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

29 Mar 2021

      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?
LB
_______________________________________________
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.