For the record, since I suggested this solution (without actually trying it):
whereIsBM boiList = case boiList of
Nothing -> Nothing
Just (Cons idx lx)
| (idx == Bacon) -> Just 1
| otherwise -> (1 +) <$> (whereIsBM lx)
The problem I did not realize here is that lx is of type BaconOrIndex, not Maybe BaconOrIndex. There are two solutions. What someone suggested of just making whereIsBM receive BaconOrIndex all the way (You're also missing the Empty case, I just realized, which maybe you confused with Nothing, so I add that one):
whereIsBM boiList = case boiList of
{
Empty -> Nothing;
Cons idx lx
| (idx == Bacon) -> Just 1
| otherwise -> (1 +) <$> (whereIsBM lx)
}
The other option is to just wrap lx in Just, but that really feels strange and not what you want, plus you'd still need to account for the Empty case.
________________________________
From: Haskell-Cafe
whereIsBM (Cons (Indx 5) (Cons (Indx 13) (Cons (Indx 2) (Cons (Indx 8) Empty)))) Nothing whereIsBM (Cons (Indx 5) (Cons (Indx 13) (Cons Bacon (Cons (Indx 8) Empty)))) Just 3
to work. Unfortunately, I couldn't get this whereIsBM boiList = go 0 where go !_ Empty = Nothing go !acc (Cons idx lx) | (idx == Bacon) = Just acc | otherwise = go (acc + 1) lx to work. Both are nearly identical, but the latter gives this error
whereIsBM (Cons (Indx 5) (Cons (Indx 13) (Cons (Indx 2) (Cons (Indx 8) Empty)))) No instance for (Show (MyList BaconOrIndex -> Maybe Integer)) : arising from a use of `print'
This also failed
whereIsBM boiList = case boiList of
Nothing -> Nothing
Just (Cons idx lx)
| (idx == Bacon) -> Just 1
| otherwise -> (1 +) <$> (whereIsBM lx)
Couldn't match type `Maybe (MyList BaconOrIndex)'
with `MyList BaconOrIndex'
Expected type: MyList BaconOrIndex -> Maybe a
Actual type: Maybe (MyList BaconOrIndex) -> Maybe a
Not sure why this didn't work. Would like to understand the whole fmap idea as applied here, though.
On Mon, Mar 29, 2021 at 4:04 AM Henning Thielemann
Thus I applaud Michael Snoyman's quest to address the absense of a basic array type in the `base` library. Perhaps more users would stop abusing lists (memoisable iterators) as an indexed store.
Data.Array actually _was_ part of base-3. However, I think we should split 'base' in more smaller parts rather than making it bigger. _______________________________________________ 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. The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. Is e buidheann carthannais a th' ann an Oilthigh Dh?n ?ideann, cl?raichte an Alba, ?ireamh cl?raidh SC005336.