[GHC] #13635: Incorrect result at runtime with list comprehensions in that case

[GHC]

#13635: Incorrect result at runtime with list comprehensions in that case
-------------------------------------+-------------------------------------
        Reporter:  vanto             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
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:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 And with `-fdefer-typed-holes`, you get the behavior that you want. (But
 be aware of
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html
 #deferred-type-errors-in-ghci this restriction] about deferred type errors
 and GHCi.)

 I argue that GHC's behavior in both cases is correct. As the
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html
 #typed-holes manual] says, a hole terminates compilation without `-fdefer-
 typed-holes`. So the GHC 8.0.2 behavior agrees with the specification. As
 there were no typed holes in GHC 7.6.3, a `_` in an expression really is
 invalid syntax, so terminating with an error is appropriate there, too.

 With `-fdefer-typed-holes`, you will still get warnings about the holes in
 your code. But you can suppress these with `-Wno-typed-holes`. My guess is
 that `{-# OPTIONS_GHC -fdefer-typed-holes -Wno-typed-holes #-}` will get
 you exactly the behavior you want.

 If you agree with this analysis, please close the ticket. Thanks!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13635#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler