[GHC] #11482: Turn -fdefer-typed-holes on by default

[GHC]

#11482: Turn -fdefer-typed-holes on by default
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           Reporter:  thomie         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #9497, #11481
Differential Rev(s):                 |         Wiki Page:
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 From http://www.devalot.com/articles/2013/07/why-haskell.html:

     You can, however, return an `undefined` value which when evaluated
 will terminate the program. Haskell programmers use this to stub out parts
 of the program during development and they’re easy to detect during a
 production build.↩

 That's terrible, they should be using typed holes!

 Hypothesis: because `ghc -fdefer-typed-holes Test.hs` is annoying to type,
 haskellers currently reach for `undefined` instead of typed holes (i.e.
 `_`), to mark the part of their code that isn't finished yet.

 Richard writes in ticket:9497#comment:2:

    I'm -1 on [enabling -fdefer-typed-holes by default]. It seems to invite
 the possibility that holes could make their way into released code.

 That's true, but the alternative is worse: developers keep using
 `undefined` instead of typed holes. Since ghc doesn't show any warnings
 for using `undefined` (#8064), we should make it as easy as possible to
 use typed holes, which does always show a warning (unless explicitly
 suppressed). Having to specify a command-line flag is too high a barrier
 (since `undefined` is so well known and easy to use)

 Proposal:
 * ghc turns `-fdefer-typed-holes` on by default.
 * build scripts specify `-fno-defer-typed-holes` to make sure no holes
 make it into releases.
 * we encourage everyone to use `_` instead of `undefined`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11482>
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