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|0005748||OCaml||OCaml typing||public||2012-08-31 22:25||2014-06-01 03:38|
|Target Version||later||Fixed in Version|
|Summary||0005748: anonymous functions with optional arguments|
|Description||In OCaml 4.00, anonymous functions with optional arguments can no longer be passed to a higher order function that doesn't know expect optional arguments. Here is an example:|
let f g = g () ;;
(* works in 3.12 and 4.00 *)
let g = fun ?opt () -> () in
Here is what no longer works in OCaml 4.00.0 but used to work in 3.12.1:
# f (fun ?opt () -> ()) ;;
Error: This function should have type unit -> 'a
but its first argument is labelled ~?opt
This breaks some of our code where the anonymous function is generated by a Camlp4 macro.
|Tags||No tags attached.|
Repeating the message of the caml-list (feedback request at the end):
OCaml 4.00 is much more agressive in propagating expected types when typing
expressions. Unfortunately, there was a conflict between this upward propagation,
and the somehow adhoc behaviour which removes optional arguments from
a function passed as argument to a function or record/variant constructor not expecting
Specifically, this is this behavior:
val f : (unit -> unit) -> unit
val g : ?x:int -> ?y:bool -> unit -> unit
let () = f g
This code triggers automatic discarding of the optional arguments of g,
so that the type matches the expected one.
But for this we need to first infer the type of the argument, to see that
it doesn't match the expected one.
I was not aware that this feature was widely used, and the behavior in 4.00.0
is to restrict this feature to expressions where upward propagation doesn't
make sense anyway:
* function applications
* message sending
* record field extraction
* wrapping of "let open" around any of those
An inline function doesn't enter this category.
I'm pondering what to do about this.
Since this feature is described in the tutorial part of the reference manual, I suppose this
qualifies as a bug. However, combining this behavior with upward propagation is difficult.
Not propagating types to function arguments seems fine, but for variant and record
constructors this is less clear-cut.
Another option is to extend the syntactic cases where the behavior is triggered, including
"let" and "let module", but your report is about inlined functions, which could benefit
How much do you rely on that?
Another option, if we add "let" to the matched expression,
would be to "let-expand" your function:
f (let g ?x ?y () = ... in g)
this way it would match the pattern, and the let is eliminated by compilation anyway.
But this workaround does not work currently.
Jacques: what do you call "upward" propagation? Propagating the expected types *down* the AST?
Wouldn't it be possible to trigger the erasure of an optional argument "on the fly" when reaching a lambda node, if the expected type asks for a function type without this optional argument?
That said, I'm not a big fan of the automatic erasure of optional arguments (it looks rather fragile, and not so useful).
Martin Jambon (reporter)
This problem occurs in two spots in a ~100K-line code base and I don't see much more need for it in the future.
The macro we're using is FILTER provided by mikmatch_pcre:
# let f = FILTER int eos;;
val f : ?share:bool -> ?pos:int -> string -> bool = <fun>
# List.filter (FILTER int eos) [ "-123"; "a"; "0"; "-1.2" ];;
- : string list = ["-123"; "0"]
|2012-08-31 22:25||Martin Jambon||New Issue|
|2012-09-04 06:12||garrigue||Note Added: 0008016|
|2012-09-04 06:12||garrigue||Assigned To||=> garrigue|
|2012-09-04 06:12||garrigue||Status||new => feedback|
|2012-09-04 19:36||frisch||Note Added: 0008021|
|2012-09-04 21:00||Martin Jambon||Note Added: 0008022|
|2012-09-04 21:00||Martin Jambon||Status||feedback => assigned|
|2012-09-06 16:43||doligez||Target Version||=> 4.00.1+dev|
|2012-09-27 14:38||doligez||Target Version||4.00.1+dev => later|
|2014-03-26 01:25||garrigue||Relationship added||related to 0006352|
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