I just filed this bug: #14963: ghci -fdefer-type-errors can't run IO action from another module https://ghc.haskell.org/trac/ghc/ticket/14963 It seems like -fdefer-type-errors and ghci just don't get along at all now. But on the good news side, ghci reloading a changed module in 8.4.1 is much much faster than 8.0.2. I don't know who did that, but good work! Reload being instant instead of having to wait a second or so makes a big difference.