_Concept
expressions	I-General_Concept
and	O
the	O
java.util.function.Supplierxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1	O
interface	O
.	O
</s>
<s>
If	O
the	O
argument	O
is	O
pure	B-Application
(	O
i.e.	O
,	O
free	O
of	O
side	O
effects	O
)	O
,	O
this	O
produces	O
the	O
same	O
results	O
as	O
call	O
by	O
name	O
,	O
saving	O
the	O
cost	O
of	O
recomputing	O
the	O
argument	O
.	O
</s>
<s>
Haskell	B-Language
is	O
a	O
well-known	O
language	O
that	O
uses	O
call-by-need	O
evaluation	O
.	O
</s>
<s>
Because	O
evaluation	O
of	O
expressions	O
may	O
happen	O
arbitrarily	O
far	O
into	O
a	O
computation	O
,	O
Haskell	B-Language
only	O
supports	O
side	O
effects	O
(	O
such	O
as	O
mutation	B-Application
)	O
via	O
the	O
use	O
of	O
monads	O
.	O
</s>
<s>
This	O
eliminates	O
any	O
unexpected	O
behavior	O
from	O
variables	O
whose	O
values	O
change	O
prior	O
to	O
their	O
delayed	B-Application
evaluation	I-Application
.	O
</s>
<s>
In	O
R	B-Language
's	O
implementation	O
of	O
call	O
by	O
need	O
,	O
all	O
arguments	O
are	O
passed	O
,	O
meaning	O
that	O
R	B-Language
allows	O
arbitrary	O
side	O
effects	O
.	O
</s>
<s>
.NET	B-Language
languages	I-Language
implement	O
call	O
by	O
need	O
using	O
the	O
type	O
Lazyxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1	O
.	O
</s>
<s>
Graph	B-Application
reduction	I-Application
is	O
an	O
efficient	O
implementation	O
of	O
lazy	O
evaluation	O
.	O
</s>
<s>
Hygienic	B-Application
macros	I-Application
avoid	O
this	O
problem	O
by	O
checking	O
for	O
and	O
replacing	O
shadowed	B-Language
variables	I-Language
that	O
are	O
not	O
parameters	O
.	O
</s>
<s>
"	O
Call	O
by	O
future	O
"	O
,	O
also	O
known	O
as	O
"	O
parallel	O
call	O
by	O
name	O
"	O
or	O
"	O
lenient	O
evaluation	O
"	O
,	O
is	O
a	O
concurrent	B-Architecture
evaluation	B-Language
strategy	I-Language
combining	O
non-strict	O
semantics	O
with	O
eager	O
evaluation	O
.	O
</s>
<s>
These	O
futures	O
are	O
computed	O
concurrently	B-Operating_System
with	O
the	O
flow	O
of	O
the	O
rest	O
of	O
the	O
program	O
.	O
</s>
<s>
Conditionals	O
block	O
until	O
their	O
condition	O
is	O
evaluated	O
,	O
and	O
lambdas	B-General_Concept
do	O
not	O
create	O
futures	O
until	O
they	O
are	O
fully	O
applied	O
.	O
</s>
<s>
If	O
implemented	O
with	O
a	O
coroutine	B-Architecture
,	O
as	O
in	O
.NET	O
async/await	O
,	O
creating	O
a	O
future	O
calls	O
a	O
coroutine	B-Architecture
(	O
an	O
async	O
function	O
)	O
,	O
which	O
may	O
yield	O
to	O
the	O
caller	O
,	O
and	O
in	O
turn	O
be	O
yielded	O
back	O
to	O
when	O
the	O
value	O
is	O
used	O
,	O
cooperatively	O
multitasking	O
.	O
</s>
<s>
The	O
strategy	O
is	O
non-deterministic	O
,	O
as	O
the	O
evaluation	O
can	O
occur	O
at	O
any	O
time	O
between	O
creation	O
of	O
the	O
future	O
(	O
i.e.	O
,	O
when	O
the	O
expression	O
is	O
given	O
)	O
and	O
use	O
of	O
the	O
future	O
's	O
value	O
.	O
</s>
<s>
However	O
,	O
call-by-future	O
may	O
perform	O
unnecessary	O
speculative	O
work	O
compared	O
to	O
call-by-need	O
,	O
such	O
as	O
deeply	O
evaluating	O
a	O
lazy	O
data	B-General_Concept
structure	I-General_Concept
.	O
</s>
<s>
Optimistic	O
evaluation	O
is	O
a	O
call-by-need	O
variant	O
where	O
the	O
function	O
's	O
argument	O
is	O
partially	O
evaluated	O
in	O
a	O
call-by-value	O
style	O
for	O
some	O
amount	O
of	O
time	O
(	O
which	O
may	O
be	O
adjusted	O
at	B-Library
runtime	I-Library
)	O
.	O
</s>
<s>
This	O
approach	O
avoids	O
some	O
of	O
call-by-need	O
'	O
s	O
runtime	B-Library
expenses	O
while	O
retaining	O
desired	O
termination	O
characteristics	O
.	O
</s>
