Presentations in Hell (pretty print any data structure)

[chrisdone]

It occurred to me recently that my adventures in present that I ultimately decided wasn’t tenable in Haskell, might actually be tenable for Hell:

1. Hell only supports built-in types, or user-defined records/variants in the file, meaning Hell is capable of knowing all possible shapes of any runtime value, and be able to print them all, structurally (without Show).
2. This could be used for either a REPL, or —eval type of function, to show any value that a Hell script can conceive of.
3. With a terminal (e.g. brick), web or emacs, one could have an interactive, colourized, lazy expansion of any data structure (including infinite lists) out of the box (think CLIM, SLIME, DevTools or nushell).
4. Built in types being Tree, List, Vector, Map, JSON, tuples, Text, ByteString, Int, etc.
5. A user function like Hell.debug :: a -> Text (note, no type class constraint) is also viable, though would depart from Haskell, so I don’t like it. It would break parametricity which is ugly. Whereas a mere command line flag would be ok.

I was watching a video about NuShell and thought, that kind of interactive display of data is actually quite viable for Hell if I combine the architectural designs from my present package and the helpful limitations of Hell.

[chrisdone]

Exciting! On my branch I have the following working:

data Node = Node { i :: Main.Yarr, label :: [Text] }
data Yarr = Yes | No
main = [
  Main.Node { i = Main.Yes, label = ["OK, go!", Error.error "Nooo", "Done." ] },
  Main.Node { i = Main.Yes, label = ["OK, go!", Error.error "Nooo", "Done." ] }
  ]

with --present, outputs:

ConsP
  (RecordP
     "Main.Node"
     [ ("i", ConstructorP "Yes" Nothing)
     , ( "label"
       , ConsP
           (TextP "OK, go!")
           (ConsP
              (ExceptionP
                 ErrorCall
                 (TextP
                    "Nooo\nCallStack (from HasCallStack):\n  error, called at /home/chris/Work/chrisdone/hell/src/Hell.hs:1455:6 in main:Main"))
              (ConsP (TextP "Done.") NilP)))
     ])
  (ConsP
     (RecordP
        "Main.Node"
        [ ("i", ConstructorP "Yes" Nothing)
        , ( "label"
          , ConsP
              (TextP "OK, go!")
              (ConsP
                 (ExceptionP
                    ErrorCall
                    (TextP
                       "Nooo\nCallStack (from HasCallStack):\n  error, called at /home/chris/Work/chrisdone/hell/src/Hell.hs:1455:6 in main:Main"))
                 (ConsP (TextP "Done.") NilP)))
        ])
     NilP)

With this I can generate a pretty-printed text output, but also a vty-based lazily-browsable interface. When I add a presentation for Value, that will offer a nice way to browse JSON values, too.