[Haskell-beginners] Reversing a list

[Francesco Ariis]

On Tue, Jul 04, 2017 at 12:06:39PM +0200, Jona Ekenberg wrote:
> Hello,
> 
> I'm currently going through the exercises in chapter 3 of Real World
> Haskell. One of the exercises challenges me to create a function which
> takes a list and makes a palindrome of it.
> 
> I tried to solve it this way:
> palindrome :: [t] -> [t]
> palindrome xs = xs ++ (reverse xs)
>   where
>     reverse []     = []
>     reverse (x:xs) = (reverse xs) ++ x

Hello Jona,

    let's analyse the error.
It points to this bit:

    palindrome xs = xs ++ (reverse xs)

And it says: I expected [[t]], but you gave me [t]. Whenever I encounter
such an error I try to write explicit type signatures so to make diagnosing
easier. In your example

    palindrome :: [t] -> [t]
    palindrome xs = xs ++ (reverse xs)
      where
          reverse :: [t] -> [t] -- explicit type signature
          reverse []     = []
          reverse (x:xs) = (reverse xs) ++ x

If we :reload ghci complains again, the offending bit being

    reverse (x:xs) = (reverse xs) ++ x
                                     ^
Expected type is [t1] but we passed t. Not it is clear! The type of `++` is:

    λ> :t (++)
    (++) :: [a] -> [a] -> [a]

and `x` is a single element. When we replace `x` with `[x]` everything works.

Does that help?
-F

P.S.: Real World Haskell is an excellent book but sometimes can be a tad
difficult to follow. If you want to integrate with another source, CIS194 [1]
is an excellent choice: free, thorough, full of home-works and interactive.

[1] http://www.cis.upenn.edu/~cis194/fall16/