[Haskell-beginners] Maybe problems converting back to number

[Francesco Ariis]

Il 06 marzo 2021 alle 09:53 Galaxy Being ha scritto:
> I've got this example from the Internet
> 
> import Data.List
> import Data.Maybe
> 
> firstFactorOf x
>     | m == Nothing = x
>     | otherwise = fromJust m
>     where m =(find p [2..x-1])
>           p y = mod x y == 0
> 
> -- myIndex :: [a] -> Int -> Maybe a
> myIndex [] _ = Nothing
> myIndex (x:xs) 0 = Just x
> myIndex (x:xs) n = myIndex xs (n-1)
> 
> I would like the Just x in the second block to actually be fromJust x as in
> the first block, i.e., I want a number returned, not a Just typed object.
> I've tried changing Just x to fromJust x but get the error when I try to
> use it

What would happen in the `Nothing` case? If an error is fine with you:

    myUnsafeIndex :: [a] -> Int -> a
    myUnsafeIndex as n =
            case myIndex as n of
              Nothing -> error "Chiare, fresche et dolci acque"
                         -- or maybe return -1? Idk
              Just r -> r

What have you written instead

> Also, my type declaration seems to be wrong too, but I don't see why.

It compiles fine here, even if I remove the comment from `myIndex`
signature
—F