Falco Static Files

program1.fs

module Program

open Falco
open Falco.Routing
open Microsoft.AspNetCore.Builder

let wapp = WebApplication.Create()

wapp.UseRouting()
    .UseDefaultFiles()
    .UseStaticFiles()
    .UseFalco([
        
    ])
    .Run(Response.ofPlainText "Not found")

program2.fs

module Program

open Falco
open Falco.Routing
open Microsoft.AspNetCore.Builder

let wapp = WebApplication.Create()

wapp.UseRouting()
    .UseDefaultFiles()
    .UseStaticFiles()
    .Run(Response.ofPlainText "Not found")

Thanks for chiming in Stuart with a solution.

Both program files fall through because they are hitting an ASP.NET extension, Microsoft.AspNetCore.Builder.RunExtensions.Run, which adds a terminal middleware to the pipeline. This has a dramatically different effect than Microsoft.AspNetCore.Builder.WebApplication.Run which runs the application and blocks the calling thread. The latter is not being called here in either example, so the host never starts up.

A lot of the older extension methods like UseStaticFiles() and UseDefaultFiles() are hung off IApplicationBuilder, where the endpoint methods are hung off WebApplication. The dichotomy makes pipelining the builder next to impossible. In C# you don't feel the sting of this as much because of void returns. To salve this, Falco comes with a couple shims to avoid an incessant amount of |> ignore calls. In both examples you actually use one, UseRouting() (required to activate Falco which depends on ASP.NET endpoints).

The most important of these are WebApplication.Use and WebApplication.UseIf which allow you to compose a pipeline entirely driven by WebApplication. Throughout the documentation I point out that the open Microsoft.AspNetCore.Builder import adds many useful extensions, many of these are complete unknown to most devs, but are the underpinning to the builder extension method. They are a bit esoteric, but they are really useful in F#-land. A quick example:

// instead of 
wapp.UseDefaultFiles()
    .UseStaticFiles()

// try:
wapp.Use(fun (appl : IApplicationBuilder) ->
    appl.UseDefaultFiles()
        .UseStaticFiles())

// or better yet,
wapp.Use(DefaultFilesExtensions.UseDefaultFiles)
    .Use(StaticFileExtensions.UseStaticFiles)

The HelloWorldMVC example shows how to use this effectively.

thanks @pimbrouwers !