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

Dustin Lee qhfgva at gmail.com
Thu Jan 8 19:01:49 UTC 2015

```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.

[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))'

thanks!

--
Dustin Lee
qhfgva=rot13(dustin)
-------------- next part --------------
An HTML attachment was scrubbed...