Indeed, there are a lot more conflicts than I'd have expected. Ignoring functions with the same type, Hoogle shows this name in the below packages. I have no sense of the overall significance of the specific packages or (equally importantly) of the `on` function within each.
haskell-gi-base:
on :: forall object info m . (GObject object, MonadIO m, SignalInfo info) => object -> SignalProxy object info -> HaskellCallbackType info -> m SignalHandlerId
brick:
on :: Color -> Color -> Attr
esqueletto:
on :: SqlExpr (Value Bool) -> SqlQuery ()
relational-query (both):
on :: MonadQuery m => Predicate Flat -> m ()
on :: MonadQuery m => QueryA m (Predicate Flat) ()
threepenny-gui:
on :: (element -> Event a) -> element -> (a -> UI void) -> UI ()
miso:
on :: MisoString -> Decoder r -> (r -> action) -> Attribute action
wild-bind:
on :: i -> v -> Binder i v ()
massiv-io:
on :: Pixel X Bit
selda-postgresql:
on :: Text -> Text -> PGConnectInfo
One note: this does conflict with some other libraries, for instance GTK2HS