Haskell - Extract Maybe in Monad -

i working on problem 9 of project euler, , have question best way extract maybe value inside monad. problem asks find 'a', 'b', 'c' satisfy:

• a^2 + b^2 = c^2
• a + b + c = 1000

i have written following code solves problem:

problem9 :: (integral a) => -> [(a, a, a)] problem9 n =             <- [1..n]         b <- [1..a]         c <- fromjustm (findc b)         guard (a + b + c == n)         return (a, b, c) 

'c' can computed analytically, but, since may not exist, return maybe value.

findc :: (integral a) => -> -> maybe findc b = ... (implementation) ... 

to extract maybe value inside list monad, have created following function:

fromjustm :: (monad m) => maybe -> m fromjustm (just a) = return fromjustm nothing = fail "" 

it seems should common operation, there standard library function this, or there more idiomatic way it?

fail not monadic operation; it's in monad type class because of historic accident / hide dirty error handling.

a more proper class monadplus, or rather applicative correspondent alternative. fail translates empty. that, signature should in fact be

fromjustm' :: alternative m => maybe -> m 

to hoogle offers

asum :: (foldable t, alternative f) => t (f a) -> f 

which fits bill: maybe foldable.

        c <- asum $ pure <$> findc b 

arguably, not in fact readable.

---

you can achieve goal easier writing

        c <- pure $ findc b 

this again use fail method: pattern-match failure in do block calls implicitly.