Question

I am writing a logger for a sorting function like this:

bubble :: (Ord a) => [a] -> Writer [String] [a]
bubble (x:y:ys)  
        | x > y = do
                tell [show x ++ " why does this not work"]
                y:bubble(x:ys)
        | otherwise = do
                tell [show y ++ " is a number"] 
                x:bubble(y:ys)
bubble [x] = do 
            tell ["nothing works"]
            return [x] 

but i get this error:

 Couldn't match expected type `WriterT
                                    [String] Data.Functor.Identity.Identity [a]'
                with actual type `[a0]'
    In a stmt of a 'do' block: y : bubble (x : ys)
    In the expression:
      do { tell [show x ++ " why does this not work"];
           y : bubble (x : ys) }
    In an equation for `bubble':
        bubble (x : y : ys)
          | x > y
          = do { tell [show x ++ " why does this not work"];
                 y : bubble (x : ys) }
          | otherwise
          = do { tell [show y ++ " is a number"];
                 x : bubble (y : ys) }
Failed, modules loaded: none.

I have read this error message word for word but I am not any closer to what the problem is?
I tried compiling with out the deceleration, for a fresh set of errors like this:

q.hs:21:17:
    No instance for (MonadWriter [[Char]] [])
      arising from a use of `tell'
    Possible fix:
      add an instance declaration for (MonadWriter [[Char]] [])
    In a stmt of a 'do' block:
      tell [show x ++ " why does this not work"]
    In the expression:
      do { tell [show x ++ " why does this not work"];
           y : bubble (x : ys) }
    In an equation for `bubble':
        bubble (x : y : ys)
          | x > y
          = do { tell [show x ++ " why does this not work"];
                 y : bubble (x : ys) }
          | otherwise
          = do { tell [show y ++ " is a number"];
                 x : bubble (y : ys) }
Failed, modules loaded: none.

Answer 1

A nod in the right direction:

1. What is the type of y?
2. What is the type of bubble(x:ys)?
3. What is the type of (:)?

---

Answers:

(In these answers, a is the same a as in bubble :: (Ord a) => [a] -> Writer [String] [a].)

1. y :: a
2. bubble(x:ys) :: Writer [String] [a]
   • bubble is a function, bubble(x:ys) is the result of applying bubble to x:ys
3. (:) :: b -> [b] -> [b]
   • : is an operator; whatever type its first operand has, its second operand must have the type “list of whatever type the first operand has”, and the result has that same list type.

Given that you have given y and bubble(x:ys) as the operands to :, can you see now what the problem is?