Generate an ok result from a random generator of values

Generate an error result from a random generator of errors

Generate an ok result or an error result with 50-50 chance

This is simply implemented as follows:

result errorGenerator okGenerator =
  frequency
    [ (1, error errorGenerator)
    , (1, ok okGenerator)
    ] (ok okGenerator)

If you want to generate results with a different frequency, tweak those numbers to your bidding in your own custom generators.