On 01/07/2014 12:54 PM, Sergey wrote:
On 05.01.2014 22:37, Adam Chlipala wrote:
More precisely, you'll run into trouble if you cause multiple
specializations of a function that defines local functions that need
to have URIs assigned to them!
But I've experimented with moving function `validator' to a global
scope, like this:
fun validator (fview : int -> url) (ferr : string -> url) (i:int)
(s:{Text:string}) : transaction page = (* the same code *)
fun form (fview : int -> url) (ferr : string -> url) (i:int) :
transaction xbody =
return
<xml>
<form>
<textarea{#Text}/>
<br/>
<submit action={validator fview ferr i}/>
</form>
</xml>
The error remains the same.
You're right. I shouldn't have brought local functions into the
picture. What actually matters is which functions, local or global, are
specialized to different argument lists, while also needing URLs assigned.
Do you plan to introduce some kind of automatic names generator for
cases
like this one?
No, I advise folks to use functors for such cases.
Functors have a weak point: looks like they don't support recursive
declarations. Imagine the following use-case: An application has
several different views, each view has a small login/password form in
it's upper-right corner. Developer wants to improve the usability of
this application by redirecting users back to the view they were at
the moment they filled up the form.
I suggest passing URLs instead of functions for this case, avoiding any
need for functors.
I am sure that most of the people would try to use a basic functional
trick here:
(* Version 1 *)
fun validate_and_apply f = return {}
fun handler u f = validate_and_apply f ; redirect u
fun form (u:url) : transaction xbody = reutrn <xml><form>...<submit
action={handler u}/></form></xml>
fun viewA (idx:int) = template ( f<- form (url (viewA idx)); return
<xml>{f}</xml>)
fun viewB (idx:int) = template ( f<- form (url (viewB idx)); return
<xml>{f}</xml>)
...
The "Input to exported function 'FormBug3/handler' involves one or
more types that are disallowed for page handler inputs: Basis.url"
will appear.
You can get around this issue by using type [string] instead of [url].
Use [show] to convert [url] to [string], then use [bless] to convert
back. I've done this before for various sites. You can even use
[Basis.currentUrl] to get the current URL, without forcing each caller
of the redirecting function to construct the URL explicitly.
Note that this is _not_ a case of unjustifiable draconian behavior by
the compiler, since [bless] will do run-time checking that URLs follow
the policy you've configured in .urp files.
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