Question

When I use fmap over a value, it essentially “unboxes” that value, applies the function to it, and boxes it back up.

For example:

-- returns Just 8
fmap (+3) (Just 5)

Is there any function that gives me the value without boxing it back up?

-- returns 8
fmap_2 (+3) (Just 5)

Granted, I don’t know how this would work for arrays, but it would be useful for Eithers and Maybes, for starters. I could also use it to mix Monads easily:

-- myfunc :: Maybe Int

-- if myfunc returned a Just, show it and then print it. Otherwise, print 'Nothing'.
putStrLn . fmap show $ myfunc

Or is there another standard way of mixing Monads?

Answer 1

For extracting Maybe use maybe:

maybe 0 (+3) $ Just 5
>>> 8
maybe 0 (+3) Nothing
>>> 0

You should use fromJust only when getting a Nothing must be considered as an abnormal situation. Don’t evolve a habit of crashing your program without a really good reason to. The Maybe and Either are there exactly to help you avoid that.

For Either there is a function either:

either (*2) (+2) $ Left 3
>>> 6
either (*2) (+2) $ Right 3
>>> 5

Please note that in correct code you won’t need to extract monads often. That’s kinda whole point about them and Haskell has to offer all the tools you may need to work with monadic values as if they were extracted.