[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:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 You make a good point. What about this design alternative:

 * ghc turns `-fdefer-typed-holes` on by default
 * Cabal / stack use `-fno-defer-typed-holes` by default.
 * We encourage everyone to use `_` instead of `undefined`.
 * If the type of a typed hole is known (that is, "pushed in", in the sense
 of bidirectional type-checking) we emit a very short, one line warning
 instead of the longer one with types. Even better, we could common up all
 of these warnings into one that says where the holes are.

 This design depends somewhat on what standard workflows are. (I have no
 clue about standard workflows, as I'm quite non-standard.) Do people use
 cabal/stack right from the get-go when starting a project? Or do they
 prototype in GHCi a bit before making a build structure? If it's the
 latter, then the design above makes sense. If it's the former, not so
 much.

 I also think the last point would be a nice part of this. The idea is that
 if you say

 {{{#!hs
 frooble :: Bool
 frooble = _   -- not sure if we're froobling today
 }}}

 we don't need a long-form warning telling us that the hole has type
 `Bool`. A short warning saying `hole at (...)` is enough.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11482#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler