add typed (formatted type) routes, like giraffe or saturn, so type of parameter can be inferred

[jkone27]

getf or postf routes with typed parameter could be useful

getf $"some/%i{x}"  fun i -> {| Bye = "bye" |}

[brianrourkeboll]

Hmm, unfortunately I don't think this would work very well with custom operation overload resolution in the current implementation, since the type parameter to Printf.StringFormat<'T> will be a curried function, e.g., int -> string or int -> float -> string.

(You can try these in F# interactive.)

While this works:

open System

[<AutoOpen>]
type T =
    static member get (pattern : Printf.StringFormat<'a -> string>, f : Func<'a, 'b>) = f

let foo = get ("foo/%i{x}", fun x -> x)

as does this:

open System

[<AutoOpen>]
type T =
    static member get (pattern : Printf.StringFormat<'a -> 'b -> string>, f : Func<'a, 'b, 'c>) = f

let foo = get ("foo/%i{x}/bar/%f{y}", fun x y -> float x + y)

this does not:

open System

[<AutoOpen>]
type T =
    static member get (pattern : Printf.StringFormat<'a -> 'b -> string>, f : Func<'a, 'b, 'c>) = f
    static member get (pattern : Printf.StringFormat<'a -> string>, f : Func<'a, 'b>) = f

let foo = get ("foo/%i{x}/bar/%f{y}", fun x y -> float x + y)

  let foo = get ("foo/%i{x}/bar/%f{y}", fun x y -> float x + y);;
  ----------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

stdin(17,11): error FS0041: No overloads match for method 'get'.

Known types of arguments: string * (^a -> float -> float) when ^a: (static member op_Explicit: ^a -> float)

Available overloads:
 - static member T.get: pattern: Printf.StringFormat<('a -> 'b -> string)> * f: Func<'a,'b,'c> -> Func<'a,'b,'c> // Argument 'pattern' doesn't match
 - static member T.get: pattern: Printf.StringFormat<('a -> string)> * f: Func<'a,'b> -> Func<'a,'b> // Argument 'pattern' doesn't match

If I recall correctly from some experimentation I did a while ago, it may be possible to work around this overload-resolution problem, but it would result in the loss of metadata like the lambda parameter names (used by, e.g., SwaggerGen).

Maybe I should get around to opening a language suggestion for supporting passing explicit type arguments to custom operations, because then we could do something like this:

webApp {
    get<int, Guid> "foo/{x}/bar/{y}" (fun x y -> …)
}

Although that's not necessarily a huge improvement over just annotating the parameters' types directly as you can already do:

webApp {
    get "foo/{x}/bar/{y}" (fun (x : int) (y : Guid) -> …)
}