[GHC] #2111: Incorrect arrow rec {} notation crashes in ghci

[GHC]

#2111: Incorrect arrow rec {} notation crashes in ghci
-------------------------+--------------------------------------------------
    Reporter:  luqui     |        Owner:       
        Type:  bug       |       Status:  new  
    Priority:  normal    |    Milestone:       
   Component:  Compiler  |      Version:  6.8.1
    Severity:  normal    |   Resolution:       
    Keywords:            |     Testcase:       
Architecture:  x86       |           Os:  Linux
-------------------------+--------------------------------------------------
Comment (by ross):

 The `-farrows` option turns on the `rec` keyword, but then you get this
 even without using arrow notation:
 {{{
 Prelude> do { rec { x <- return ('a':x); }; putStrLn (take 20 x) }
 ghc-6.8.2: panic! (the 'impossible' happened)
   (GHC version 6.8.2 for i386-unknown-linux):
         tcDoStmt: unexpected Stmt
     rec {(x{v aon}) <- base:GHC.Base.return{v 01T}
                          ('a' base:GHC.Base.:{(w) d 64} x{v aon})}

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}
 but if `do` is replaced with `mdo`, it works:
 {{{
 Prelude> mdo { rec { x <- return ('a':x); }; putStrLn (take 20 x) }
 aaaaaaaaaaaaaaaaaaaa
 }}}
 This is because tcDoStmt does not handle RecStmt.  In the original
 example, the correct behaviour would be to object to the use of arrow
 application in an expression, as it does with `mdo`:
 {{{
 Prelude> mdo { rec { x <- undefined -< x }; undefined -< x }

 <interactive>:1:17:
     The arrow command
       undefined -< x
     was found where an expression was expected
     In an 'mdo' expression: x <- undefined -< x
     In an 'mdo' expression: rec {x <- undefined -< x}
     In the expression:
         mdo rec {x <- undefined -< x}
             undefined -< x
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2111#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler