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

29 Mar 2021

      A bit of post-mortem...

I got this

data MyList a = Empty | Cons a (MyList a) deriving (Eq,Ord,Show)
data BaconOrIndex = Bacon | Indx Int deriving (Eq,Ord,Show)

import Data.Maybe
whereIsBM = whereIsBM' 1
whereIsBM' _ Empty = Nothing
whereIsBM' !n (Cons Bacon _) = Just n
whereIsBM' !n (Cons _ lx) = whereIsBM' (succ n) lx
...
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 <
lemming@henning-thielemann.de> wrote:
...
On Mon, 29 Mar 2021, Viktor Dukhovni wrote:
...
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.
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Re: [Haskell-cafe] Maybe won't let me count

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