Hello everyone, 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 Is it really a bug or is it something I really shouldn't be doing? I've found this (fixed) bug (https://ghc.haskell.org/trac/ghc/ticket/8557), with a similar error output, but it does not seem to be the same case. Thanks Emilio