<s>
In	O
type	O
theory	O
and	O
programming	O
languages	O
,	O
a	O
type	B-Application
variable	I-Application
is	O
a	O
mathematical	O
variable	O
ranging	O
over	O
types	O
.	O
</s>
<s>
Even	O
in	O
programming	O
languages	O
that	O
allow	O
mutable	O
variables	O
,	O
a	O
type	B-Application
variable	I-Application
remains	O
an	O
abstraction	O
,	O
in	O
the	O
sense	O
that	O
it	O
does	O
not	O
correspond	O
to	O
some	O
memory	O
locations	O
.	O
</s>
<s>
Programming	O
languages	O
that	O
support	O
parametric	O
polymorphism	O
make	O
use	O
of	O
universally	O
quantified	O
type	B-Application
variables	I-Application
.	O
</s>
<s>
Languages	O
that	O
support	O
existential	O
types	O
make	O
use	O
of	O
existentially	B-Algorithm
quantified	I-Algorithm
type	B-Application
variables	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
the	O
following	O
OCaml	B-Language
code	O
defines	O
a	O
polymorphic	O
identity	O
function	O
that	O
has	O
a	O
universally	O
quantified	O
type	O
,	O
which	O
is	O
printed	O
by	O
the	O
interpreter	O
on	O
the	O
second	O
line	O
:	O
</s>
<s>
In	O
mathematical	O
notation	O
,	O
the	O
type	O
of	O
the	O
function	O
id	O
is	O
,	O
where	O
is	O
a	O
type	B-Application
variable	I-Application
.	O
</s>
