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

[Galaxy Being]

I'm not getting past

whereIsBM boiList = case boiList of
                      Nothing -> Nothing
                      Just (Cons idx lx)
                        | (idx == Bacon) -> Just 1
                        | otherwise -> Just (1 + whereIsBM lx)

...and a few other attempts.

On Sun, Mar 28, 2021 at 10:37 PM Jon Purdy <evincarofautumn at gmail.com>
wrote:

> ‘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’:
>
> 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
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