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IDProjectCategoryView StatusDate SubmittedLast Update
0007443OCamltypingpublic2016-12-28 01:352016-12-29 20:08
Assigned Tooctachron 
PlatformOSOS Version
Product Version4.05.0+dev 
Target VersionFixed in Version4.05.0+dev 
Summary0007443: Doubtful "unused open" warning on module used for record field disambiguation
DescriptionThe following code fails to compile with "-w @a" (setting all warnings as errors):

  module M = struct
    type t = { x : int; y : int }

  let sum : M.t -> int = function
    | M.{ x; y } -> x + y

The compiler complains that the opening "M." in the pattern M.{ x; y } is unused. But if I remove "M.", with the warning-as-errors setting, I still get a compilation error!

I believe that access to a module record field for disambiguation purposes should be counted as a "use" of this module. One might argue that this needs not be the case if neither warnings 40 or 42 are set, but I would personally still count it as a "use" in those cases -- I want to be able to write code that respects the 40-as-an-error principle even if 40 is not an error or even not set.
Additional InformationThis issue can easily be worked around by writing the pattern { M.x; y } instead, but I personally find M.{ x; y } more elegant and, beside, the warning behavior may surprise innocent bystanders -- hence it is arguably a usability bug.
TagsNo tags attached.
Attached Files

- Relationships
related to 0007386acknowledged In or-patterns, propagate path disambiguation from one prefixed constructor to other constructors 

-  Notes
gasche (developer)
2016-12-28 02:48

I just noticed (in real code) that this also happens with variant constructor disambiguation:

  module M = struct
    type t = A | B | C

  let sum : M.t -> unit = function
    | M.(A | B | C) -> ()

Again the compiler complains that "M." is unused. This is again wrong and, furthermore, there is no workaround as in the record case -- at least as long as 7386 is not resolved.
octachron (developer)
2016-12-28 17:59

I fear that this nothing but a simple mistake on my side when introducing pattern open: see the short fix in [^] .
gasche (developer)
2016-12-29 20:08

I can confirm that the fix by octachron in GPR#990 fixed the problem -- I tested those two examples.

I should have realized that this was not an issue with "open" in general, but with the new pattern-open feature, as "let open M in function A | B | C -> ()" does not warn -- I failed to try.

Thanks octachron for the quick fix and the nice feature.

- Issue History
Date Modified Username Field Change
2016-12-28 01:35 gasche New Issue
2016-12-28 02:48 gasche Note Added: 0017048
2016-12-28 02:48 gasche Relationship added related to 0007386
2016-12-28 17:59 octachron Note Added: 0017053
2016-12-29 20:08 gasche Note Added: 0017054
2016-12-29 20:08 gasche Status new => resolved
2016-12-29 20:08 gasche Fixed in Version => 4.05.0+dev
2016-12-29 20:08 gasche Resolution open => fixed
2016-12-29 20:08 gasche Assigned To => octachron
2017-02-23 16:45 doligez Category OCaml typing => typing

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