Maybe problems converting back to number
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 and this as a crude return the nth element of a list import Data.List import Data.Maybe -- 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
myIndex [1,2,3,4,5] 3
* Non type-variable argument : in the constraint: Num (Maybe (Maybe a)) : (Use FlexibleContexts to permit this) : * When checking the inferred type : it :: forall a. Num (Maybe (Maybe a)) => Maybe a What am I missing here? Also, my type declaration seems to be wrong too, but I don't see why. LB
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
Here's what I finally did
myIndex'' l n
| m == Nothing = error "No list."
| otherwise = fromJust m
where m = mI l n
mI [] _ = Nothing
mI (h:t) n | n == 0 = Just h
| otherwise = mI t (n-1)
but then I can't say why I went to this extra step.
On Sat, Mar 6, 2021 at 10:53 AM 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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
participants (2)
-
Francesco Ariis -
Galaxy Being