On Fri, Oct 01, 2021 at 11:52:53AM +0200, Ben Franksen wrote:
Well a balanced Tree can have an efficient corecursive foldl, or a performant 'foldr`', and Sets can know their size statically, and `elem` runs in linear time even in structures that potentially support faster search.
All true, and I think it is important to document these things. The question is: where?
I disagree that everything one should know about Data.Foldable is adequately described in Data.List. At least not without a new overview for Data.List that would cover some of the same ground in that specialised context, and could then be imported by reference. A reader who wants to better understand folds should learn the difference between strict reduction and corecursion, and certainly Data.List is not the best place to discuss tips for construction of Foldable instances. Perhaps the overview could start with a concise version that explains thinking about folds in terms of lists, and notes quickly that one cat typically get by with understanding "foldr", "foldl'" and foldMap. But ultimately one should understand why foldl', how to define instances, why `elem` is stuck doing linear lookup for `Set`, ... Would you like to contribute the "short version" introductory text for the impatient? Different readers will come to the documentation for different needs, most will come for just the synopses, and won't read the Overview, that's fine. If there's a need for a shorter blurb, please contribute. Perhaps the best path forward is to get MR 6555 done and dusted, and then additional MRs can be filed on top of that by those who'd like to see further improvements? -- Viktor.