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http://dev.sourcefabric.org/browse/LS-617?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=22947#action_22947
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David Baelde commented on LS-617:
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My bad. Subtyping is not allowed, but a type annotation would totally fix your
problem (both the simple one from the original description, and the real
problem). Basically, you would do:
def apply(f:(?x:'a)->'a,x) = f(x=x) end
# then you can pass a function with optional x to apply
def foo(~data,~headers,~uri,response:(?code:int,string)->string) = ... end
# then you can pass the good response function to that foo
> 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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