Boolean inference from 'yes'/'no' in JSON breaks roundtrip

[piaste]

The JSON Provider, iirc like others, infers a boolean data type from yes or no strings (case insensitive).

However, the boolean representation as a yes/no string is not preserved, and generating a new JSON string via .ToString() will contain a JSON true/false field, which won't typically be compatible with the original JSON payload.

Repro script:

#r "nuget: fsharp.data"
open FSharp.Data

type Yes = JsonProvider<""" { "Foo" : "Yes" } """>
let yes = Yes.Root(foo = true)
printfn "%O" yes

(*
{
  "Foo": true
}
*)

Workaround : Use the InferTypesFromValues = false parameter and disable type inference entirely.

[github-actions]

🤖 This is an automated response from Repo Assist.

This is a genuine design tension with boolean inference from string values.

Why it happens: JsonProvider infers bool at compile time from the string values "yes", "no" (and "true", "false", "1", "0"). Once the field is typed as bool, the generated serializer outputs canonical JSON true/false, losing all trace of the original "Yes" representation. There is no way for the erased type system to remember "this bool should serialize as a string yes/no" — the erased type is simply System.Boolean.

The workaround you've identified (InferTypesFromValues = false) is the correct one. With inference disabled, "Yes" stays as a string and serialises as-is.

A narrower alternative is using the SampleIsList = true parameter with a sample that only contains true/false booleans, so the provider never learns the yes/no pattern and keeps those fields as strings in production data.

A more complete fix would require a new "string-encoded bool" virtual type in the inference engine — significantly more work. This may be worth tracking as a future enhancement if there is broad user demand.

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[dsyme]

Question answered 👍