<s>
Ur	B-Language
also	O
called	O
Ur/Web	B-Language
is	O
a	O
free	B-Application
and	O
open-source	B-Application
functional	B-Language
programming	I-Language
language	I-Language
specific	O
for	O
web	O
development	O
,	O
created	O
by	O
Adam	O
Chlipala	O
at	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
that	O
from	O
a	O
single	O
program	O
produces	O
server	B-Application
code	O
,	O
browser	B-Application
client	O
code	O
and	O
SQL	B-Language
code	O
specific	O
for	O
the	O
chosen	O
database	O
backend	B-Architecture
.	O
</s>
<s>
Ur	B-Language
supports	O
a	O
powerful	O
kind	O
of	O
metaprogramming	B-Application
based	O
on	O
row	O
types	O
.	O
</s>
<s>
Ur/Web	B-Language
is	O
Ur	B-Language
plus	O
a	O
special	O
standard	O
library	O
and	O
associated	O
rules	O
for	O
parsing	B-Language
and	O
optimization	O
.	O
</s>
<s>
Ur/Web	B-Language
supports	O
construction	O
of	O
dynamic	O
web	O
applications	O
backed	O
by	O
SQL	B-Language
databases	I-Language
.	O
</s>
<s>
The	O
signature	O
of	O
the	O
standard	O
library	O
is	O
such	O
that	O
well-typed	O
Ur/Web	B-Language
programs	O
"	O
do	O
n't	O
go	O
wrong	O
"	O
in	O
a	O
very	O
broad	O
sense	O
.	O
</s>
<s>
This	O
type	B-Language
safety	I-Language
is	O
just	O
the	O
foundation	O
of	O
the	O
Ur/Web	B-Language
methodology	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
use	O
metaprogramming	B-Application
to	O
build	O
significant	O
application	O
pieces	O
by	O
analysis	O
of	O
type	O
structure	O
.	O
</s>
<s>
The	O
Ur/Web	B-Language
compiler	O
also	O
produces	O
very	O
efficient	O
object	O
code	O
that	O
does	O
not	O
use	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
The	O
implementation	O
of	O
all	O
this	O
is	O
open	B-Application
source	I-Application
.	O
</s>
<s>
SQL	B-Language
syntax	O
templates	O
embedded	O
in	O
the	O
language	O
facilitate	O
the	O
handling	O
of	O
tables	O
.	O
</s>
<s>
Although	O
the	O
syntax	O
is	O
based	O
on	O
Standard	B-Language
ML	I-Language
the	O
language	O
includes	O
concepts	O
from	O
Haskell	B-Language
with	O
additional	O
type	O
manipulation	O
.	O
</s>
<s>
Ajax	B-General_Concept
call/response	O
is	O
serialized	O
through	O
a	O
monad	O
called	O
transaction	O
(	O
corresponds	O
to	O
Haskell	B-Language
's	O
IO	O
)	O
and	O
its	O
marshalling	B-Application
and	O
decoding	O
is	O
encapsulated	O
in	O
the	O
rpc	O
function	O
.	O
</s>
<s>
The	O
browser	B-Application
client	O
side	O
includes	O
functional	B-Language
reactive	B-Architecture
programming	I-Architecture
facilities	O
using	O
the	O
(	O
source	O
a	O
)	O
type	O
and	O
a	O
signal	O
monad	O
.	O
</s>
<s>
This	O
is	O
a	O
demo	O
program	O
showing	O
client	O
,	O
server	B-Application
and	O
database	O
code	O
with	O
Ajax	B-General_Concept
communication	O
,	O
from	O
the	O
web	O
demos	O
,	O
with	O
extra	O
comments	O
to	O
outline	O
each	O
of	O
the	O
components	O
:	O
</s>
<s>
Interface	O
file	O
(	O
ML-like	O
signature	O
)	O
with	O
extension	O
:	O
</s>
<s>
Implementation	O
file	O
(	O
.ur	O
extension	O
)	O
:	O
</s>
<s>
server	B-Application
side	O
,	O
page	O
retrieving	O
functions	O
with	O
no	O
side	O
effects	O
(	O
http	O
GET	O
method	O
)	O
are	O
accessible	O
through	O
a	O
URL	O
as	O
;	O
they	O
should	O
have	O
type	O
(	O
unit	O
->	O
transaction	O
page	O
)	O
.	O
</s>
<s>
urweb	O
module1	O
#	O
looks	O
for	O
module1.urpExecute	O
as	O
a	O
web	B-Application
server	I-Application
(	O
other	O
modes	O
are	O
CGI	B-Language
,	O
FastCGI	O
,	O
...	O
)	O
:	O
</s>
