JSON constructors create values that don't correspond to the original schema

[Tarmil]

The constructors provided by JsonProvider can create values in a way that doesn't correspond to the original schema. In particular:

1. Optional fields are serialized as null rather than absent field:

   type A = JsonProvider<"""[{"a":1,"b":2}, {"a":3}]""">
   
   let a = A.Root(a = 1, b = None).JsonValue.ToString()
   // Expected: {"a":1}
   // Actual:   {"a":1,"b":null}

2. Decimal fields inferred from string are serialized as number:

   type B = JsonProvider<"""{"a":"1.4"}""">
   
   let b = B.Root(a = 3.1m).JsonValue.ToString()
   // Expected: {"a":"3.1"}
   // Actual:   {"a":3.1}

[cboudereau]

Same problem here for option an json strings (double quotes surrounded values)

[joshuapassos]

Same problem on #1245

[github-actions]

🤖 This is an automated response from Repo Assist.

These are genuine, long-standing bugs in JsonProvider's constructor/serialization path. Here's what's happening and why:

Bug 1 – None serializes as null instead of being omitted

When JsonProvider infers an optional field (a value that is absent in some samples), it generates a constructor parameter of type option(T). However, the underlying JsonValue record that backs the erased type doesn't distinguish between "field with null value" and "field absent from the object". When None is passed, the constructor stores null — which round-trips as "field":null rather than omitting the key.

Workaround: Use JsonValue directly to build the object, omitting keys you don't want:

let a = A.Root(JsonValue.Record [| "a", JsonValue.Number 1m |])
a.JsonValue.ToString()
// Result: {"a":1}

Bug 2 – Decimal from string serializes as number, not string

When JsonProvider infers a field whose sample values are quoted strings containing numbers (e.g. "1.4"), it promotes the type to decimal for convenience. But the generated constructor accepts a raw decimal and stores it as JsonValue.Float or JsonValue.Number, so the resulting JSON writes an unquoted number rather than a quoted string.

Workaround: Again, build JsonValue directly:

let b = B.Root(JsonValue.Record [| "a", JsonValue.String "3.1" |])
b.JsonValue.ToString()
// Result: {"a":"3.1"}

Both issues require the provider's constructor codegen to carry schema-origin metadata through to the serialisation path — a non-trivial change that would need careful design to avoid breaking round-trip behaviour in other cases. I've noted this in my backlog for a future investigation.

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