[GHC] #14549: RecordWildCards doesn't work properly in repl

[GHC]

#14549: RecordWildCards doesn't work properly in repl
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        Reporter:  akegalj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  GHCi              |              Version:  8.2.1
      Resolution:                    |             Keywords:  repl,
                                     |  recordwildcards
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by AntC):

 I would expect an error; the question is what error?

 If you write that code as a program, the compiler doesn't go past the
 second line: `Multiple declarations of ‘t1’`

 * The declaration for `Test` puts name `t1` in scope -- as a field
 selector function.
 * The `t1 = 2` (tries to) create a different declaration of `t1` ===>
 fail.

 In the repl, I guess the `t1 = 2` shadows `Test`'s declaration of `t1`, so
 you don't get a failure at that point.

 This works:
 {{{
 data Test = Test { t1 :: Int, t2 :: Int } deriving Show

 tt = let t1 = 2 in Test { t2 = 2, ..}

 main = print tt
 }}}

 You might also get your code to work if you compiled the declaration for
 `Test` into a separate module and imported it. (In effect that's what I'm
 doing with the `let`.) Then `t1` would refer to your local variable, and
 wouldn't clash with imported `t1`. In an expression using record syntax
 `Test { t2 = 2, t1 = t1 }`, the lhs `t1` must be a field label, the rhs
 `t1` must be your local variable. Then `Test { t2 = 2, .. }` should work.

 In general, the repl is a tool for hacking quick-and-dirty. Don't expect
 that if it accepts some line of code you have an overall coherent program.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14549#comment:1>
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