Question

I have a function which signature is String -> String -> IO (Maybe String)
Now, I use this function to build values for a dictionary and I end up with: [(String,IO (Maybe String))]

I have to analyze the values in the dictionary and based on the result return the appropriate key. I was hoping to just use filter on the Map to step through it but I can’t think of a way to extract that IO action on the fly. So how do I map/filter through the dictionary running the IO action and based on the result of the IO action’s value return appropriate key of the dictionary?? Is there an easy way of doing it or I just got myself in a mess?

Thanks.

Answer 1

Perhaps the solution is to use sequence with something like

sequence . map (\(a,mb) -> (mb >>= \b -> return (a,b)))

then you can simply use liftM to apply your filter to the resulting IO [(String,Maybe String)].

liftM is in Control.Monad. Alternatively, in do notation

myFilter :: (String,(Maybe String)) -> Bool) 
            -> [(String,IO (Maybe String))] 
            -> IO [(String,(Maybe String))]
myFilter f ml =
   do
       l <- sequence . map (\(a,mb) -> (mb >>= \b -> return (a,b))) $ ml
       return $ filter f ml

Perhaps some refactoring is in order. Often when working with monads you want to use mapM instead of map.

There is also a filterM function in Control.Monad. It might be what you need.

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Edit: it was pointed out in comments that

sequence . map (\(a,mb) -> (mb >>= \b -> return (a,b))) $ ml

is equivalent to

mapM (\(a,mb) -> fmap ((,) a) mb) ml

so

myFiter' f = (liftM $ filter f) .  mapM (\(a,mb) -> fmap ((,) a) mb)