Question

I am trying to solve problem 10 from the ninety nine haskell problems.
Here is my solution that I believe is correct.

The pack function (from question 9) is already correct. The problem is with the encode function.

pack :: (Eq a) => [a] -> [[a]]
pack [] = []
pack (x:xs) = (x : takeWhile (== x) xs) : (pack $ dropWhile (== x) xs)

encode :: (Eq a) => [a] -> [(Int, a)]
encode [] = []
encode list = (encode' $ head packed) : (encode $ tail packed)
    where packed = pack list
          encode' l = (length l, head l) 

When I load the file from ghci, this is the error:

encode.hs:6:0:
Occurs check: cannot construct the infinite type: a = [a]
When generalising the type(s) for `encode’

Line 6 is the line containing encode [] = []

What’s wrong with my encode function? I keep checking the types of the variables used and I believe there is nothing wrong.

Function usage example (assuming code is working correctly):

pack "aaaabbbbccccccddddddd"
> ["aaaa","bbbb","cccccc","ddddddd"]
encode "aaaabbbbccccccddddddd"
> [(4,'a'),(4,'b'),(6,'c'),(7,'d')]

Answer 1

encode $ tail packed

We have

packed :: [[a]]
tail packed :: [[a]]

But we need to pass an [a] to encode.

(Think about it like this: list needs to go through pack. packed is the output from pack, but in the recursive call it will be passed to pack again.)