[Haskell-beginners] help understanding error from attempt at CIS 194 : Homework 7 - indexJ

[Dustin Lee]

Here's what I have so far for JoinList.hs:   (
http://www.seas.upenn.edu/~cis194/spring13/hw/07-folds-monoids.pdf)

====
module JoinList

where

import Data.Monoid
import Sized

data JoinList m a = Empty
                  | Single m a
                  | Append m (JoinList m a) (JoinList m a)
     deriving (Eq, Show)


tag :: Monoid m => JoinList m a -> m
tag Empty           = mempty
tag (Single m a)    = m
tag (Append m  _ _) = m

(+++) :: Monoid m => JoinList m a -> JoinList m a -> JoinList m a
(+++) x y = Append (tag x <> tag y) x y


indexJ :: (Sized b, Monoid b) => Int -> JoinList b a -> Maybe a
indexJ _ Empty  = Nothing
indexJ i (Single m a)
    | i == 0    = Just a
    | otherwise = Nothing
indexJ i (Append m x y)
    | (getSize (tag x)) >= i  = indexJ i x
    | otherwise               = indexJ (i - (getSize (tag x))) y

=====

Here is the error I'm getting.   Haven't been able to make sense of it yet.

*Sized> :load "JoinList.hs"
[1 of 2] Compiling Sized            ( Sized.hs, interpreted )
[2 of 2] Compiling JoinList         ( JoinList.hs, interpreted )

JoinList.hs:50:21:
    Could not deduce (b ~ Size)
    from the context (Sized b, Monoid b)
      bound by the type signature for
                 indexJ :: (Sized b, Monoid b) => Int -> JoinList b a ->
Maybe a
      at JoinList.hs:44:11-63
      `b' is a rigid type variable bound by
          the type signature for
            indexJ :: (Sized b, Monoid b) => Int -> JoinList b a -> Maybe a
          at JoinList.hs:44:11
    Expected type: JoinList Size a
      Actual type: JoinList b a
    In the first argument of `tag', namely `x'
    In the first argument of `getSize', namely `(tag x)'
    In the first argument of `(>=)', namely `(getSize (tag x))'
Failed, modules loaded: Sized.



thanks!

-- 
Dustin Lee
qhfgva=rot13(dustin)
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