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Today's Topics:
1. Maybe problems converting back to number (Galaxy Being)
2. Re: Maybe problems converting back to number (Francesco Ariis)
3. Re: Maybe problems converting back to number (Galaxy Being)
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Message: 1
Date: Sat, 6 Mar 2021 09:53:30 -0600
From: Galaxy Being <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Maybe problems converting back to number
Message-ID:
<CAFAhFSVen27BZfwD2Vo=4Mi4cC=ouof9pmi_uwyu1b1unwv...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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
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Message: 2
Date: Sat, 6 Mar 2021 17:52:16 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Maybe problems converting back to
number
Message-ID: <20210306165216.GB2849@extensa>
Content-Type: text/plain; charset=utf-8
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
------------------------------
Message: 3
Date: Sat, 6 Mar 2021 22:17:26 -0600
From: Galaxy Being <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Maybe problems converting back to
number
Message-ID:
<CAFAhFSVPkBfVOBxR-KzwO3GGoNKV+9-AV691AGSSbhmk=x-...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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 <[email protected]> wrote:
> 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
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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