[Ur] runST monad in ur/web

[Artella Coding]

Hi, is there something like a haskell runST monad (
https://wiki.haskell.org/Monad/ST) in ur/web, so that I can perform
mutations within a pure function?

For example I might want to have my pure function make a copy of a large
array, and then perform the relevant mutations on this copy, before
returning this copy. The function is pure in that it does not modify the
input nor does it require data from the "outside world", and yet relies on
mutations of the internal copy (which it employs for the sake of
efficiency).

Thanks
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