Unwrapping a monad

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Regarding monads:

Yes, Either a is a monad. So simplifying the problem, you are basically asking for this:

main = print $ magicMonadUnwrap v

v :: Either String Int
v = Right 3

magicMonadUnwrap :: (Monad m) => m a -> a
magicMonadUnwrap = undefined

How do you define magicMonadUnwrap? Well, you see, it’s different for each monad. Each one needs its own unwrapper. Many of these have the word “run” in them, for example, runST, runCont, or runEval. However, for some monads, it might not be safe to unwrap them (hence the need for differing unwrappers).

One implementation for lists would be head. But what if the list is empty? An unwrapper for Maybe is fromJust, but what if it’s Nothing?

Similarly, the unwrapper for the Either monad would be something like:

fromRight :: Either a b -> b
fromRight (Right x) = x

But this unwrapper isn’t safe: what if you had a Left value instead? (Left usually represents an error state, in your case, a parse error). So the best way to act upon an Either value it is to use the either function, or else use a case statement matching Right and Left, as Daniel Wagner illustrated.

tl;dr: there is no magicMonadUnwrap. If you’re inside that same monad, you can use <-, but to truly extract the value from a monad…well…how you do it depends on which monad you’re dealing with.

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