I am writing Haskell code practicing tail recursion to inverse a list and have came up with this solution:
reverseh' [] list = list
reverseh' (x:xs) list = reverseh' (xs) (x ++ list)
reverse' x = reverseh' x []
It only works with list of list but I wanted it to be have type signature [a] -> [a].
Could you please explain what I am doing wrong here?
If you don't get the expected type, it's a good idea to add an explicit type signature to tell the compiler what you want the type to be, here:
reverseh' :: [a] -> [a] -> [a]
Then you get a compilation error:
Couldn't match expected type `[a]' with actual type `a'
`a' is a rigid type variable bound by
the type signature for reverseh' :: [a] -> [a] -> [a]
at reverseh.hs:1:14
Relevant bindings include
list :: [a] (bound at reverseh.hs:3:18)
xs :: [a] (bound at reverseh.hs:3:14)
x :: a (bound at reverseh.hs:3:12)
reverseh' :: [a] -> [a] -> [a] (bound at reverseh.hs:2:1)
In the first argument of `(++)', namely `x'
In the second argument of reverseh', namely `(x ++ list)'
This tells you that in (x ++ list), x needs to be of type [a] in order to typecheck, but with the type signature you have in mind, x really is of type a. So you want to replace ++ with a function of type a -> [a] -> [a], and so you go with (x : list).