On Mon, Nov 10, 2014 at 10:25:45AM -0800, Greg Weber wrote:
What is the goal of this project? To pretty-print output? Or to be a slightly friendlier version of Show?
From my perspective display would be akin to str()/#to_s in Python/Ruby. So I was not thinking to tackle pretty-printing in general.
My *very personal* main use case, that I don't see other good solutions for, is string interpolation. Currently I have a crude hack in interpolate [1] that does the right thing for String, Text and many prelude types (see also [2] if you are curious). But even more important to me is that we come up with something that is general, has minimal dependencies and is not controversial, so that people are willing to depend on it and provide instances. If there is a way to "add extension points" (what ever that may mean) for pretty-printing or do something else that is useful in the context of pretty-printing libraries, I think we should discuss it. But it's not my main objective.
To put it another way, what happens with more complicated structures? As an example, what should the output be for this type [[String]] ?
[["abc", "def"], ["ghi"], ["jkl"]]
Or
[ ["abc", "def"] , ["ghi"] , ["jkl"] ]
In case of doubt I would lean towards falling back to `show`. That would probably mean [["abc","def"],["ghi"],["jkl"]] for consistency with `show` (even though personally I prefer to have a space after the comma). By adding `displayList` to the class, individual instances would still be free to override this behavior. On the implementation side, I think we could either use showList and put that into a builder _or_ use show on all the list elements and construct the final "displayed" list output using a builder. The second may be more efficient, but for nested lists show would still be invoked on lists (I think we have a similar situation for other nested data types, unless we invoke displayBuilder recursively, which I think gives surprising results). I'm not entirely sure. An other option could be to just skip support for lists, not sure? Cheers, Simon [1] https://github.com/sol/interpolate/blob/9be1f6d15722c58e467e7ebc0cb6dc6fb2c4... [2] http://sol.github.io/interpolate-talk/