Basically, what I want to do is to prefill a cell with an automatically
calculated default value, but I want to do the calculation only until it
succeeds.

Idea:

- A1 is a text field
- B1 is some value that's requested based on A1 via HTTP

However, I do not want the HTTP request to be done again when A1 is
changed again, or when all cells get recalculated. So basically, I want
a B1 to "collapse" to a string value without formula as soon as A1 once
was value.

So, the "primitive" I need is:

B1: =RetryUntilNonempty(SomeFunction(A1))

As soon as SomeFunction returns anything other than "", B1 will collapse
to the return value and no longer contain a formula.

Internally (but that's the part of which I know how it works),
SomeFunction will check A1 for validity, if invalid, return "", and as
soon as it is valid, issue the HTTP request and return some nonempty
string.

How to do that?