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Nesting path combinators #570

slogsdon opened this Issue Jan 22, 2017 · 5 comments


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slogsdon commented Jan 22, 2017

As a newcomer to Suave, I've been working to move past copying and pasting examples I come across, and I've started building an application from scratch. During this process, I attempted to do something like this:

let app =
  path "/" >=> OK "index"
  path "/api"
    >=> choose [
          path "" >=> NO_CONTENT
          path "/users" >=> OK """{"api": "users"}"""

expecting a request to /api would return a 204 No Content response and a request to /api/users would return the JSON-encoded string. To my surprise, I received a 404 Not Found for both. After doing some digging, I discovered that path tests if the given path matches the request path, so with path "/users" above, it compares a request to /api/users with the given /users, fails, and returns None.

I wasn't able to find anything in the existing code base to handle this, so I added the following as a helper module in my application to achieve the expected behavior:

let optionally pred value =
  if pred then Some value else None

let getCurrentRoot ctx =
  match ctx.userState.TryFind("rootPath") with
  | None -> ""
  | Some p -> string p

let rootPath (part : string) (ctx : HttpContext) =
  let root = getCurrentRoot ctx
  { ctx with userState = ctx.userState.Add("rootPath", root + part) }
  |> Some
  |> async.Return

let subPath (part : string) (ctx : HttpContext) =
  let fullPath = (getCurrentRoot ctx) + part
  |> optionally (fullPath = ctx.request.path)
  |> async.Return

allowing the following to work as expected:

let app =
  path "/" >=> OK "index"
  rootPath "/api"
    >=> choose [
          // handles requests to `/api`
          subPath "" >=> NO_CONTENT
          // handles requests to `/api/users`
          subPath "/users" >=> OK """{"api": "users"}"""

I took to Twitter to see if there was an official way of doing this, and the official Twitter handle suggested I open a PR to start a discussion (see: I'm a a big fan of using issues for these types of discussions, but if something like the above is desired for Suave, I'd be happy to submit a PR for any changes.

Does something like this exist today? If not, would it be useful to include this functionality as a part of Suave?


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caindy commented Feb 12, 2017

Not a Suave expert, but if I understand correctly, you simply want something like:

let apiPath = ((+) "/api") >> path

So you can do

choose [ apiPath "" >=> NO_CONTENT; apiPath "/users" >=> OK "...json..." ]

Later maybe you'd extend your apiPath to check the mime-type or something.


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slogsdon commented Feb 13, 2017

@caindy Thanks for the suggestion, but there would still be some duplication within a developer's application that could stand to be removed. A developer need to create and manage these helper functions, so a benefit here would be reducing the amount of code the developer's application would need to hold itself.

@ademar ademar added the feature label Feb 28, 2017


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danieljsummers commented Apr 24, 2017

Are you looking for something like Express.js's router/module paradigm? (briefly described here)

Something like...

let apiRoutes =
  router [
    routePath "/" >=> NO_CONTENT
    routePath "/users" >=> OK """{"api": "users"}"""

let app =
  choose [
    path "/" >=> OK "index"
    path "/api" >=> apiRoutes

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czifro commented Jul 28, 2017

I too found this to be a minor let down. However, I (am no way a Suave expert) can see why path does not do what you ( @slogsdon ) would want/expect. It isn't really idiomatic. It would make more sense to have a separate function do what you are talking about. @danieljsummers snippet actually looks really nice. I think that path (pun!) seems more idiomatic.


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haf commented Aug 7, 2017

+1 to PRs where we can discuss this over code.

@ademar ademar added the you-take-it label Oct 19, 2018

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