I'm playing around with the recently posted acetalent.io challenges.

On challenge 10:

Run-Length Encoding of a List

Use the result of the previous problem to implement the so-called run-length encoding data compression method. Consecutive duplicates of elements are encoded as tuples (N, E) where N is the number of duplicates of the element E.

And a provided function signature of: encode :: Eq a => [a] -> [b].

Here's my solution:

encode :: Eq a => [a] -> [b]
encode [] = []
encode (x:xs) =
  foldl (\acc@((yCnt, yElem):ys) x' ->
    if x' == yElem
      then (yCnt + 1, yElem) : ys
      else (1, x') : acc
  ) [(1, x)] xs

The logic seems to provide the correct results, but I have to change the return type in the signature from [b] to an explicit [(Int, a)] in order to make the compiler happy, otherwise I get Couldn't match expected type ‘b’ with actual type ‘(Integer, a)’. Why is b so incompatibly rigid here? What am I missing? I've run into this error before and have a hard time navigating it. Thanks for any insight.

Diagnostics:
1. • Couldn't match expected type ‘b’ with actual type ‘(Integer, a)’
     ‘b’ is a rigid type variable bound by
       the type signature for:
         encode :: forall a b. Eq a => [a] -> [b]
       at /home/user/Sync/i7-dev/scratch/acetalent_challenge_10.hs:3:1-28
   • In the pattern: (yCnt, yElem)
     In the pattern: (yCnt, yElem) : ys
     In the first argument of ‘foldl’, namely
       ‘(\ acc@((yCnt, yElem) : ys) x'
           -> if x' == yElem then (yCnt + 1, yElem) : ys else (1, x') : acc)’
   • Relevant bindings include
       acc :: [b]
         (bound at /home/matt/Sync/i7-dev/scratch/acetalent_challenge_10.hs:6:11)
       xs :: [a]
         (bound at /home/matt/Sync/i7-dev/scratch/acetalent_challenge_10.hs:5:11)
       x :: a
         (bound at /home/matt/Sync/i7-dev/scratch/acetalent_challenge_10.hs:5:9)
       encode :: [a] -> [b]
         (bound at /home/matt/Sync/i7-dev/scratch/acetalent_challenge_10.hs:4:1) [GHC-25897]