[GHC] #12167: <<loop>> when zip + unzipping a shadowed Vector type variable

8 Jun 2016

      #12167: <<loop>> when zip + unzipping a shadowed Vector type variable
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           Reporter:  markog         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Windows
       Architecture:  x86_64         |   Type of failure:  Incorrect result
  (amd64)                            |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 module Main where
   import Data.Vector.Unboxed (Vector)
   import qualified Data.Vector.Unboxed as V

   (|>) :: a -> (a -> b) -> b
   x |> f = f x

   main = do
     s <- do
       x <- return $ V.fromList [1,2,3,4] :: IO (Vector Int)
       d <- return $ V.fromList [1,2,3,4] :: IO (Vector Int)
       let
         xd :: (Vector Int, Vector Int)
         xd =
           V.zip x d
           |> V.unzip
         (x,d) = xd  -- here is where the error happens
                     -- returning xd works
                     -- removing the shadowing also works
         in return x
     print s
 }}}

 I do not see how the above code warrants a <<loop>> error as there is
 really no recursion in it. The linter always complains when I shadow
 variables, but I often use the above style in F# to reduce the namespace
 bloat. Shadowing is not a problem when the variables have different types.

 Is the above really a compiler error?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12167
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