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# general
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m
You can do this by using the output of the 
apply
as an input to 
pB
.
e.g. if 
gate
is the 
apply
you want to gate on and 
foo
is the actual input you need, you can do this;
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const gate = /* some apply */;
const foo = /* some value */;
const pB = new B("b", {
    ...
    input: pulumi.all([foo, gate]).apply(([f]) => f);
});
f
@microscopic-florist-22719 What if 
B
doesn't need to use the output of 
pulumi.all
?
m
Does 
B
have any inputs? If so, you can mix it into one of those
If not, then this gets trickier
f
It has inputs but none that have to do with the result of the apply.
m
That's okay
The snippet I posted above throws away the result of the apply
The only reason the 
all
is in there is to ensure that the apply has finished before 
pB
is created
Let me decompose that example into two steps
say we start with this for `pB`:
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const pB = new B("b", {
    ...
    input: someInput,
});
then we write an 
apply
that needs to run before 
pB
is registered:
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const gate = pA.foo.apply(foo => ...);
Now we mix the result of the apply in to `pB`'s inputs using an `all`:
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const pB = new B("b", {
    ...
    input: pulumi.all([someInput, gate]).apply(([i]) => i),
});
this expression:
f
Oh I see what's happening. Are other users taking this approach?
m
IIRC @creamy-potato-29402 blogged about it
and I have one or two uses of this in the wild
hm, maybe he did not post his example as a blog. I know that he has an example of using this technique to gate k8s updates on Prometheus checks.
f
Cool. I'll try it out.
@microscopic-florist-22719 If I wanted to use this multiple times, would a function like this work?
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function waitForDatomic<T>(val: T): pulumi.Output<T> {
    return pulumi.all([val, datomicGate]).apply(([v]) => v);
}
Hmm the return type there is quite right because 
pulumi.all
returns 
Output<Unwrap<T>>
. Not sure what the 
Unwrap
does...
m
The short answer on "what 
Unwrap
does is "it's complicated" 🙂. @lemon-spoon-91807 can provide a longer answer.
But a function like that would work, yes
f
Where is the 
Unwrap
type declared?
l
f
How do I use it? My IDE says namespace has no exported member 'Unwrap'.
l
the idea behind 'unwrap' is is deep clone of your object with 'complex' types removed.
f
When doing 
pulumi.Unwrap
.
l
so, for example, if you have: 
{ foo: Promise<number>, bar: Promise<Promise<bool>[]> }
then 'Unwrap' of that type would be 
{ foo: number, bar: bool[] }
right now they're not exported types. You can just not give a return type for this method and TS will iner the right type
we could export them to help out here. haven't had a need yet, but i don't see anything wrong with that given that they are in the signatures of helper functions we do export
(i actually wish TS would warn/error about this).
because it's possible to export signatures that refer to types that are themselves not exported.
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