[#20] Add Zippable type class with zip and zipWith methods

[JanGe]

Issue #20

• zip, zipWith, unzip
• zip{2,7}, zipWith{2,7}, unzip{2,7}
• Provide custom method definitions for Array, UArray and String with better performance
• tests

[DavidM-D]

I think it's spelt Zippable
https://en.wiktionary.org/wiki/zippable

[JanGe]

Damn it. GHC HEAD won't let me make String an instance of Zippable as zip can't be used with it:

Core/String/UTF8.hs:105:10: error:
• Couldn't match type ‘Char’ with ‘(C.Element a, C.Element b)’
  Inaccessible code in
    the type signature for:
      C.zip :: (C.Sequential a, C.Sequential b,
                C.Element String ~ (C.Element a, C.Element b)) =>
               a -> b -> String
• In the instance declaration for ‘C.Zippable String’

[JanGe]

For tuples sizes >= 3 I didn't implement a type specific zipWith so far. Are they needed?

[vincenthz]

wow @JanGe. didn't have time to read it all yet, but this seems great ! Will try to process over the weekend

[JanGe]

Well, actually the BoxedZippable type class doesn't work yet for tuples sizes >= 3.

$ zip3 (fromList "abc" :: String) (fromList "abc" :: String) (fromList "abc" :: String) :: Array (Char, Char, Char)

<interactive>:5:1:
    Couldn't match type ‘(Char, Char, Char)’
                   with ‘(Element String, Element String)’
    In the expression:
        zip3
          (fromList "abc" :: String)
          (fromList "abc" :: String)
          (fromList "abc" :: String) ::
          Array (Char, Char, Char)
    In an equation for ‘it’:
        it
          = zip3
              (fromList "abc" :: String)
              (fromList "abc" :: String)

(And GHC HEAD catches that at compile time!)

Still thinking about how I could best solve this...

Edit: Working now. My polymorphic BoxedZippable instance didn't work with bigger tuples anymore (obviously):

instance ( Sequential a, Sequential b
         , Zippable col, Element col ~ (Element a, Element b))
        => BoxedZippable col a b

Is there any way I could require Element col to support a boxed types?

[vincenthz]

Nice work. we can improve on the building side of things later on I think