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

A simple solution: data MyList a = Empty | Cons a (MyList a) deriving (Eq,Ord,Show) data BaconOrIndex = Bacon | Indx Int deriving (Eq,Ord,Show) whereIsBM = whereIsBM' 0 whereIsBM' _ Empty = Nothing whereIsBM' !n (Cons Bacon _) = Just n whereIsBM' !n (Cons _ lx) = whereIsBM' (succ n) lx You can omit the ! if you want. The result will be the same, but the computation will use more memory because the program will first construct the unevaluated data structure (called 'thunk') suc (suc (suc (... 0) ...)) and then compute it instead of computing succ 0 to 1, then succ 1 to 2, etc., step by step, in constant memory. succ n is n+1 but faster than the function (+). Best, Antonio Regidor Garcia El Sun, Mar 28, 2021 at 11:27:48PM -0500, Galaxy Being escribió:

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 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 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?

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