[
http://dev.sourcefabric.org/browse/LS-617?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=22945#action_22945
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Romain Beauxis commented on LS-617:
-----------------------------------
I am trying to define a wrapper for HTTP callbacks that would make it optional
to return some informations. Here's some code:
# Master endpoints controler
def handler(~method,~protocol,~data,~headers,uri) =
def f(current_handler, element) =
x = fst(element)
endpoint = fst(x)
target_method = snd(x)
handler = snd(element)
if string.match(pattern=endpoint, uri) and method == target_method then
handler
else
current_handler
end
end
handler = list.fold(f, fourofour, !endpoints)
# Response callback, use default values if
# no override is passed by the handler.
def response(~code=200, ~headers=headers, data) =
http_response(
protocol=protocol,
code=code,
headers=headers,
data=data)
end
handler(data=data, headers=headers, uri=uri, response)
end
harbor.http.register(port=port, "^#{prefix}.*", handler)
In the response callback, I use the information already available (headers) or
expected by default (code) so that the handler would not have to return them if
it does not need to.
Such a handler could then be:
def foo(~data, ~headers, ~uri, response)
response("foo!")
end
Or, for a 404 handler:
def not_found(~data, ~headers, ~uri, response)
resonse(code=404, "Not found, dude!")
end
The problem is that response has type (?code:int, ...) ->... while in
not_found, its inferred type is
(code:int, ...) -> ... so this would not work..
Actually, even with type annotation this would still be annoying has each
handler definition would have to annotate the response's callback type..
If you have a better idea I'd be very interested :-)
> We need type annotations..
> --------------------------
>
> Key: LS-617
> URL: http://dev.sourcefabric.org/browse/LS-617
> Project: Liquidsoap
> Issue Type: New Feature
> Affects Versions: 1.0
> Reporter: Romain Beauxis
> Priority: Important
>
> Here's why:
> # def apply(f, x) = f(x=x) end;;
> apply : (((x:'a)->'b),'a)->'b = <fun>
> # def optional(~x="f") = "f" end;;
> optional : (?x:string)->string = fun (~x="f") -> "f"
> # apply(optional, "foo");;
> At line 316, char -125:
> this value has type
> (?x:_)->_ (infered at line 316, char -139--110)
> but it should be a subtype of (the type of the v
> In OCaml:
> # let apply f x = f ~x:x ;;
> val apply : (x:'a -> 'b) -> 'a -> 'b = <fun>
> # let optional ?(x="foo") = x;;
> Warning 16: this optional argument cannot be erased.
> val optional : ?x:string -> string = <fun>
> # apply optional "gni";;
> Error: This expression has type ?x:string -> string
> but an expression was expected of type x:'a -> 'b
> BUT:
> # let apply (f : ?x:'a -> 'b) x = f ~x:x ;;
> val apply : (?x:'a -> 'b) -> 'a -> 'b = <fun>
> # apply optional "gni";;
> - : string = "gni"
> In case you're wondering, I have a much more useful example that cannot work
> without that...
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