until -package:numeric-prelude -package:base-prelude -package:distribution-opensuse -package:extra -package:llvm-hs-pure -package:snap-core package:free -is:exact

Repeatedly run a computation until it produces a Just value. This can be useful when paired with a monad that has side effects. For example, we may have genId :: IO (Maybe Id) that uses a random number generator to allocate ids, but fails if it finds a collision. We can repeatedly run this with
retract (untilJust genId) :: IO Id