I'm quite intrigued about these different behaviors between GHCs 7.6.3 and 7.8.3:


GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> let x = 3 + 4
Prelude> :print x
x = (_t1::Integer)
Prelude> :force x
x = 7
Prelude> print _t1
7
Prelude>


GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> let x = 3 + 4
Prelude> :print x
x = (_t1::Num a => a)
Prelude> :force x
x = _
Prelude> print _t1
ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.3 for x86_64-unknown-linux):
    tcTyVarDetails a{tv atm} [tv]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug