<s>
In	O
functional	B-Language
programming	I-Language
,	O
a	O
monad	B-Application
transformer	I-Application
is	O
a	O
type	O
constructor	O
which	O
takes	O
a	O
monad	O
as	O
an	O
argument	O
and	O
returns	O
a	O
monad	O
as	O
a	O
result	O
.	O
</s>
<s>
Monad	B-Application
transformers	I-Application
can	O
be	O
used	O
to	O
compose	O
features	O
encapsulated	O
by	O
monads	O
–	O
such	O
as	O
state	O
,	O
exception	B-General_Concept
handling	I-General_Concept
,	O
and	O
I/O	O
–	O
in	O
a	O
modular	O
way	O
.	O
</s>
<s>
Typically	O
,	O
a	O
monad	B-Application
transformer	I-Application
is	O
created	O
by	O
generalising	O
an	O
existing	O
monad	O
;	O
applying	O
the	O
resulting	O
monad	B-Application
transformer	I-Application
to	O
the	O
identity	O
monad	O
yields	O
a	O
monad	O
which	O
is	O
equivalent	O
to	O
the	O
original	O
monad	O
(	O
ignoring	O
any	O
necessary	O
boxing	O
and	O
unboxing	O
)	O
.	O
</s>
<s>
A	O
monad	B-Application
transformer	I-Application
consists	O
of	O
:	O
</s>
<s>
Given	O
any	O
monad	O
,	O
the	O
option	O
monad	B-Application
transformer	I-Application
(	O
where	O
denotes	O
the	O
option	O
type	O
)	O
is	O
defined	O
by	O
:	O
</s>
<s>
Given	O
any	O
monad	O
,	O
the	O
exception	O
monad	B-Application
transformer	I-Application
(	O
where	O
is	O
the	O
type	O
of	O
exceptions	O
)	O
is	O
defined	O
by	O
:	O
</s>
<s>
Given	O
any	O
monad	O
,	O
the	O
reader	O
monad	B-Application
transformer	I-Application
(	O
where	O
is	O
the	O
environment	O
type	O
)	O
is	O
defined	O
by	O
:	O
</s>
<s>
Given	O
any	O
monad	O
,	O
the	O
state	O
monad	B-Application
transformer	I-Application
(	O
where	O
is	O
the	O
state	O
type	O
)	O
is	O
defined	O
by	O
:	O
</s>
<s>
Given	O
any	O
monad	O
,	O
the	O
writer	O
monad	B-Application
transformer	I-Application
(	O
where	O
is	O
endowed	O
with	O
a	O
monoid	O
operation	O
with	O
identity	O
element	O
)	O
is	O
defined	O
by	O
:	O
</s>
<s>
Given	O
any	O
monad	O
,	O
the	O
continuation	O
monad	B-Application
transformer	I-Application
maps	O
an	O
arbitrary	O
type	O
into	O
functions	O
of	O
type	O
,	O
where	O
is	O
the	O
result	O
type	O
of	O
the	O
continuation	O
.	O
</s>
