instances Display List, Tuples ...

[amigalemming]

How about Display instances for lists and tuples? This would simplify displaying the results of functions that return multiple results. E.g. the tuple members could be displayed in tiers.

[HeinrichApfelmus]

Yes please, although that would require figuring out how to deal with compositionality.

[amigalemming]

On Mon, 18 Mar 2019, Heinrich Apfelmus wrote: Yes please, although that would require figuring out how to deal with compositionality.

In the current Text-based design we could combine the objects with '<br>'. However, Xhtml would require '<br/>'. But hm, for lists of numbers you currently use showList, what makes sense, too. I guess, there should be a DisplayList class such that for every element type there can be a custom list instance.

[HeinrichApfelmus]

A DisplayList class? I would like to stick to the trick that the Haskell Prelude uses, namely that for every type, we also implements a showList function that is responsible for showing a list of these things.

[amigalemming]

On Tue, 19 Mar 2019, Heinrich Apfelmus wrote: A DisplayList class? I would like to stick to the trick that the Haskell Prelude uses, namely that for every type, we also implements a showList function that is responsible for showing a list of these things.

I think the 'showList' design was the answer to the problem, that the Haskell creators wanted the single [Char] instance to behave differently from all other list instances. Is this the case here, too? My DisplayList approach would be this one: class DisplayList a where displayList :: [a] -> Graphic instance (DisplayList a) => Display [a] where display = displayList This would also avoid FlexibleInstances. Maybe with a default implementation: class (Display a) => DisplayList a where displayList :: [a] -> Graphic displayList = Text.intersperse (Text.pack "<br>") . map display For graphical objects we might use the stacking with <br> and for numbers we would use the list notation, i.e. [1,2,3].

[HeinrichApfelmus]

I think we have the same problem: Some lists should be displayed differently from the default display, just like [Char] should be displayed differently from the default [a]. The only difference is that in our case, it applies to many instances, not just a single one.

We do not have to introduce a DefaultList class, the displayList function can be added to the Display class with exactly the same code and effect.

class Display a where
    display     ::  a  -> Graphic
    displayList :: [a] -> Graphic

    displayList = Text.intersperse (Text.pack "<br>") . map display

instance Display a => Display [a] where
    display = displayList