## #10248 closed bug (fixed)

# GHC panic when used with deferred type errors, again

Reported by: | Iceland_jack | Owned by: | |
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Priority: | normal | Milestone: | |

Component: | GHCi | Version: | 7.10.1-rc3 |

Keywords: | Cc: | hvr | |

Operating System: | Linux | Architecture: | x86 |

Type of failure: | Other | Test Case: | |

Blocked By: | Blocking: | ||

Related Tickets: | Differential Rev(s): | ||

Wiki Page: |

### Description (last modified by )

Similar to previous report of mine #9834. Given `error.hs`

data I a = I a

I get the following result:

$ ghci -fdefer-type-errors -ignore-dot-ghci error.hs GHCi, version 7.10.0.20150316: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( error.hs, interpreted ) Ok, modules loaded: Main. *Main> I <$> _ I <$> _ <interactive>:2:7: Warning: Found hole ‘_’ with type: IO a0 Where: ‘a0’ is an ambiguous type variable In the second argument of ‘(<$>)’, namely ‘_’ In the first argument of ‘GHC.GHCi.ghciStepIO :: IO a_auR -> IO a_auR’, namely ‘I <$> _’ In a stmt of an interactive GHCi command: it <- GHC.GHCi.ghciStepIO :: IO a_auR -> IO a_auR (I <$> _) ghc: panic! (the 'impossible' happened) (GHC version 7.10.0.20150316 for i386-unknown-linux): nameModule $dShow_aIT Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug *Main>

### Change History (7)

### comment:1 Changed 3 years ago by

Description: | modified (diff) |
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### comment:2 Changed 3 years ago by

### comment:3 Changed 3 years ago by

I encountered the following on GHC 7.10.1, looks like it might be the same bug:

% ghc -ignore-dot-ghci -fdefer-typed-holes -e 'map (\a b -> undefined) _' <interactive>:1:25: Warning: Found hole ‘_’ with type: [a0] Where: ‘a0’ is an ambiguous type variable Relevant bindings include it :: [t -> t1] (bound at <interactive>:1:1) In the second argument of ‘map’, namely ‘_’ In the expression: map (\ a b -> undefined) _ In an equation for ‘it’: it = map (\ a b -> undefined) _ <interactive>:1:25: Warning: Found hole ‘_’ with type: [a0] Where: ‘a0’ is an ambiguous type variable Relevant bindings include it :: [t -> t2] (bound at <interactive>:1:1) In the second argument of ‘map’, namely ‘_’ In the expression: map (\ a b -> undefined) _ In an equation for ‘it’: it = map (\ a b -> undefined) _ <interactive>: <interactive>: panic! (the 'impossible' happened) (GHC version 7.10.1 for x86_64-unknown-linux): nameModule $dShow_aMc Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

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Well that's odd. It works fine in HEAD.

Simon