[GHC] #7169: Warning for incomplete record field label used as function

[GHC]

#7169: Warning for incomplete record field label used as function
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.7
      Resolution:                    |             Keywords:  Warnings,
                                     |  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I agree with nakaji-dayo here. Emitting a warning upon every use of `quux`
 feels like the wrong design choice.

 To address goldfire's point above:

 > We want to be able to have fields such as `quux` that are used only with
 some (but not all) constructors of a type

 I agree. I think a reasonable design would be:

 * Introduce a flag for this purpose (proposal: `-Wpartial-records`)
 * If the flag is enabled and you define a partial record selector (e.g.,
 `data A = B1 { b :: Int } | B2 { b :: Int, wat :: Char }`, emit a warning
 of the definition site (and only the definition site).
 * One can disable this warning by prefixing the partial record selector
 with an underscore (e.g., `data A = B1 { b :: Int } | B2 { b :: Int, _wat
 :: Char }`). This should allow you to use the partial record for scenarios
 where it's handy, but it also gives a visual indication that something is
 fishy about that function.

 A separate question is if this flag should be implied by `-Wall`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7169#comment:17>
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