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/