ofJson<'T> fails to deserialize DateTime and returns a string instead

[Zaid-Ajaj]

Description

Given a record of type A that has with a field of type DateTime, then ofJson<seq<A>> fails to deserialize the DateTime field correctly and returns a string instead

Repro code

The following code will not work because formatDate is getting a string as an input and will throw error: Uncaught TypeError: d.getFullYear is not a function because it is applying functions of Date on string

open System
open Fable.Core
open Fable.Core.JsInterop

type NewsItem = { 
    Title : string
    Published : DateTime
}

let serializedNewsItems = """
     [{ 
         "Title": "Breaking News",
         "Published": "2018-02-18T18:44:36.0000000"
     }]
"""
let formatDate (date: DateTime) = 
    // HERE THE ERROR!!!
    sprintf "%d/%d/%d" date.Year date.Month date.Day

let newsItems = ofJson<seq<NewsItem>> serializedNewsItems

for item in newsItems do printfn "%s - %s" item.Title (formatDate item.Published)

Workaround

Change ofJson<seq<NewsItem>> to ofJson<list<NewsItem>> then it works just fine

Expected and actual results

Expected to deserialize correctly but it instead fails

Related information

• Fable version (dotnet fable --version): 1.3.8
• Operating system: Ubuntu 17.0

[alfonsogarciacaro]

This is tricky because ofJson doesn't support interfaces. I think it's the same for Newtonsoft.Json, as it's not possible to know which constructor should be used when deserializing. But maybe they default to arrays for IEnumerable, I haven't checked it.

[Zaid-Ajaj]

@alfonsogarciacaro I don't think the problem is using seq because the JSON is getting deserialized, the issue is that DateTime field of the items is correctly deserialized but it is returned as a string instead. If it is tricky to implement, we can always just let the user know to use list instead of seq either with compiler warnings or a console message during runtime (coming from ofJson<'T>)

[alfonsogarciacaro]

I need to check if there's a solution, but right now Fable doesn't keep generic info for interfaces. So if you have ofJson<seq<NewsItem>>, at runtime Fable doesn't know that's a sequence of NewsItem. It just works because it deserializes the JSON as a plain JS object, but it doesn't know Published is a date, so it won't try to parse it as such.

I haven't decided yet how to represent reflection info in Fable 2 and how to make it work with ofJson and friends. Maybe we can find a nice way to handle these cases.