[Haskell-cafe] Re: Re: [Haskell] View patterns in GHC: Request for feedback

[Benjamin Franksen]

Dan Licata wrote:
> On Jul25, apfelmus wrote:
>> The point is to be
>> able to define both  zip  and  pairs  with one and the same operator :< .
> 
> There's actually a quite simple way of doing this.  You make the view
> type polymorphic, but not in the way you did:
> 
>    type Queue elt
>    empty :: Queue elt
>    cons :: elt -> Queue elt -> Queue elt
> 
>    data ViewL elt rec = EmptyL
>                     | elt :< rec
> 
>    view :: Queue elt -> ViewL elt (Queue elt)
>    view2 :: Queue elt -> ViewL elt (ViewL elt (Queue elt))

This is cool! The more so because 'view2' can quite easily be defined in
terms of 'view'

view2 q = case view q of
  EmptyL -> EmptyL
  x :< q' -> x :< view q'

so it suffices to provide the one-level 'view' as a library function. Does
this scale to views containing multiple nestable constructors?

It would, of course, be nice to get rid of having to write view2
altogether...

Cheers
Ben