Random.elm

-- I've successfully written a random generator for Circles.
randomCircle : Random.Generator Circle
randomCircle =
    Random.map4
        Circle
        (Random.pair (Random.float -200 200) (Random.float -200 200))
        (Random.float minimumRadiusLength maximumRadiusLength)
        randomColor
        (Random.float 0 maximumAlpha)


-- But now, I want a function that takes a Circle and either 
-- returns that Circle or generates a random one.
sometimesRandomCircle : Circle -> Random.Generator Circle
sometimesRandomCircle circle =
--  This approach doesn't work. What might?
    if Random.bool == True then
        circle
    else
        randomCircle

-- I understand how to create a *fully* random Circle (or List of random Circles).
-- I want to return a random Circle half the time, and return the existing Circle the other half.

@xarvh - Thank you so much!

I'd been banging my head against this for a few hours, so it was so awesome to get unstuck.

This got me thinking a Hoogle for Elm would have helped. Happy to see there is one, and it even finds Random.Extra.flattenList when I query for List (Random.Generator a) -> Random.Generator (List a).

@jmitchell - Nice find! That would indeed have helped.

Here's how I'd do it:

andThen = flip Random.andThen

swapSome : List Circle -> Random.Generator (List Circle)
swapSome circles =
  let
    doSwap oldCircle bool =
      if bool then
        Random.Extra.constant oldCircle
      else
        randomCircle
  in
    Random.list (List.length circles) Random.bool -- or use a non-uniform generator of booleans
    |> andThen (\bools -> List.map2 doSwap circles bools |> Random.Extra.flattenList)

Tricky little problem though!