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

[GHC]

#11482: Turn -fdefer-typed-holes on by default
-------------------------------------+-------------------------------------
        Reporter:  thomie            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9497, #11481     |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by thomie):

 * failure:  None/Unknown => Incorrect warning at compile-time


@@ -24,1 +24,1 @@
- use typed holes, which does show a warning (unless explicitly suppressed
+ use typed holes, which do show a warning (unless explicitly suppressed

New description:

 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
 [https://downloads.haskell.org/~ghc/7.10.3/docs/html/users_guide/typed-
 holes.html 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 do show a warning (unless explicitly suppressed
 with `-fno-warn-typed-holes`). 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#comment:2>
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