# [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

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’:

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
> _______________________________________________