Global inference mode for JsonProvider

[mjgpy3]

Excellent library, thanks for all the hard work!

Is there a way to specify that a nested JSON value is of the same type as wrapper object?

For example, today I was trying to use the provider on an object that looks like

{
  "id": 1234,
  "children": [
    {
      "id": 789,
      "children": []
    }
  ]
}

Where everything in the object follows that basic form any number of levels deep or wide.

Do you think it's possible to provide a type that looks basically like?

type Root = { Id: int; Children: Root [] }

I look forward to hearing your thoughts, thank you.

[realvictorprm]

I think this could be better coupled to a json schema 🤔

[ovatsus]

XmlProvider has support for this with the Global static param, the same could be done for Jsonprovider

[github-actions]

🤖 This is an automated response from Repo Assist.

Great question! Recursive JSON types (like tree structures with children arrays that contain nodes of the same shape) are a known limitation of JsonProvider. The provider uses structural type inference — it infers types from a concrete sample at compile time — which doesn't naturally produce recursive type definitions.

Current workaround

For recursive structures, the most reliable approach is to use JsonValue directly:

open FSharp.Data

// Parse manually and recurse
let rec parseNode (json: JsonValue) =
    let id = json.["id"].AsInteger()
    let children = 
        json.["children"].AsArray() 
        |> Array.map parseNode
    (id, children)

let root = JsonValue.Parse("""{"id": 1234, "children": [{"id": 789, "children": []}]}""")
let tree = parseNode root

What it would take to fix this in JsonProvider

As @ovatsus noted above, XmlProvider has a [Global static parameter]((fsprojects.github.io/redacted) that enables this: when Global=true, all elements sharing the same name are inferred to have the same type, which allows recursive element structures. The same approach could be applied to JsonProvider — objects that share the same JSON "shape" (same field names) would be unified into a single generated type, enabling recursive references.

This would be a meaningful enhancement but would require non-trivial changes to the JSON inference engine (JsonInference.fs) to detect and unify structurally-similar object types. If someone wants to tackle this, JsonInference.fs and XmlInference.fs (for comparison with the Global mode implementation) are good starting points.

For now, JsonValue + manual parsing remains the practical workaround for recursive JSON structures.

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