Add vs-constr-render to ValueSkeleton for Custom Rendering

[sidprasad]

This PR introduces a new variant to the ValueSkeleton data type: vs-constr-render.

vs-constr-render enables more flexible rendering of value skeletons by allowing the attachment of renderer functions to constructor-like skeletons.

[blerner]

I noticed this PR a few days ago, but I don't currently understand it (though I'm sure we discussed this design at some point, I just don't recall it right now) - is there an example of using this new construction?

[sidprasad]

@blerner

I noticed this PR a few days ago, but I don't currently understand it (though I'm sure we discussed this design at some point, I just don't recall it right now) - is there an example of using this new construction?

Thanks for checking in! There's an example of how this might hook up to CPO here: brownplt/code.pyret.org#597

I don't have a CLI-specific example, but in general the idea is to be able to say "I'm going to take control of how this value is rendered".

import valueskeleton as VS
# dom-render might be some way of rendering this in the DOM / CPO
import dom-render as DR

# Some custom rendering logic for the CLI
fun render(args):
  for raw-array-fold(str from "", elt from args, i from 0):
    str + elt + "cli"
  end
end

data RBNod:
  | Black(value, left, right)
  | Red(value, left, right)
  | Leaf(value)
 sharing:
    method _output(self):
   # Register how you'd like things to be rendered in both CLI and CPO scenarios
    VS.vs-constr-render("RBNod", [list: ], { cli: render, cpo: lam(a):  DR.genlayout( self, ...) end })
end
end

[blerner]

These aren't actually guaranteed to be strings, though, right? (It could be arbitrary HTML or something?)

[blerner]

What happens when this app returns a value that's not of the expected type? E.g. returns a DOM node when it's supposed to be a raw string? Or if it crashes/throws an error?

In general, I'm concerned about allowing arbitrary Pyret code to run during rendering, such that if the Pyret code is buggy/unexpected, the overall rendering algorithm will choke and produce no output at all, or the dreaded "error while rendering; details logged to console" message... Do you have a fallback here for what to do if that .app fails for whatever reason?

[blerner]

This feels very brittle to me. I also don't see where this gets changed for other contexts -- you have it marked as a const, so line 221 will never look for anything other than cli context, right? (In the PR you linked to, it's a const of cpo, which again is rigid.

Also -- how does this interact with to-repr and to-string?