Generating an href value and route pattern from a common string - f#

Question

In mvc-movie-giraffe I have a line like this in a view to generate a link to an edit page:
a [ _href ("/Movies/Edit/" + (string elt.Id)) ] [ encodedText "Edit" ]
Then where I setup the routes, I have the following:
routef "/Movies/Edit/%i" edit_handler
It would be nice to factor out that string "/Movies/Edit/".
So I added it to a Urls module along with some others:
module Urls =
let movies = "/Movies"
let movies_create = "/Movies/Create"
let movies_edit = "/Movies/Edit/%i"
For the href value:
a [ _href ("/Movies/Edit/" + (string elt.Id)) ] [ encodedText "Edit" ]
we can do this instead:
a [ _href (sprintf (Printf.StringFormat<int->string>(Urls.movies_edit)) elt.Id) ] [ encodedText "Edit" ]
and for the route case:
routef "/Movies/Edit/%i" edit_handler
we can do this:
routef (PrintfFormat<obj, obj, obj, obj, int>(Urls.movies_edit)) edit_handler
And this does appear to work!
My question is, is there a better way? Perhaps something more concise? The above is a bit syntax heavy. Or is there already some established best practice around this?
I looked through the Giraffe samples and didn't notice anything like this technique being used.
Update - Approach #2
Here's one approach. Basically, factor out those longer expressions as well:
module Urls =
let movies = "/Movies"
let movies_create = "/Movies/Create"
let movies_edit = "/Movies/Edit/%i"
let movies_edit_href = sprintf (Printf.StringFormat<int->string>(movies_edit))
let movies_edit_route = PrintfFormat<obj, obj, obj, obj, int>(movies_edit)
And then reference them in views:
a [ _href (Urls.movies_edit_href elt.Id) ] [ encodedText "Edit" ]
and when setting up routes:
routef Urls.movies_edit_route edit_handler
Update - approach #3
With approach #2, I ended up with this:
module Urls =
let movies = "/Movies"
let movies_create = "/Movies/Create"
let movies_edit = "/Movies/Edit/%i"
let movies_edit_href = sprintf (Printf.StringFormat<int->string>(movies_edit))
let movies_edit_route = PrintfFormat<obj, obj, obj, obj, int>(movies_edit)
let movies_details = "/Movies/Details/%i"
let movies_details_href = sprintf (Printf.StringFormat<int->string>(movies_details))
let movies_details_route = PrintfFormat<obj, obj, obj, obj, int> (movies_details)
let movies_delete = "/Movies/Details/%i"
let movies_delete_href = sprintf (Printf.StringFormat<int->string>(movies_delete))
let movies_delete_route = PrintfFormat<obj, obj, obj, obj, int> (movies_delete)
There's some repetition there so I defined these extensions:
type String with
member this.route = PrintfFormat<obj, obj, obj, obj, int>(this)
member this.href = sprintf (Printf.StringFormat<int->string>(this))
Now, the Urls module is just:
module Urls =
let movies = "/Movies"
let movies_create = "/Movies/Create"
let movies_edit = "/Movies/Edit/%i"
let movies_details = "/Movies/Details/%i"
let movies_delete = "/Movies/Details/%i"
And then to get the href string for movies_edit:
a [ _href (Urls.movies_edit.href elt.Id) ] [ encodedText "Edit" ]
And similar for the route:
routef Urls.movies_edit.route edit_handler

Answer 1

I definitely do this at work to share url route templating.
type Route<'t> = PrintfFormat<obj, obj, obj, obj, 't>
let Details = Route<Guid> ("/Details/%O")
and then reference this in my route handlers as well as in my views to generate routes. There are a few caveats though:
for values (like let bindings), you can't leave the generic parameters of the PrintfFormat type 'open'. So they get unified to obj, which is what you're seeing.
if you need to keep the generic parameters open for some reason, the workaround is to convert the value to an inline function with a unit parameter, then invoke that function at each callsite:
let inline Details () = PrintfFormat<_,_,_,_,Guid> "/Details/%O"
this gets past the 'value restriction' that many ML-family languages have. You can look up that term for literature on why the generic parameters have to remain open on a value-binding.