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

[divyanshu ranjan]

Hi Dustin,

(tag x) return something of type b which implements Sized and Monoid.
Where as getSize takes value of type Size.
Hence the error. b might not be equal to Size. Thus compiler is
complaining.

Given b is (Sized b), how one can convert it into Size ?



Thanks
Divyanshu Ranjan


On Fri, Jan 9, 2015 at 12:31 AM, Dustin Lee <qhfgva at gmail.com> wrote:

> 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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>
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