Better Support for F# Option<'T> in Swagger

README.md

Description

This is a Minimal API written for a net6.0 ASP.NET Web server. the Program.fs file will be the only F# file you need to add to a new F# ASP.NET Web app that can be generated with:

dotnet new web -lang F#

Intent

To replace the Option<'T> type with the 'T type in generated Swagger schemas

Output

swagger.json is the generated swagger document from running Program.fs

The Parent type will change from this output:

{
  "child": {
    "value": {
      "text": "string"
    }
  }
}

To this output:

{
  "child": {
    "text": "string"
  }
}

Program.fs

open System
open Microsoft.AspNetCore.Builder
open Microsoft.Extensions.Hosting
open Microsoft.Extensions.DependencyInjection
open Microsoft.FSharp.Core
open Microsoft.OpenApi.Models
open Swashbuckle.AspNetCore.SwaggerGen

type Child = { Text: string }
type Parent = { Child: Child option }

// Replaces all Option<'T> types and types with Properties that have Option<'T> with the corresponding 'T type
type OptionSchemaFilter() =
    let isFSharpOption (t:Type) =
        // There's got to be a better way to test if it's the generic Option<'T> type,
        // but I couldn't figure it out and settled for this instead
        t.Name = "FSharpOption`1" && t.Namespace = "Microsoft.FSharp.Core"
    interface ISchemaFilter with
        member x.Apply(schema: OpenApiSchema, context: SchemaFilterContext) =
            if isFSharpOption context.Type then
                let argumentType = context.Type.GetGenericArguments()[0]
                let argumentSchema = context.SchemaGenerator.GenerateSchema(argumentType, context.SchemaRepository)
                schema.Reference <- argumentSchema.Reference
            else
                for propertyInfo in context.Type.GetProperties() do
                    if isFSharpOption propertyInfo.PropertyType then
                        let argumentType = propertyInfo.PropertyType.GetGenericArguments()[0]
                        let argumentSchema = context.SchemaGenerator.GenerateSchema(argumentType, context.SchemaRepository)
                        
                        // There is probably a better way to generate the property name.
                        // This seems like it could have edge cases                        
                        let camelCasePropertyName = String [|
                            yield Char.ToLower(propertyInfo.Name[0])
                            yield! propertyInfo.Name[1..]
                        |]
                        
                        schema.Properties[camelCasePropertyName].Reference <- argumentSchema.Reference

// Removes the "*FSharpOption" added types from the schema list
type OptionDocumentFilter() =
    interface IDocumentFilter with
        member this.Apply(swaggerDoc, _) = 
            for key in swaggerDoc.Components.Schemas.Keys do
                if key.EndsWith("FSharpOption") then
                    swaggerDoc.Components.Schemas.Remove(key) |> ignore

[<EntryPoint>]
let main args =
    let builder = WebApplication.CreateBuilder(args)
    
    builder.Services.AddSwaggerGen(fun options ->
        options.SchemaFilter<OptionSchemaFilter>()
        options.DocumentFilter<OptionDocumentFilter>()    
    ) |> ignore
    
    builder.Services.AddEndpointsApiExplorer() |> ignore

    let app = builder.Build()

    app.UseSwagger() |> ignore
    app.UseSwaggerUI() |> ignore
    
    app.MapPost("/test", Func<_,_>(
        fun ([<FromBody>]s:Parent) -> 
            System.Text.Json.JsonSerializer.Serialize(s))
    ) |> ignore
    
    app.Run()

    0

swagger.json

{
  "openapi": "3.0.1",
  "info": {
    "title": "FsWebApp",
    "version": "1.0"
  },
  "paths": {
    "/test": {
      "post": {
        "tags": [
          "Pipe #5 input at line 62@62"
        ],
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "$ref": "#/components/schemas/Parent"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Success",
            "content": {
              "text/plain": {
                "schema": {
                  "type": "string"
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Child": {
        "type": "object",
        "properties": {
          "text": {
            "type": "string",
            "nullable": true
          }
        },
        "additionalProperties": false
      },
      "Parent": {
        "type": "object",
        "properties": {
          "sub": {
            "$ref": "#/components/schemas/Child"
          }
        },
        "additionalProperties": false
      }
    }
  }
}

Has anyone taken a stab at this for .NET 10 / OpenApi 2.0? The biggest issue I'm having in converting it is that References can't be assigned anymore and I'm not sure what has taken their places.