<s>
The	O
history	O
of	O
the	O
programming	O
language	O
Scheme	B-Language
begins	O
with	O
the	O
development	O
of	O
earlier	O
members	O
of	O
the	O
Lisp	B-Language
family	O
of	O
languages	O
during	O
the	O
second	O
half	O
of	O
the	O
twentieth	O
century	O
.	O
</s>
<s>
During	O
the	O
design	O
and	O
development	O
period	O
of	O
Scheme	B-Language
,	O
language	O
designers	O
Guy	O
L	O
.	O
Steele	O
and	O
Gerald	O
Jay	O
Sussman	O
released	O
an	O
influential	O
series	O
of	O
Massachusetts	O
Institute	O
of	O
Technology	O
(	O
MIT	O
)	O
AI	B-Application
Memos	O
known	O
as	O
the	O
Lambda	O
Papers	O
(	O
1975	O
–	O
1980	O
)	O
.	O
</s>
<s>
Much	O
of	O
the	O
history	O
of	O
Scheme	B-Language
has	O
been	O
documented	O
by	O
the	O
developers	O
themselves	O
.	O
</s>
<s>
The	O
development	O
of	O
Scheme	B-Language
was	O
heavily	O
influenced	O
by	O
two	O
predecessors	O
that	O
were	O
quite	O
different	O
from	O
one	O
another	O
:	O
Lisp	B-Language
provided	O
its	B-Application
general	O
semantics	O
and	O
syntax	O
,	O
and	O
ALGOL	B-Language
provided	O
its	B-Application
lexical	B-Language
scope	I-Language
and	O
block	O
structure	O
.	O
</s>
<s>
Scheme	B-Language
is	O
a	O
dialect	O
of	O
Lisp	B-Language
but	O
Lisp	B-Language
has	O
evolved	O
;	O
the	O
Lisp	B-Language
dialects	O
from	O
which	O
Scheme	B-Language
evolved	O
—	O
although	O
they	O
were	O
in	O
the	O
mainstream	O
at	O
the	O
time	O
—	O
are	O
quite	O
different	O
from	O
any	O
modern	O
Lisp	B-Language
.	O
</s>
<s>
Lisp	B-Language
was	O
invented	O
by	O
John	O
McCarthy	O
in	O
1958	O
while	O
he	O
was	O
at	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
(	O
MIT	O
)	O
.	O
</s>
<s>
McCarthy	O
published	O
its	B-Application
design	O
in	O
a	O
paper	O
in	O
Communications	O
of	O
the	O
ACM	O
in	O
1960	O
,	O
entitled	O
"	O
Recursive	O
Functions	O
of	O
Symbolic	O
Expressions	O
and	O
Their	O
Computation	O
by	O
Machine	O
,	O
Part	O
I	O
"	O
(	O
Part	O
II	O
was	O
never	O
published	O
)	O
.	O
</s>
<s>
The	O
use	O
of	O
s-expressions	B-Protocol
which	O
characterize	O
the	O
syntax	O
of	O
Lisp	B-Language
was	O
initially	O
intended	O
to	O
be	O
an	O
interim	O
measure	O
pending	O
the	O
development	O
of	O
a	O
language	O
employing	O
what	O
McCarthy	O
called	O
"	O
m-expressions	B-Language
"	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
the	O
m-expression	B-Language
car[cons[A,B]]	O
is	O
equivalent	O
to	O
the	O
s-expression	B-Protocol
.	O
</s>
<s>
S-expressions	B-Protocol
proved	O
popular	O
,	O
however	O
,	O
and	O
the	O
many	O
attempts	O
to	O
implement	O
m-expressions	B-Language
failed	O
to	O
catch	O
on	O
.	O
</s>
<s>
The	O
first	O
implementation	O
of	O
Lisp	B-Language
was	O
on	O
an	O
IBM	B-Device
704	I-Device
by	O
Steve	O
Russell	O
,	O
who	O
read	O
McCarthy	O
's	O
paper	O
and	O
coded	O
the	O
eval	O
function	O
he	O
described	O
in	O
machine	O
code	O
.	O
</s>
<s>
The	O
familiar	O
(	O
but	O
puzzling	O
to	O
newcomers	O
)	O
names	O
CAR	B-Language
and	I-Language
CDR	I-Language
used	O
in	O
Lisp	B-Language
to	O
describe	O
the	O
head	O
element	O
of	O
a	O
list	O
and	O
its	B-Application
tail	O
,	O
evolved	O
from	O
two	O
IBM	B-Device
704	I-Device
assembly	O
language	O
commands	O
:	O
Contents	O
of	O
Address	O
Register	O
and	O
Contents	O
of	O
Decrement	O
Register	O
,	O
each	O
of	O
which	O
returned	O
the	O
contents	O
of	O
a	O
15-bit	O
register	O
corresponding	O
to	O
segments	O
of	O
a	O
36-bit	O
IBM704	O
instruction	O
word	O
.	O
</s>
<s>
The	O
first	O
complete	O
Lisp	B-Language
compiler	B-Language
,	O
written	O
in	O
Lisp	B-Language
,	O
was	O
implemented	O
in	O
1962	O
by	O
Tim	O
Hart	O
and	O
Mike	O
Levin	O
at	O
MIT	O
.	O
</s>
<s>
This	O
compiler	B-Language
introduced	O
the	O
Lisp	B-Language
model	O
of	O
incremental	O
compilation	B-Language
,	O
in	O
which	O
compiled	B-Language
and	O
interpreted	O
functions	O
can	O
intermix	O
freely	O
.	O
</s>
<s>
The	O
two	O
variants	O
of	O
Lisp	B-Language
most	O
significant	O
in	O
the	O
development	O
of	O
Scheme	B-Language
were	O
both	O
developed	O
at	O
MIT	O
:	O
LISP	B-Language
1.5	I-Language
developed	O
by	O
McCarthy	O
and	O
others	O
,	O
and	O
Maclisp	B-Language
–	O
developed	O
for	O
MIT	O
's	O
Project	O
MAC	O
,	O
a	O
direct	O
descendant	O
of	O
LISP	B-Language
1.5	I-Language
.	O
which	O
ran	O
on	O
the	O
PDP-10	B-Device
and	O
Multics	B-Application
systems	O
.	O
</s>
<s>
Since	O
its	B-Application
inception	O
,	O
Lisp	B-Language
was	O
closely	O
connected	O
with	O
the	B-Application
artificial	I-Application
intelligence	I-Application
(	O
AI	B-Application
)	O
research	O
community	O
,	O
especially	O
on	O
PDP-10	B-Device
.	O
</s>
<s>
The	O
36-bit	O
word	O
size	O
of	O
the	O
PDP-6	B-Device
and	O
PDP-10	B-Device
was	O
influenced	O
by	O
the	O
usefulness	O
of	O
having	O
two	O
Lisp	B-Language
18-bit	B-General_Concept
pointers	O
in	O
one	O
word	O
.	O
</s>
<s>
ALGOL	B-Language
58	I-Language
,	O
originally	O
to	O
be	O
called	O
IAL	B-Language
for	O
"	O
International	B-Language
Algorithmic	I-Language
Language	I-Language
"	O
,	O
was	O
developed	O
jointly	O
by	O
a	O
committee	O
of	O
European	O
and	O
American	O
computer	O
scientists	O
in	O
a	O
meeting	O
in	O
1958	O
at	O
ETH	O
Zurich	O
.	O
</s>
<s>
ALGOL	B-Language
60	I-Language
,	O
a	O
later	O
revision	O
developed	O
at	O
the	O
ALGOL	B-Language
60	I-Language
meeting	O
in	O
Paris	O
and	O
now	O
commonly	O
named	O
ALGOL	B-Language
,	O
became	O
the	O
standard	O
for	O
the	O
publication	O
of	O
algorithms	O
and	O
had	O
a	O
profound	O
effect	O
on	O
future	O
language	O
development	O
,	O
despite	O
the	O
language	O
's	O
lack	O
of	O
commercial	O
success	O
and	O
its	B-Application
limitations	O
.	O
</s>
<s>
Tony	O
Hoare	O
has	O
remarked	O
:	O
"	O
Here	O
is	O
a	O
language	O
so	O
far	O
ahead	O
of	O
its	B-Application
time	O
that	O
it	O
was	O
not	O
only	O
an	O
improvement	O
on	O
its	B-Application
predecessors	O
but	O
also	O
on	O
nearly	O
all	O
its	B-Application
successors.	O
"	O
</s>
<s>
ALGOL	B-Language
introduced	O
the	O
use	O
of	O
block	O
structure	O
and	O
lexical	B-Language
scope	I-Language
.	O
</s>
<s>
It	O
was	O
also	O
notorious	O
for	O
its	B-Application
difficult	O
call	O
by	O
name	O
default	O
parameter	O
passing	O
mechanism	O
,	O
which	O
was	O
defined	O
so	O
as	O
to	O
require	O
textual	O
substitution	O
of	O
the	O
expression	O
representing	O
the	O
working	O
parameter	O
in	O
place	O
of	O
the	O
formal	O
parameter	O
during	O
execution	O
of	O
a	O
procedure	O
or	O
function	O
,	O
causing	O
it	O
to	O
be	O
re-evaluated	O
each	O
time	O
it	O
is	O
referenced	O
during	O
execution	O
.	O
</s>
<s>
ALGOL	B-Language
implementors	O
developed	O
a	O
mechanism	O
they	O
called	O
a	O
thunk	B-Application
,	O
which	O
captured	O
the	O
context	O
of	O
the	O
working	O
parameter	O
,	O
enabling	O
it	O
to	O
be	O
evaluated	O
during	O
execution	O
of	O
the	O
procedure	O
or	O
function	O
.	O
</s>
<s>
In	O
1971	O
Sussman	O
,	O
Drew	O
McDermott	O
,	O
and	O
Eugene	O
Charniak	O
had	O
developed	O
a	O
system	O
called	O
Micro-Planner	O
which	O
was	O
a	O
partial	O
and	O
somewhat	O
unsatisfactory	O
implementation	O
of	O
Carl	O
Hewitt	O
's	O
ambitious	O
Planner	B-Application
project	O
.	O
</s>
<s>
Sussman	O
and	O
Hewitt	O
worked	O
together	O
along	O
with	O
others	O
on	O
Muddle	O
,	O
later	O
renamed	O
MDL	B-Language
,	O
an	O
extended	O
Lisp	B-Language
which	O
formed	O
a	O
component	O
of	O
Hewitt	O
's	O
project	O
.	O
</s>
<s>
Drew	O
McDermott	O
,	O
and	O
Sussman	O
in	O
1972	O
developed	O
the	O
Lisp-based	O
language	O
Conniver	O
,	O
which	O
revised	O
the	O
use	O
of	O
automatic	O
backtracking	O
in	O
Planner	B-Application
which	O
they	O
thought	O
was	O
unproductive	O
.	O
</s>
<s>
Hewitt	O
was	O
dubious	O
that	O
the	O
"	O
hairy	O
control	O
structure	O
"	O
in	O
Conniver	O
was	O
a	O
solution	O
to	O
the	O
problems	O
with	O
Planner	B-Application
.	O
</s>
<s>
In	O
November	O
1972	O
,	O
Hewitt	O
and	O
his	O
students	O
invented	O
the	O
Actor	B-Application
model	I-Application
of	O
computation	O
as	O
a	O
solution	O
to	O
the	O
problems	O
with	O
Planner	B-Application
.	O
</s>
<s>
A	O
partial	O
implementation	O
of	O
Actors	O
was	O
developed	O
called	O
Planner-73	O
(	O
later	O
called	O
PLASMA	O
)	O
.	O
</s>
<s>
Steele	O
,	O
then	O
a	O
graduate	O
student	O
at	O
MIT	O
,	O
had	O
been	O
following	O
these	O
developments	O
,	O
and	O
he	O
and	O
Sussman	O
decided	O
to	O
implement	O
a	O
version	O
of	O
the	O
Actor	B-Application
model	I-Application
in	O
their	O
own	O
"	O
tiny	O
Lisp	B-Language
"	O
developed	O
on	O
Maclisp	B-Language
,	O
to	O
understand	O
the	O
model	O
better	O
.	O
</s>
<s>
PLASMA	O
's	O
use	O
of	O
lexical	B-Language
scope	I-Language
was	O
similar	O
to	O
the	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
Sussman	O
and	O
Steele	O
decided	O
to	O
try	O
to	O
model	O
Actors	O
in	O
the	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
They	O
called	O
their	O
modeling	O
system	O
Schemer	O
,	O
eventually	O
changing	O
it	O
to	O
Scheme	B-Language
to	O
fit	O
the	O
six-character	O
limit	O
on	O
the	O
ITS	B-Application
file	O
system	O
on	O
their	O
DEC	B-Device
PDP-10	I-Device
.	O
</s>
<s>
They	O
eliminated	O
what	O
they	O
regarded	O
as	O
redundant	O
code	O
and	O
,	O
at	O
that	O
point	O
,	O
discovered	O
that	O
they	O
had	O
written	O
a	O
very	O
small	O
and	O
capable	O
dialect	O
of	O
Lisp	B-Language
.	O
</s>
<s>
Hewitt	O
remained	O
critical	O
of	O
the	O
"	O
hairy	O
control	O
structure	O
"	O
in	O
Scheme	B-Language
and	O
considered	O
primitives	O
(	O
e.g.	O
,	O
START	O
!	O
PROCESS	O
,	O
STOP	O
!	O
PROCESS	O
,	O
and	O
EVALUATE	O
!	O
UNINTERRUPTIBLY	O
)	O
used	O
in	O
the	O
Scheme	B-Language
implementation	O
to	O
be	O
a	O
backward	O
step	O
.	O
</s>
<s>
25	O
years	O
later	O
,	O
in	O
1998	O
,	O
Sussman	O
and	O
Steele	O
reflected	O
that	O
the	O
minimalism	O
of	O
Scheme	B-Language
was	O
not	O
a	O
conscious	O
design	O
goal	O
,	O
but	O
rather	O
the	O
unintended	O
outcome	O
of	O
the	O
design	O
process	O
.	O
</s>
<s>
"	O
We	O
were	O
actually	O
trying	O
to	O
build	O
something	O
complicated	O
and	O
discovered	O
,	O
serendipitously	O
,	O
that	O
we	O
had	O
accidentally	O
designed	O
something	O
that	O
met	O
all	O
our	O
goals	O
but	O
was	O
much	O
simpler	O
than	O
we	O
had	O
intended	O
...	O
we	O
realized	O
that	O
the	O
lambda	B-Language
calculus	I-Language
—	O
a	O
small	O
,	O
simple	O
formalism	O
—	O
could	O
serve	O
as	O
the	O
core	O
of	O
a	O
powerful	O
and	O
expressive	O
programming	O
language.	O
"	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
Hewitt	O
remained	O
critical	O
of	O
the	O
lambda	B-Language
calculus	I-Language
as	O
a	O
foundation	O
for	O
computation	O
writing	O
"	O
The	O
actual	O
situation	O
is	O
that	O
the	O
λ-calculus	B-Language
is	O
capable	O
of	O
expressing	O
some	O
kinds	O
of	O
sequential	O
and	O
parallel	O
control	O
structures	O
but	O
,	O
in	O
general	O
,	O
not	O
the	O
concurrency	O
expressed	O
in	O
the	O
Actor	B-Application
model	I-Application
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
the	O
Actor	B-Application
model	I-Application
is	O
capable	O
of	O
expressing	O
everything	O
in	O
the	O
λ-calculus	B-Language
and	O
more.	O
"	O
</s>
<s>
He	O
has	O
also	O
been	O
critical	O
of	O
aspects	O
of	O
Scheme	B-Language
that	O
derive	O
from	O
the	O
lambda	B-Language
calculus	I-Language
such	O
as	O
reliance	O
on	O
continuation	O
functions	O
and	O
the	O
lack	O
of	O
exceptions	O
.	O
</s>
<s>
Between	O
1975	O
and	O
1980	O
Sussman	O
and	O
Steele	O
worked	O
on	O
developing	O
their	O
ideas	O
about	O
using	O
the	O
lambda	B-Language
calculus	I-Language
,	O
continuations	O
and	O
other	O
advanced	O
programming	O
concepts	O
such	O
as	O
optimization	O
of	O
tail	B-Language
recursion	I-Language
,	O
and	O
published	O
them	O
in	O
a	O
series	O
of	O
AI	B-Application
Memos	O
which	O
have	O
become	O
collectively	O
termed	O
the	O
Lambda	O
Papers	O
.	O
</s>
<s>
Scheme	B-Language
was	O
the	O
first	O
dialect	O
of	O
Lisp	B-Language
to	O
choose	O
lexical	B-Language
scope	I-Language
.	O
</s>
<s>
It	O
had	O
a	O
large	O
impact	O
on	O
the	O
effort	O
that	O
led	O
to	O
the	O
development	O
of	O
its	B-Application
sister-language	O
,	O
Common	B-Language
Lisp	I-Language
,	O
to	O
which	O
Guy	O
Steele	O
was	O
a	O
contributor	O
.	O
</s>
<s>
The	O
Scheme	B-Language
language	I-Language
is	O
standardized	O
in	O
the	O
official	O
Institute	O
of	O
Electrical	O
and	O
Electronics	O
Engineers	O
(	O
IEEE	O
)	O
standard	O
,	O
and	O
a	O
de	O
facto	O
standard	O
called	O
the	O
Revisedn	O
Report	O
on	O
the	O
Algorithmic	B-Language
Language	I-Language
Scheme	B-Language
(	O
RnRS	B-Language
)	O
.	O
</s>
<s>
The	O
most	O
widely	O
implemented	O
standard	O
is	O
R5RS	B-Language
(	O
1998	O
)	O
,	O
and	O
a	O
new	O
standard	O
,	O
R6RS	O
,	O
was	O
ratified	O
in	O
2007	O
.	O
</s>
<s>
Besides	O
the	O
RnRS	B-Language
standards	O
there	O
are	O
also	O
Scheme	B-Language
Requests	I-Language
for	I-Language
Implementation	I-Language
documents	O
,	O
that	O
contain	O
additional	O
libraries	O
that	O
may	O
be	O
added	O
by	O
Scheme	B-Language
implementations	O
.	O
</s>
