<s>
Elm	B-Language
is	O
a	O
domain-specific	B-Language
programming	I-Language
language	I-Language
for	O
declaratively	B-Language
creating	O
web	O
browser-based	O
graphical	B-Application
user	I-Application
interfaces	I-Application
.	O
</s>
<s>
Elm	B-Language
is	O
purely	B-Application
functional	I-Application
,	O
and	O
is	O
developed	O
with	O
emphasis	O
on	O
usability	B-General_Concept
,	O
performance	O
,	O
and	O
robustness	B-Application
.	O
</s>
<s>
It	O
advertises	O
"	O
no	O
runtime	B-General_Concept
exceptions	I-General_Concept
in	O
practice	O
"	O
,	O
made	O
possible	O
by	O
the	O
Elm	B-Language
compiler	O
's	O
static	O
type	O
checking	O
.	O
</s>
<s>
Elm	B-Language
was	O
initially	O
designed	O
by	O
Evan	O
Czaplicki	O
as	O
his	O
thesis	O
in	O
2012	O
.	O
</s>
<s>
The	O
first	O
release	O
of	O
Elm	B-Language
came	O
with	O
many	O
examples	O
and	O
an	O
online	O
editor	O
that	O
made	O
it	O
easy	O
to	O
try	O
out	O
in	O
a	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
Evan	O
joined	O
Prezi	B-Application
in	O
2013	O
to	O
work	O
on	O
Elm	B-Language
,	O
and	O
in	O
2016	O
moved	O
to	O
NoRedInk	B-Application
as	O
an	O
Open	O
Source	O
Engineer	O
,	O
also	O
starting	O
the	O
Elm	B-Language
Software	O
Foundation	O
.	O
</s>
<s>
The	O
initial	O
implementation	O
of	O
the	O
Elm	B-Language
compiler	O
targets	O
HTML	B-Language
,	O
CSS	B-Language
,	O
and	O
JavaScript	B-Language
.	O
</s>
<s>
The	O
set	O
of	O
core	O
tools	O
has	O
continued	O
to	O
expand	O
,	O
now	O
including	O
a	O
REPL	B-Application
,	O
package	B-Application
manager	I-Application
,	O
time-travelling	O
debugger	O
,	O
and	O
installers	O
for	O
macOS	O
and	O
Windows	O
.	O
</s>
<s>
Elm	B-Language
also	O
has	O
an	O
ecosystem	O
of	O
and	O
,	O
an	O
advanced	O
online	O
editor	O
that	O
allows	O
saved	O
work	O
and	O
inclusion	O
of	O
community	O
libraries	O
.	O
</s>
<s>
Elm	B-Language
has	O
a	O
small	O
set	O
of	O
language	O
constructs	O
,	O
including	O
traditional	O
if-expressions	O
,	O
let-expressions	O
for	O
local	O
state	O
,	O
and	O
case-expressions	O
for	O
pattern	B-Language
matching	I-Language
.	O
</s>
<s>
As	O
a	O
functional	B-Language
language	I-Language
,	O
it	O
supports	O
anonymous	B-General_Concept
functions	I-General_Concept
,	O
functions	O
as	O
arguments	O
,	O
and	O
functions	O
can	O
return	O
functions	O
,	O
the	O
latter	O
often	O
by	O
partial	O
application	O
of	O
curried	O
functions	O
.	O
</s>
<s>
Its	O
semantics	O
include	O
immutable	B-Application
values	I-Application
,	O
stateless	B-Application
functions	I-Application
,	O
and	O
static	O
typing	O
with	O
type	O
inference	O
.	O
</s>
<s>
Elm	B-Language
programs	O
render	O
HTML	B-Language
through	O
a	O
virtual	O
DOM	B-General_Concept
,	O
and	O
may	O
interoperate	O
with	O
other	O
code	O
by	O
using	O
"	O
JavaScript	B-Language
as	O
a	O
service	O
"	O
.	O
</s>
<s>
All	O
values	O
in	O
Elm	B-Language
are	O
immutable	B-Application
,	O
meaning	O
that	O
a	O
value	O
cannot	O
be	O
modified	O
after	O
it	O
is	O
created	O
.	O
</s>
<s>
Elm	B-Language
uses	O
persistent	B-Application
data	I-Application
structures	I-Application
to	O
implement	O
its	O
arrays	O
,	O
sets	O
,	O
and	O
dictionaries	O
in	O
the	O
standard	O
library	O
.	O
</s>
<s>
Elm	B-Language
is	O
statically	O
typed	O
.	O
</s>
<s>
Elm	B-Language
uses	O
a	O
single	O
colon	O
to	O
mean	O
"	O
has	O
type	O
"	O
.	O
</s>
<s>
Custom	B-Language
types	I-Language
allow	O
the	O
programmer	O
to	O
create	O
custom	B-Language
types	I-Language
to	O
represent	O
data	O
in	O
a	O
way	O
that	O
matches	O
the	O
problem	O
domain	O
.	O
</s>
<s>
There	O
are	O
a	O
few	O
special	O
types	O
that	O
programmers	O
create	O
to	O
interact	O
with	O
the	O
Elm	B-Language
runtime	B-Library
.	O
</s>
<s>
For	O
example	O
,	O
Html	B-Language
Msg	O
represents	O
a	O
(	O
virtual	O
)	O
DOM	B-General_Concept
tree	I-General_Concept
whose	O
event	O
handlers	O
all	O
produce	O
messages	O
of	O
type	O
Msg	O
.	O
</s>
<s>
Rather	O
than	O
allow	O
any	O
value	O
to	O
be	O
implicitly	O
nullable	O
(	O
such	O
a	O
JavaScript	B-Language
's	O
undefined	O
or	O
a	O
null	O
pointer	O
)	O
,	O
Elm	B-Language
's	O
standard	O
library	O
defines	O
a	O
Maybe	O
a	O
type	O
.	O
</s>
<s>
Elm	B-Language
provides	O
a	O
limited	O
number	O
of	O
built-in	O
type	O
classes	O
:	O
number	O
which	O
includes	O
Int	O
and	O
Float	O
to	O
facilitate	O
the	O
use	O
of	O
numeric	O
operators	O
such	O
as	O
( +	O
)	O
or	O
( *	O
)	O
,	O
comparable	O
which	O
includes	O
numbers	O
,	O
characters	O
,	O
strings	O
,	O
lists	O
of	O
comparable	O
things	O
,	O
and	O
tuples	O
of	O
comparable	O
things	O
to	O
facilitate	O
the	O
use	O
of	O
comparison	O
operators	O
,	O
and	O
appendable	O
which	O
includes	O
strings	O
and	O
lists	O
to	O
facilitate	O
concatenation	O
with	O
( ++	O
)	O
.	O
</s>
<s>
Elm	B-Language
does	O
not	O
provide	O
a	O
mechanism	O
to	O
include	O
custom	B-Language
types	I-Language
into	O
these	O
type	O
classes	O
or	O
create	O
new	O
type	O
classes	O
(	O
see	O
Limitations	O
section	O
)	O
.	O
</s>
<s>
Elm	B-Language
has	O
a	O
module	B-Architecture
system	I-Architecture
that	O
allows	O
users	O
to	O
break	O
their	O
code	O
into	O
smaller	O
parts	O
called	O
modules	B-Architecture
.	O
</s>
<s>
Modules	B-Architecture
can	O
hide	O
implementation	O
details	O
such	O
as	O
helper	O
functions	O
,	O
and	O
group	O
related	O
code	O
together	O
.	O
</s>
<s>
Modules	B-Architecture
serve	O
as	O
a	O
namespace	O
for	O
imported	O
code	O
,	O
such	O
as	O
Bitwise.and	O
.	O
</s>
<s>
Third	O
party	O
libraries	O
(	O
or	O
packages	O
)	O
consist	O
of	O
one	O
or	O
more	O
modules	B-Architecture
,	O
and	O
are	O
available	O
from	O
the	O
.	O
</s>
<s>
Elm	B-Language
uses	O
an	O
abstraction	O
called	O
ports	O
to	O
communicate	O
with	O
JavaScript	B-Language
.	O
</s>
<s>
It	O
allows	O
values	O
to	O
flow	O
in	O
and	O
out	O
of	O
Elm	B-Language
programs	O
,	O
making	O
it	O
possible	O
to	O
communicate	O
between	O
Elm	B-Language
and	O
JavaScript	B-Language
.	O
</s>
<s>
Elm	B-Language
has	O
a	O
library	O
called	O
elm/html	O
that	O
a	O
programmer	O
can	O
use	O
to	O
write	O
HTML	B-Language
and	O
CSS	B-Language
within	O
Elm	B-Language
.	O
</s>
<s>
It	O
uses	O
a	O
virtual	O
DOM	B-General_Concept
approach	O
to	O
make	O
updates	O
efficient	O
.	O
</s>
<s>
Elm	B-Language
does	O
not	O
officially	O
support	O
server-side	O
development	O
.	O
</s>
<s>
Nevertheless	O
,	O
there	O
are	O
several	O
independent	O
projects	O
,	O
which	O
attempt	O
to	O
explore	O
possibilities	O
to	O
use	O
Elm	B-Language
for	O
the	O
back-end	O
.	O
</s>
<s>
The	O
projects	O
are	O
mainly	O
stuck	O
on	O
Elm	B-Language
version	O
0.18.0	O
since	O
newer	O
ones	O
do	O
not	O
support	O
"	O
native	O
"	O
code	O
and	O
some	O
other	O
utilized	O
features	O
.	O
</s>
<s>
There	O
are	O
two	O
attempts	O
to	O
use	O
Elm	B-Language
with	O
BEAM	O
(	O
Erlang	O
virtual	O
machine	O
)	O
.	O
</s>
<s>
One	O
of	O
the	O
projects	O
executes	O
Elm	B-Language
directly	O
on	O
the	O
environment	O
while	O
another	O
one	O
compiles	O
it	O
to	O
Elixir	O
.	O
</s>
<s>
Also	O
,	O
there	O
was	O
an	O
attempt	O
to	O
create	O
a	O
back-end	O
framework	O
for	O
Elm	B-Language
powered	O
by	O
Node.js	O
infrastructure	O
.	O
</s>
<s>
The	O
Elm	B-Language
Architecture	O
is	O
a	O
pattern	O
for	O
building	O
interactive	O
web	O
applications	O
.	O
</s>
<s>
Elm	B-Language
applications	O
are	O
naturally	O
constructed	O
in	O
that	O
way	O
,	O
but	O
other	O
projects	O
may	O
find	O
the	O
concept	O
useful	O
.	O
</s>
<s>
An	O
Elm	B-Language
program	O
is	O
always	O
split	O
into	O
three	O
parts	O
:	O
</s>
<s>
Those	O
are	O
the	O
core	O
of	O
the	O
Elm	B-Language
Architecture	O
.	O
</s>
<s>
The	O
view	O
function	O
would	O
be	O
defined	O
with	O
the	O
Html	B-Language
library	O
and	O
display	O
the	O
number	O
and	O
button	O
.	O
</s>
<s>
In	O
the	O
Elm	B-Language
Architecture	O
,	O
sending	O
messages	O
to	O
update	O
is	O
the	O
only	O
way	O
to	O
change	O
the	O
state	O
.	O
</s>
<s>
In	O
more	O
sophisticated	O
applications	O
,	O
messages	O
may	O
come	O
from	O
various	O
sources	O
:	O
user	O
interaction	O
,	O
initialization	O
of	O
the	O
model	O
,	O
internal	O
calls	O
from	O
update	O
,	O
subscriptions	O
to	O
external	O
events	O
(	O
window	O
resize	O
,	O
system	O
clock	O
,	O
JavaScript	B-Language
interop	O
...	O
)	O
and	O
URL	O
changes	O
and	O
requests	O
.	O
</s>
<s>
Elm	B-Language
does	O
not	O
support	O
higher-kinded	O
polymorphism	O
,	O
which	O
related	O
languages	O
Haskell	B-Language
and	O
PureScript	B-Language
offer	O
,	O
nor	O
does	O
Elm	B-Language
support	O
the	O
creation	O
of	O
type	O
classes	O
.	O
</s>
<s>
This	O
means	O
that	O
,	O
for	O
example	O
,	O
Elm	B-Language
does	O
not	O
have	O
a	O
generic	O
map	O
function	O
which	O
works	O
across	O
multiple	O
data	O
structures	O
such	O
as	O
List	O
and	O
Set	O
.	O
</s>
<s>
In	O
Elm	B-Language
,	O
such	O
functions	O
are	O
typically	O
invoked	O
qualified	O
by	O
their	O
module	B-Architecture
name	O
,	O
for	O
example	O
calling	O
List.map	O
and	O
Set.map	O
.	O
</s>
<s>
In	O
Haskell	B-Language
or	O
PureScript	B-Language
,	O
there	O
would	O
be	O
only	O
one	O
function	O
map	O
.	O
</s>
<s>
Another	O
outcome	O
is	O
a	O
large	O
amount	O
of	O
boilerplate	O
code	O
in	O
medium	O
to	O
large	O
size	O
projects	O
as	O
illustrated	O
by	O
the	O
author	O
of	O
"	O
Elm	B-Language
in	O
Action	O
"	O
in	O
their	O
single	O
page	O
application	O
example	O
with	O
almost	O
identical	O
fragments	O
being	O
repeated	O
in	O
update	O
,	O
view	O
,	O
subscriptions	O
,	O
route	O
parsing	O
and	O
building	O
functions	O
.	O
</s>
