Question

I’m basically looking for the find analogue of filterM:

findM :: (a -> m Bool) -> [a] -> m (Maybe a)

but I haven’t been able to write it myself using find and some sort of lifting function. I’m currently doing it via Data.Maybe.listToMaybe:

x <- filterM f list
return $ listToMaybe x

which works, but it seems like I should be able to do it directly with find.

Edit: Found some stuff on hackage: Control.Monad.Loops.firstM and Control.Monad.TM.findM both do what I want

Answer 1

Well, you could write it “from scratch” like

findM p ls =
    case ls of
         []   -> return Nothing
         x:xs -> do
            ok <- p x
            if ok
               then return (Just x)
               else findM p xs

though I realize that is much less elegant than what you are asking for. I could not figure out how to do it with some sort of lifting function. I feel like the lifting would have to occur at the recursive step of find, which is hidden inside the function and not available to be lifted. Though I would love to see someone prove me wrong.