[GHC] #9046: Panic in GHCi when using :print

28 Apr 2014


      #9046: Panic in GHCi when using :print
------------------------------------+-------------------------------------
       Reporter:  quchen            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:  7.8.3
      Component:  GHCi              |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  GHCi crash
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 When using names generated by :print, GHCi panics sometimes. According to
 #ghc, this is reproducible on ''x86_64-apple-darwin'' as well, but is not
 an issue in ''7.6.3 x86_64 linux''.

 {{{
...
let a = [1]
...
:print a
 a = (_t1::Num t => [t])
...
:t _t1
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.8.2 for x86_64-unknown-linux):
         tcTyVarDetails t{tv avD} [tv]
...
_t1
 <same panic>
 }}}
From playing around with it a bit, this seems to have something to do with
 polymorphic values.

 {{{
 -- No panic
...
let a = [1 :: Int]
let a = 1 :: Int
let a = "hello"
-- Panic
...
let a = [1]
let a = 1
let a = "hello" -- With -XOverloadedStrings
 }}}
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9046
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