[Haskell-cafe] Library for Sparse Vectors?

[Alexander Solla]

On Tue, Jul 26, 2011 at 3:27 PM, dokondr <dokondr at gmail.com> wrote:

> Thanks for the detailed reply and example!
> Using IntMap as a vector seems to be a good idea.
> In your example:
> 1)  I would use:
>
>        dot = dot' * dot'
>        dot'  = sum . elems . intersectionWith (*)
>        norm = sum . fmap (**2) . elems
>
>  instead of:
>
>         dot  = sum . elems . intersectionWith (*)
>         norm = (**0.5) . sum . fmap (**2) . elems
>
>
Your dot' is a function, so (dot' * dot') wouldn't type check.


> 2)  I don't understand the syntax:
>
> cosineSimilarity <$> lookup x space
>                  <*> lookup y space
>
> What are <$> and <*>?


(lookup x space) has type (Maybe something).  So does (lookup y space).  We
are using "applicative functors" to pull out values from (lookup x space)
and (lookup y space), apply cosineSimilarity to the values we pulled out,
and wrapping it all back up in a Maybe, depending on whether the lookups
found something or not.

<$> is exactly the same thing as fmap.
<*> is harder to explain.  But it is just plumbing.


http://learnyouahaskell.com/functors-applicative-functors-and-monoids
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