[Haskell-beginners] type problems with my little code

Gesh gesh at gesh.uni.cx
Sun Apr 20 14:01:33 UTC 2014

On April 20, 2014 3:35:52 PM GMT+03:00, raffa f <freitasraffa at gmail.com> wrote:
>hi everyone, i'm a complete beginner on programming and i've been
>the 99 haskell problems and i've come into a situation. just like all
>problems with haskell, it's about types. i wrote a little thing that
>a list and puts all the equal characters together, like [1,1,3,2,3]
>will be
>[  [1,1],  [3,3], [2]  ]. here it is:
>pack xs = pack' xs
>         where
>         pack' :: [a] -> [[a]]
>         pack' [] = []
>         pack' (x:y) = extra x xs ++ pack' y
>extra x xs = [sum' (map (remover x) (map ((==) x) xs))]
>remover :: a -> Bool -> [a]
>remover y x = if x then [y] else []
>sum' :: [[a]] -> [a]
>sum' [] = []
>sum' (x:xs) = x ++ sum' xs
>i know, i know, this code is probably terrible and i'm sure there are
>clever ways to do this...but i wanna understand what's wrong. the
>function works perfectly, it takes a variable and then looks at how
>times it's presented on a list and outputs a list with a list of that
>variable the amount of times that it was presented on the original
>however, pack does not work quite right. first of all, it'll use extra
>repeated characters, creating repeated lists, but that doesn't matter
>because i have a function that fixes that issue and i'll use it after i
>figure out what's wrong with pack. now, the real problem is with types.
>here's what haskell says when i try to load it:
>    Couldn't match type `a' with `a1'
>      `a' is a rigid type variable bound by
>          the inferred type of pack :: [a] -> [[a]] at noventa.hs:73:1
>      `a1' is a rigid type variable bound by
>           the type signature for pack' :: [a1] -> [[a1]] at
>    Expected type: [a1]
>      Actual type: [a]
>    In the second argument of `extra', namely `xs'
>    In the first argument of `(++)', namely `extra x xs'
>    In the expression: extra x xs ++ pack' y
>Failed, modules loaded: none.
>i don't understand this at all! if i replace [a] with String and [[a]]
>[String], it works! but i want pack to work for lists of numbers too...
>types are so confusing. can anyone help me?
>Beginners mailing list
>Beginners at haskell.org

I'm a bit fuzzy on this part of GHC's type checker, so if someone could correct me, that'd be awesome.
If I understand correctly, what's happening here is that the type signature you gave pack' implicitly told GHC that pack' can work with any list, independently of the type of xs.
However, this is incorrect, as the call to extra forces the type of the input to pack' to be a list with the same type of elements as those of xs. Thus, GHC complains that you told it pack' accepts more stuff than it actually does.
This can be fixed in one of a few ways - this list might not be exhaustive, though.
- Keep the type of pack' as it currently is, but fix its implementation to actually have that type. Since our problem was caused by passing xs to extra restricting the type, you'd need to replace xs in that expression by something else, e.g. x:y
- Fix the type of pack' to what you actually meant. This can be done either by removing the signature, allowing GHC to fill in the correct type, or by turning on ScopedTypeVariables, writing pack's signature and making sure you use the same type variable for the elements of the parameters of pack and pack'.

Besides all of the above, there are a few more comments I'd like to make on your code.
First, note that sum' = concat
Next, extra x xs = [filter (==x) xs]
Then, noting that [x] ++ xs = x:xs, you obtain that
pack' (x:xs) = filter (==x) xs : pack' xs

In addition, the type signature you gave pack' lacks an Eq a constraint, which is needed as you are testing the elements of the parameter for equality.

Finally, pack doesn't do what you want it to do, as it will replace every element of its parameter by a list containing as many occurrences of that element as there are occurrences of that element in the parameter. Can you think of a way to solve this problem?

Good luck learning! I hope you'll find this journey interesting and beautiful.

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