[GHC] #15855: Warn about incomplete NamedFieldPuns patterns

[GHC]

#15855: Warn about incomplete NamedFieldPuns patterns
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           Reporter:  Artyom.Kazak   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In certain situations it's nice to be able to ensure that record pattern
 matches are exhaustive:

 {{{#!haskell
 data R = R {a, b :: Int}

 -- should warn
 f R{a} = a

 -- should not warn
 g R{a, b} = a + b

 -- should not warn
 h R{a, ..} = a + b

 -- should not warn
 i R{a, b = _} = a
 }}}

 It's useful when there are several functions that inspect a record (e.g.
 handlers that update a subset of fields in a database) and we want to make
 certain that when a new field is added to the record, the programmer won't
 forget to update functions working with that record (either by handling
 the field or explicitly discarding it). Currently the only non-hacky way
 to do it is writing `f (R a _) = ...`, which is obviously not ideal.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15855>
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