<s>
In	O
computer	B-General_Concept
programming	I-General_Concept
,	O
M-expressions	B-Language
(	O
or	O
meta-expressions	B-Language
)	O
were	O
an	O
early	O
proposed	O
syntax	O
for	O
the	O
Lisp	B-Language
programming	I-Language
language	I-Language
,	O
inspired	O
by	O
contemporary	O
languages	O
such	O
as	O
Fortran	B-Application
and	O
ALGOL	B-Language
.	O
</s>
<s>
Compared	O
to	O
S-expressions	B-Protocol
,	O
M-expressions	B-Language
introduce	O
function	O
notation	O
,	O
infix	O
operators	O
(	O
including	O
a	O
operator	O
)	O
,	O
and	O
shorthands	O
for	O
and	O
into	O
the	O
language	O
.	O
</s>
<s>
John	O
McCarthy	O
published	O
the	O
first	O
paper	O
on	O
Lisp	B-Language
in	O
1960	O
while	O
a	O
research	O
fellow	O
at	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
.	O
</s>
<s>
In	O
it	O
he	O
described	O
a	O
language	O
of	O
symbolic	O
expressions	O
(	O
S-expressions	B-Protocol
)	O
that	O
could	O
represent	O
complex	O
structures	O
as	O
lists	O
.	O
</s>
<s>
Then	O
he	O
defined	O
a	O
set	O
of	O
primitive	O
operations	O
on	O
the	O
S-expressions	B-Protocol
,	O
and	O
a	O
language	O
of	O
meta-expressions	B-Language
(	O
M-expressions	B-Language
)	O
that	O
could	O
be	O
used	O
to	O
define	O
more	O
complex	O
operations	O
.	O
</s>
<s>
Finally	O
,	O
he	O
showed	O
how	O
the	O
meta-language	O
itself	O
could	O
be	O
represented	O
with	O
S-expressions	B-Protocol
,	O
resulting	O
in	O
a	O
system	O
that	O
was	O
potentially	O
self-hosting	O
.	O
</s>
<s>
McCarthy	O
had	O
planned	O
to	O
develop	O
an	O
automatic	O
Lisp	B-Language
compiler	O
(	O
LISP	B-Language
2	I-Language
)	O
using	O
M-expressions	B-Language
as	O
the	O
language	O
syntax	O
and	O
S-expressions	B-Protocol
to	O
describe	O
the	O
compiler	O
's	O
internal	O
processes	O
.	O
</s>
<s>
Stephen	O
B	O
.	O
Russell	O
read	O
the	O
paper	O
and	O
suggested	O
to	O
him	O
that	O
S-expressions	B-Protocol
were	O
a	O
more	O
convenient	O
syntax	O
.	O
</s>
<s>
Although	O
McCarthy	O
disapproved	O
of	O
the	O
idea	O
,	O
Russell	O
and	O
colleague	O
Daniel	O
J	O
.	O
Edwards	O
hand-coded	O
an	O
interpreter	B-Application
program	O
that	O
could	O
execute	O
S-expressions	B-Protocol
.	O
</s>
<s>
This	O
program	O
was	O
adopted	O
by	O
McCarthy	O
's	O
research	O
group	O
,	O
establishing	O
S-expressions	B-Protocol
as	O
the	O
dominant	O
form	O
of	O
Lisp	B-Language
.	O
</s>
<s>
McCarthy	O
reflected	O
on	O
the	O
fate	O
of	O
M-expressions	B-Language
in	O
1979	O
:	O
</s>
<s>
A	O
form	O
of	O
sugared	O
M-expressions	B-Language
has	O
been	O
implemented	O
in	O
the	O
Wolfram	B-Language
language	I-Language
of	O
Wolfram	B-Application
Mathematica	I-Application
since	O
1988	O
:	O
</s>
<s>
MLisp	B-Language
was	O
a	O
contemporary	O
(	O
1968-1973	O
)	O
project	O
to	O
implement	O
an	O
M-expression-like	O
frontend	O
for	O
Lisp	B-Language
.	O
</s>
<s>
A	O
few	O
extra	O
features	O
like	O
hygienic	B-Application
macros	I-Application
,	O
pattern	B-Language
matching	I-Language
,	O
and	O
backtracking	O
were	O
incorporated	O
.	O
</s>
<s>
M-LISP	O
(	O
MetaLISP	O
)	O
from	O
1989	O
was	O
another	O
attempt	O
to	O
blend	O
M-expressions	B-Language
with	O
Scheme	B-Language
.	O
</s>
<s>
A	O
parser	O
for	O
the	O
"	O
AI	O
Memo	O
8	O
"	O
M-expression	B-Language
is	O
available	O
in	O
Common	B-Language
Lisp	I-Language
,	O
but	O
the	O
author	O
intends	O
it	O
as	O
a	O
case	O
against	O
M-expressions	B-Language
due	O
to	O
its	O
perceived	O
inability	O
to	O
cope	O
with	O
macros	O
.	O
</s>
<s>
The	O
K	B-Application
(	O
programming	O
language	O
)	O
also	O
includes	O
M-Expressions	B-Language
,	O
in	O
addition	O
to	O
the	O
more	O
terse	O
notation	O
in	O
the	O
APL-tradition	O
.	O
</s>
<s>
A	O
CGOL	B-Language
(	O
1977	O
)	O
was	O
implemented	O
in	O
MacLisp	B-Language
and	O
follows	O
a	O
similar	O
goal	O
of	O
introducing	O
Algol-like	B-Language
syntax	O
with	O
infix	O
operators	O
.	O
</s>
<s>
It	O
is	O
known	O
to	O
work	O
on	O
Armed	O
Bear	O
Common	B-Language
Lisp	I-Language
.	O
</s>
<s>
A	O
more	O
recent	O
(	O
circa	O
2003	O
)	O
variant	O
is	O
the	O
I-expression	O
,	O
which	O
use	O
indentation	B-Language
to	O
indicate	O
parentheses	O
implicitly	O
,	O
and	O
are	O
thus	O
in	O
some	O
ways	O
intermediate	O
between	O
S-expressions	B-Protocol
and	O
M-expressions	B-Language
.	O
</s>
<s>
I-expressions	O
were	O
introduced	O
in	O
Scheme	B-Language
Request	I-Language
For	I-Language
Implementation	I-Language
49	O
as	O
an	O
auxiliary	O
syntax	O
for	O
Scheme	B-Language
,	O
but	O
they	O
have	O
not	O
been	O
widely	O
adopted	O
.	O
</s>
<s>
Like	O
I-expressions	O
,	O
t-expressions	O
are	O
only	O
a	O
simple	O
transformation	O
away	O
from	O
S-expressions	B-Protocol
,	O
so	O
that	O
theoretically	O
they	O
can	O
be	O
used	O
on	O
any	O
Lisp	B-Language
dialect	O
and	O
not	O
interfere	O
with	O
features	O
like	O
macros	O
.	O
</s>
<s>
Additional	O
syntax-related	O
include	O
Apple	O
's	O
Dylan	B-Language
(	O
Algol-like	B-Language
tokens	O
)	O
and	O
Clojure	B-Language
's	O
addition	O
of	O
other	O
literal	O
syntaxes	O
.	O
</s>
