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Routes are how Saturn connect all the HTTP requests to the different actions. Think of route as the URL of the application. The site is but you may have an route for your about page such as

In Saturn, Routers contain all the routes of your application. Router is a list of routes. A website can have a router that handles the different routes to your page like so:
    ├── "/"   
    ├── "/about"
    ├── "/contact"
    ├── "/news"
    └── "/investors"

Since this is at the root, this is your router at "" path. You can then add a router inside another router to have the following:
    ├── books 
    |   ├── list
    |   ├── add
    |   ├── update
    |   └── delete
    ├── about 
    ├── contact
    ├── news  
    └── investors

Now you have router for the "/books" path inside another router for the "" path.

Now to see it in code, create a Saturn project from the template and you will have a Router.fs file like this:

module Router

open Saturn
open Giraffe.Core
open Giraffe.ResponseWriters

let browser = pipeline {
    plug acceptHtml
    plug putSecureBrowserHeaders
    plug fetchSession
    set_header "x-pipeline-type" "Browser"

let defaultView = router {
    get "/" (htmlView Index.layout)
    get "/index.html" (redirectTo false "/")
    get "/default.html" (redirectTo false "/")

let browserRouter = router {
    not_found_handler (htmlView NotFound.layout) //Use the default 404 webpage
    pipe_through browser //Use the default browser pipeline

    forward "" defaultView //Use the default view

//Other scopes may use different pipelines and error handlers

// let api = pipeline {
//     plug acceptJson
//     set_header "x-pipeline-type" "Api"
// }

// let apiRouter = router {
//     error_handler (text "Api 404")
//     pipe_through api
//     forward "/someApi" someScopeOrController
// }

let appRouter = router {
    // forward "/api" apiRouter
    forward "" browserRouter

First, take a look at the router function.

let appRouter = router {
    forward "" browserRouter

The appRouter function is a router. Inside is the forward "" defaultView line. The forward function need a path and a router. In this case, the path is an empty string and the router is browserRouter. That means that the browserRouter router will handles the routes at the current location. Since, appRouter is the first router called, the current location is the root of the application.

Now let's look at browserRouter:

let browserRouter = router {
    not_found_handler (htmlView NotFound.layout)
    pipe_through browser

    forward "" defaultView

There are three lines. The first line, not_found_handler (htmlView NotFound.layout) tells browserRouter to displays a not found page if the user enter a route that the application does not handles. The second line tell the application to use the browser pipeline defined above. The pipeline is a list of settings on how the website will deliver the pages. Lastly, forward "" defaultView is like forward "" defaultView from the appRouter. Again, browserRouter does not contain any routes but it told the defaultView router to handle them. Finally, we get to the part where the application is told how to handle the routes. Inside defaultView, we created 3 routes.

let defaultView = router {
    get "/" (htmlView Index.layout)
    get "/index.html" (redirectTo false "/")
    get "/default.html" (redirectTo false "/")

Here, we see that get is used to define the routes. There are 3 routes here but 2 that redirect to the first route. To illustrate, the routes are:
    └── "" (router)
        └── "" (browserRouter)
            └── "" (defaultView)
                ├── "/"       
                ├── "/index.html"       -redirect to
                └── "/default.html"     -redirect to

Looking at the first line inside defaultView, get "/" (htmlView Index.layout) tells the application to displays Index.layout at the root of the application. The get correspond to the HTTP verb GET so when you type in a link, the browser try to GET the page. The first parameter of get is "/" so basicly when getting the root, the get function will return something. The second parameter is (htmlView Index.layout) so the get function return a html page specified by Index.layout. The second and third line have (redirectTo false "/"), tell the application to go to "" when going to "" or ""

Best Practices

You can combine all 3 routers into one router like so:

let appRouter = router {
    not_found_handler (htmlView NotFound.layout)
    pipe_through browser

    get "/" (htmlView Index.layout)
    get "/index.html" (redirectTo false "/")
    get "/default.html" (redirectTo false "/")

The template splits them into 3 to encourages good practices. In the first router, you can see the commented out code forward "/api" apiRouter. This is a good suggestin in the template to have a separate router to handle your api routes. We set up how to deliever the webpage with pipe_through browser in browserRouter. The setting are important for a browser to know how to handle your routes but not for a different applications to access your routes as an api.

The template provided an example of how to set up the api routes in the commented out code, which I copied below:

let api = pipeline {
    plug acceptJson
    set_header "x-pipeline-type" "Api"

let apiRouter = router {
    error_handler (text "Api 404")
    pipe_through api

    forward "/someApi" someScopeOrController

Here we have the apiRouter router which does not return a 404 page but a 404 text instead which is appropriate for an api. The router also use a pipeline that is more appropriate for an api such as accepting JSON inputs instead of HTML as in the browser pipeline.

Format Strings

You might be wondering how to make a routes that accept a numerical id. You can make multiple routes for each id like so

get "/1" (getApplication 1)
get "/2" (getApplication 2)
get "/3" (getApplication 3)

But this is impracticle because there can be a large amount of items or new items are constantly being created with new ids. Instead the solution is to use format strings. Remember that in the Adding Pages Guide, we used getf "/%s" index2Action to pass a string to page.

Format Char Type
%b bool
%c char
%s string
%i int
%d int64
%f float/double
%O Guid

For a numerical id, we want to pass an int which is %i in the list above so you can replace the lines aboves with

getf "/%i" getApplication

Notice that getf is used instead of get. This is a separate version of get that handle format characters.

You can use format strings with "forward" too by using "forwardf"

There are a lot of functionalities within routers and you can view all of them here.