FSharp.Data.Json serializes some floats as ints

[pbiggar]

The float 100.0 gets serialized as 100, not as "100.0". This throws away the fact that it's a float, and violates the downstream contracts which expect a float here. AFAICT, there is no way to make it print as a float.

To reproduce in fsi:

> FSharp.Data.JsonValue.Parse("100.0").ToString();;
val it : string = "100.0"

> FSharp.Data.JsonValue.Float(100.0).ToString();;
val it : string = "100"

[jcmrva]

AsFloat() in FSharp.Data.JsonExtensions will give you a float, but I'm not sure FSharp.Data is the only issue:

>  JsonValue.Float(100.0).AsFloat();;
val it: float = 100.0

> string 100.0;;
val it: string = "100"

>  JsonValue.Float(100.0).AsFloat().ToString();;
val it: string = "100"

It does seem to parse correctly, but comes through as a decimal unless you create it as a float:

> JsonValue.Parse("""{"flt":100.0}""") |> fun j -> j?flt |> function | JsonValue.Number dec -> true | _ -> false;;
val it: bool = true

>  JsonValue.Float(100.0) |> function | JsonValue.Float fl -> true | _ -> false;;
val it: bool = true

I guess it makes sense from a .NET perspective but kind of wrong because JSON doesn't have multiple number types.

[bongjunj]

Float is a fallback of Number

FSharp.Data/src/FSharp.Data.Json.Core/JsonValue.fs

Lines 34 to 46 in bbe54ed

	/// Represents a JSON value. Large numbers that do not fit in the
	/// Decimal type are represented using the Float case, while
	/// smaller numbers are represented as decimals to avoid precision loss.
	[<RequireQualifiedAccess>]
	[<StructuredFormatDisplay("{_Print}")>]
	type JsonValue =
	| String of string
	| Number of decimal
	| Float of float
	| Record of properties: (string * JsonValue)[]
	| Array of elements: JsonValue[]
	| Boolean of bool
	| Null

Please read the comment above.

FSharp.Data parses a number into a decimal value first, but if it fails, it parses the number into a float value.
This is because the range of a decimal value can represent is limited.

> let n1 = FSharp.Data.JsonValue.Parse("100.0");;                                                                 
val n1: FSharp.Data.JsonValue = 100.0

> let n2 = FSharp.Data.JsonValue.Parse("123432472368476238462472002348732428734628462746823.034343343433343343434333433");;
val n2: FSharp.Data.JsonValue = 1.2343247236847624E+50

> n1.GetType().FullName;;
val it: string = "FSharp.Data.JsonValue+Number"

> n2.GetType().FullName;;
val it: string = "FSharp.Data.JsonValue+Float"

Therefore, basically, the problem is that ToString method of System.Decimal is not working as expected.

> decimal 100.0;;
val it: decimal = 100M {Scale = 0uy;}

> it.ToString ();;
val it: string = "100"

Solutions

To fix the problem you described, I think we should

• store the precision of the original data. For example, we should store the number of all digits and below the point of "100.0": 4, 1.
• Any other idea?

[github-actions]

🤖 This is an automated response from Repo Assist, the repository's AI assistant.

I've identified the root cause and created a fix in a draft PR.

Root cause: JsonValue.Float(100.0).ToString() returns "100" instead of "100.0" because the serializer writes floats using w.Write(number) which calls .ToString() — and (100.0 : float).ToString() in .NET drops the trailing zero since IEEE 754 doubles carry no scale information.

The fix formats floats using the "R" round-trip format and appends ".0" when the output has no decimal point or exponent, making it unambiguously a JSON float.

PR: Fix JsonValue.Float serialization: append '.0' for whole-number floats

All 2841 FSharp.Data.Core.Tests pass (including 3 new regression tests) — 1 pre-existing network test failure unrelated to this change.

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