This works fine keyboardAct a d p (SpecialKey KeyLeft) Down Modifiers {shift=Down, ctrl=Up, alt=Up} = do (x,y) <- get p p $= (x-0.1, y) This keyboardAct a d p (SpecialKey KeyLeft) Down shiftDown = do (x,y) <- get p p $= (x-0.1, y) where, shiftDown = Modifiers {shift=Down, ctrl=Up, alt=Up} gives me : KeyBindings.hs:32:1: Warning: Pattern match(es) are overlapped In an equation for `keyboardAct': keyboardAct angle delta p (SpecialKey KeyLeft) Down Modifiers {shift = Up, ctrl = Up, alt = Up} = ... keyboardAct is used several times via pattern matching, e.g. keyboardAct angle delta p (SpecialKey KeyLeft) Down Modifiers {shift=Up, ctrl=Up, alt=Up} = do (x,y) <- get p a <- get angle d <- get delta angle $= a+d and there are several more. So clearly, the pattern match is overlapped, just like the warning says. The question is why I don't get the same warning before I make use of shiftDown. Thanks Brian