[Haskell-cafe] Maybe won't let me count

Jon Purdy evincarofautumn at gmail.com
Mon Mar 29 03:37:13 UTC 2021

‘whereIsBM’ returns a Maybe-wrapped value, so applying ‘1 + …’ to it would
require ‘Maybe a’ to be in ‘Num’, hence the error message.
‘FlexibleContexts’ (ditto ‘FlexibleInstances’) is a pretty benign
extension, but it won’t help here, since it just kicks the error down the
road a bit.

The basic thing you need to do is match on the Maybe and return ‘Nothing’
if it was ‘Nothing’, or ‘Just (1 + x)’ if it was ‘Just x’ for some x. That
can be written quite literally as a ‘case’ expression:

case whereIsBM lx of
  Just x -> Just (1 + x)
  Nothing -> Nothing

Which could also be written with ‘do’:

  x <- whereIsBM lx
  pure (1 + x)

But this pattern is very common, so it’s already packaged up and
generalised as ‘fmap’, a.k.a. ‘<$>’

fmap (1 +) (whereIsBM lx)
-- or
(1 +) <$> whereIsBM lx

On Sun, Mar 28, 2021, 8:13 PM Galaxy Being <borgauf at gmail.com> wrote:

> I've got this
> import Data.Maybe
> data MyList a = Empty | Cons a (MyList a) deriving (Eq,Ord,Show)
> data BaconOrIndex = Bacon | Indx Int deriving (Eq,Ord,Show)
> whereIsBM Empty = Nothing
> whereIsBM (Cons idx lx) = if (idx == Bacon) then Just 1 else (whereIsBM lx)
> which I would like to tell me where the Bacon is (index), not just if
> there's Bacon, which is what it does now. That is, I need this to happen
> > whereIsBM (Cons (Indx 5) (Cons Bacon (Cons (Indx 2) (Cons (Indx 8)
> Empty))))
> Just 2
> So I need to traverse a BaconOrIndex list and count how deep I went to
> find the Bacon variable. I get the above code to evaluate error-free, but
> obviously I'm only returning a Just 1 when it sees Bacon. What I need is
> to have the last part be
>  . . . else (1 + whereIsBM lx)
>  work; but it keeps giving the error
> Non type-variable argument in the constraint: Num (Maybe a)
>       (Use FlexibleContexts to permit this)
>     • When checking the inferred type
>         whereIsBM :: forall a.
>                      (Num a, Num (Maybe a)) =>
>                      MyList BaconOrIndex -> Maybe a
> I haven't a clue what this means. Eventually, I'll wrap this in something
> that handles the Nothing and  does fromJust on the alternative. This
> whole effort is because if I didn't use the Maybe strategy, and said
> whereIsBM Empty = 0
> ...
> it would never give back 0 if it didn't find Bacon, rather, it would
> simply return the whole countdown to Empty. What can I do to make Maybe work
> here?
> LB
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20210328/fb773f4a/attachment.html>

More information about the Haskell-Cafe mailing list