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

[Dustin Lee]

Ahhh....  I'm using the size function now and at least I don't get any
compile errors anymore.....   Now to check the code actually works.

Thanks!

On Thu, Jan 8, 2015 at 12:26 PM, divyanshu ranjan <
idivyanshu.ranjan at gmail.com> wrote:

> 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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-- 
Dustin Lee
qhfgva=rot13(dustin)
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