JSON/fold example in the README doesn't work

Hi there 👋

When I attempt to paste the following snippet from the README into the Grace browser:

JSON/fold
  { "bool": \b -> if b then 1 else 0
  , "natural": \x -> x
  , "integer": Integer/abs
  , "real": \_ -> 1
  , "string": \_ -> 2
  , "null": 3
  , "object": List/length
  , "array": \vs -> List/fold vs (\x -> \y -> x + y : Natural) 0
  }
  [ true, 1, [ -2, false, "" ], null, { foo: { } } ]

I get:

 Not a subtype

The following type:

  List a?

(input):1:1: 
  │
1 │ JSON/fold
  │ ↑

… cannot be a subtype of:

  { cons: b? -> c? -> c?, nil: c? }

(input):9:21: 
  │
9 │   , "array": \vs -> List/fold vs (\x -> \y -> x + y : Natural) 0
  │                     ↑

I can’t understand how I might fix this given that Grace doesn’t support recursion, which seems like it’d be necessary in order to get the result that the README suggests: 10. In fact I’m having trouble understanding the usefulness of JSON/fold at all given that recursion isn’t supported…

[Gabriella439]

Right. The idea is that a JSON value like this:

  [ true, 1, [ -2, false, "" ], null, { "foo": { } } ]

… is modeled as:

\constructors ->
    constructors.array
      [ constructors.bool true
      , constructors.natural 1
      , constructors.array [ constructors.integer -2, constructors.bool false, constructors.string "" ]
      , constructors.null
      , constructors.object [ { key = "foo", value = constructors.object [ ] } ]
      ]

… and the “handlers” that you’re passing to JSON/fold become the constructors argument to that function.

[Gabriella439]

Also, this blog post of mine might help for others who want to improve their intuition of how folds in general work:

https://www.haskellforall.com/2021/02/folds-are-constructor-substitution.html

There is also a useful Dhall-specific guide to recursion that is relevant, too (since Dhall has the same limitations as Grace):

https://docs.dhall-lang.org/howtos/How-to-translate-recursive-code-to-Dhall.html