#5689: The 'impossible' happened: type-inference succeeds somehow in code which
isn't type-safe
------------------------------+---------------------------------------------
Reporter: nicolast | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.0.4 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
------------------------------+---------------------------------------------
While trying to figure out how Haskell handles OCaml's value restriction,
I created some code which I thought shouldn't type-check.
It did, though, and makes GHCi panic on execution, or makes GHC fail
during compilation.
Minor changes to the code makes it no longer type-check, as expected.
Here's the code, including some comments which show when compilation does
fail as expected:
{{{
{-# LANGUAGE ScopedTypeVariables #-}
import Data.IORef
main :: IO ()
main = do
(r :: IORef (t -> t)) <- newIORef id
-- r <- newIORef i -- => Type-check error
writeIORef r (\v -> if v then False else True)
c <- readIORef r
print $ c True
print $ c 1234
-- print $ c Nothing -- => Type-check error
-- print $ c (1 :: Int) -- => Type-check error
}}}
When replacing the "print $ c 1234" line with one of the last 2 lines,
type-checking fails.
When removing the explicit type-annotation on 'r', type-checking fails
when "print $ c 1234" is left in place.
Here's the GHCi and GHC output:
{{{
Prelude> :load demo1.hs
[1 of 1] Compiling Main ( demo1.hs, interpreted )
Ok, modules loaded: Main.
*Main> main
ghc: panic! (the 'impossible' happened)
(GHC version 7.0.4 for x86_64-unknown-linux):
nameModule $dNum{v ann}
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
{{{
$ ghc --make demo1.hs
[1 of 1] Compiling Main ( demo1.hs, demo1.o )
ghc: panic! (the 'impossible' happened)
(GHC version 7.0.4 for x86_64-unknown-linux):
initC: srt_lbl
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5689>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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