<s>
In	O
Lisp	B-Language
programming	I-Language
languages	I-Language
,	O
a	O
fexpr	B-Language
is	O
a	O
function	O
whose	O
operands	O
are	O
passed	O
to	O
it	O
without	O
being	O
evaluated	O
.	O
</s>
<s>
When	O
a	O
fexpr	B-Language
is	O
called	O
,	O
only	O
the	O
body	O
of	O
the	O
fexpr	B-Language
is	O
evaluated	O
;	O
no	O
other	O
evaluations	O
take	O
place	O
except	O
when	O
explicitly	O
initiated	O
by	O
the	O
fexpr	B-Language
.	O
</s>
<s>
In	O
contrast	O
,	O
when	O
an	O
ordinary	O
Lisp	B-Language
function	O
is	O
called	O
,	O
the	O
operands	O
are	O
evaluated	O
automatically	O
,	O
and	O
only	O
the	O
results	O
of	O
these	O
evaluations	O
are	O
provided	O
to	O
the	O
function	O
;	O
and	O
when	O
a	O
(	O
traditional	O
)	O
Lisp	B-Language
macro	O
is	O
called	O
,	O
the	O
operands	O
are	O
passed	O
in	O
unevaluated	O
,	O
but	O
whatever	O
result	O
the	O
macro	O
function	O
returns	O
is	O
automatically	O
evaluated	O
.	O
</s>
<s>
In	O
early	O
Lisp	B-Language
,	O
the	O
environment	O
mapped	O
each	O
symbol	O
to	O
an	O
association	B-Data_Structure
list	I-Data_Structure
,	O
rather	O
than	O
directly	O
to	O
a	O
value	O
.	O
</s>
<s>
The	O
only	O
difference	O
between	O
a	O
FEXPR	B-Language
and	O
an	O
EXPR	O
was	O
whether	O
the	O
operands	O
were	O
automatically	O
evaluated	O
.	O
</s>
<s>
In	O
strict	O
original	O
usage	O
,	O
a	O
FEXPR	B-Language
is	O
therefore	O
a	O
user-defined	O
function	O
whose	O
operands	O
are	O
passed	O
unevaluated	O
.	O
</s>
<s>
However	O
,	O
in	O
later	O
usage	O
the	O
term	O
fexpr	B-Language
may	O
describe	O
any	O
first-class	B-Application
function	I-Application
whose	O
operands	O
are	O
passed	O
unevaluated	O
,	O
regardless	O
of	O
whether	O
the	O
function	O
is	O
primitive	O
or	O
user-defined	O
.	O
</s>
<s>
As	O
a	O
simple	O
illustration	O
of	O
how	O
fexprs	B-Language
work	O
,	O
here	O
is	O
a	O
fexpr	B-Language
definition	O
written	O
in	O
the	O
Kernel	O
programming	O
language	O
,	O
which	O
is	O
similar	O
to	O
Scheme	B-Language
.	O
</s>
<s>
(	O
By	O
convention	O
in	O
Kernel	O
,	O
the	O
names	O
of	O
fexprs	B-Language
always	O
start	O
with	O
.	O
)	O
</s>
<s>
This	O
definition	O
provides	O
a	O
fexpr	B-Language
called	O
,	O
which	O
takes	O
three	O
operands	O
.	O
</s>
<s>
When	O
the	O
fexpr	B-Language
is	O
called	O
,	O
a	O
local	O
environment	O
is	O
created	O
by	O
extending	O
the	O
static	O
environment	O
where	O
the	O
fexpr	B-Language
was	O
defined	O
.	O
</s>
<s>
Local	O
bindings	O
are	O
then	O
created	O
:	O
symbols	O
,	O
,	O
and	O
are	O
bound	O
to	O
the	O
three	O
operands	O
of	O
the	O
call	O
to	O
the	O
fexpr	B-Language
,	O
and	O
symbol	O
is	O
bound	O
to	O
the	O
dynamic	O
environment	O
from	O
which	O
the	O
fexpr	B-Language
is	O
being	O
called	O
.	O
</s>
<s>
The	O
body	O
of	O
the	O
fexpr	B-Language
,	O
...	O
,	O
is	O
then	O
evaluated	O
in	O
this	O
local	O
environment	O
,	O
and	O
the	O
result	O
of	O
that	O
evaluation	O
becomes	O
the	O
result	O
of	O
the	O
call	O
to	O
the	O
fexpr	B-Language
.	O
</s>
<s>
This	O
example	O
is	O
statically	B-Language
scoped	I-Language
:	O
the	O
local	O
environment	O
is	O
an	O
extension	O
of	O
the	O
static	O
environment	O
.	O
</s>
<s>
Before	O
about	O
1980	O
,	O
the	O
Lisp	B-Language
languages	I-Language
that	O
supported	O
fexprs	B-Language
were	O
mainly	O
dynamically	B-Language
scoped	I-Language
:	O
the	O
local	O
environment	O
was	O
an	O
extension	O
of	O
the	O
dynamic	O
environment	O
,	O
rather	O
than	O
of	O
the	O
static	O
environment	O
.	O
</s>
<s>
Fexpr	B-Language
support	O
continued	O
in	O
Lisp	B-Language
1.5	I-Language
,	O
the	O
last	O
substantially	O
standard	O
dialect	O
of	O
Lisp	B-Language
before	O
it	O
fragmented	O
into	O
multiple	O
languages	O
.	O
</s>
<s>
In	O
the	O
1970s	O
,	O
the	O
two	O
dominant	O
Lisp	B-Language
languages	I-Language
MacLisp	B-Language
and	O
Interlisp	B-Language
both	O
supported	O
fexprs	B-Language
.	O
</s>
<s>
At	O
the	O
1980	O
Conference	O
on	O
Lisp	B-Language
and	O
Functional	O
Programming	O
,	O
Kent	O
Pitman	O
presented	O
a	O
paper	O
"	O
Special	O
Forms	O
in	O
Lisp	B-Language
"	O
in	O
which	O
he	O
discussed	O
the	O
advantages	O
and	O
disadvantages	O
of	O
macros	O
and	O
fexprs	B-Language
,	O
and	O
ultimately	O
condemned	O
fexprs	B-Language
.	O
</s>
<s>
His	O
central	O
objection	O
was	O
that	O
,	O
in	O
a	O
Lisp	B-Language
dialect	O
that	O
allows	O
fexprs	B-Language
,	O
static	O
analysis	O
cannot	O
determine	O
generally	O
whether	O
an	O
operator	O
represents	O
an	O
ordinary	O
function	O
or	O
a	O
fexpr	B-Language
therefore	O
,	O
static	O
analysis	O
cannot	O
determine	O
whether	O
or	O
not	O
the	O
operands	O
will	O
be	O
evaluated	O
.	O
</s>
<s>
Since	O
the	O
decline	O
of	O
MacLisp	B-Language
and	O
Interlisp	B-Language
,	O
the	O
two	O
Lisp	B-Language
languages	I-Language
that	O
had	O
risen	O
to	O
dominance	O
by	O
1993	O
Scheme	B-Language
and	O
Common	B-Language
Lisp	I-Language
do	O
not	O
support	O
fexprs	B-Language
.	O
</s>
<s>
newLISP	B-Language
does	O
support	O
fexprs	B-Language
,	O
but	O
calls	O
them	O
"	O
macros	O
"	O
.	O
</s>
<s>
In	O
Picolisp	B-Language
all	O
built-in	O
functions	O
are	O
,	O
while	O
Lisp-level	O
functions	O
are	O
exprs	O
,	O
fexprs	B-Language
,	O
,	O
or	O
a	O
mixture	O
of	O
those	O
.	O
</s>
<s>
Starting	O
with	O
Brian	O
Smith	O
's	O
3-Lisp	O
in	O
1982	O
,	O
several	O
experimental	O
Lisp	B-Language
dialects	O
have	O
been	O
devised	O
to	O
explore	O
the	O
limits	O
of	O
computational	B-Language
reflection	I-Language
.	O
</s>
<s>
To	O
support	O
reflection	O
,	O
these	O
Lisps	B-Language
support	O
procedures	O
that	O
can	O
reify	O
various	O
data	O
structures	O
related	O
to	O
the	O
call	O
to	O
them	O
including	O
the	O
unevaluated	O
operands	O
of	O
the	O
call	O
,	O
which	O
makes	O
these	O
procedures	O
fexprs	B-Language
.	O
</s>
<s>
By	O
the	O
late	O
1990s	O
,	O
fexprs	B-Language
had	O
become	O
associated	O
primarily	O
with	O
computational	B-Language
reflection	I-Language
.	O
</s>
<s>
Some	O
theoretical	O
results	O
on	O
fexprs	B-Language
have	O
been	O
obtained	O
.	O
</s>
<s>
In	O
1993	O
,	O
John	O
C	O
.	O
Mitchell	O
used	O
Lisp	B-Language
with	O
fexprs	B-Language
as	O
an	O
example	O
of	O
a	O
programming	O
language	O
whose	O
source	O
expressions	O
cannot	O
be	O
formally	O
abstract	O
(	O
because	O
the	O
concrete	O
syntax	O
of	O
a	O
source	O
expression	O
can	O
always	O
be	O
extracted	O
by	O
a	O
context	O
in	O
which	O
it	O
is	O
an	O
operand	O
to	O
a	O
fexpr	B-Language
)	O
.	O
</s>
<s>
In	O
1998	O
,	O
Mitchell	O
Wand	O
showed	O
that	O
adding	O
a	O
fexpr	B-Language
device	O
to	O
lambda	B-Language
calculus	I-Language
a	O
device	O
that	O
suppresses	O
rewriting	O
of	O
operands	O
produces	O
a	O
formal	O
system	O
with	O
a	O
trivial	O
equational	O
theory	O
,	O
rendering	O
it	O
impossible	O
to	O
make	O
source-to-source	O
optimizations	O
without	O
a	O
whole-program	O
analysis	O
.	O
</s>
<s>
In	O
2007	O
,	O
John	O
N	O
.	O
Shutt	O
proposed	O
an	O
extension	O
of	O
lambda	B-Language
calculus	I-Language
that	O
would	O
model	O
fexprs	B-Language
without	O
suppressing	O
rewriting	O
of	O
operands	O
,	O
avoiding	O
Wand	O
's	O
result	O
.	O
</s>
