narrow ()
narrow
<
U
>
(
guard
)
:
(
<
T
>
(
x
)
=>
Opt
<
U
>
)
Type Parameters
U
Parameters
guard
:
(
(
value
)
=>
value
is
U
)
(
value
)
:
value
is
U
Parameters
value
:
any
Returns
value
is
U
Returns
(
<
T
>
(
x
)
=>
Opt
<
U
>
)
<
T
>
(
x
)
:
Opt
<
U
>
Type Parameters
T
Parameters
x
:
Opt
<
T
>
Returns
Opt
<
U
>
See
Opt.narrow
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See
Opt.narrow