splitHsFunType orig_ty@(L _ (HsAppTy t1 t2))
  = go t1 [t2]
  where  -- Look for (->) t1 t2, possibly with parenthesisation
    go (L _ (HsTyVar fn))    tys | fn == funTyConName
                                 , [t1,t2] <- tys
                                 , (args, res) <- splitHsFunType t2
                                 = (t1:args, res)
    go (L _ (HsAppTy t1 t2)) tys = go t1 (t2:tys)
    go (L _ (HsParTy ty))    tys = go ty tys
    go _                     _   = ([], orig_ty)  -- Failure to match
splitHsFunType orig_ty@(L _ (HsAppTy t1 t2))
  = go t1 [t2]
  where  -- Look for (->) t1 t2, possibly with parenthesisation
    go (L _ (HsTyVar fn))    tys | fn == funTyConName
                                 , [t1,t2] <- tys
                                 , (args, res) <- splitHsFunType t2
                                 = (t1:args, res)
    go (L _ (HsAppTy t1 t2)) tys = go t1 (t2:tys)
    go (L _ (HsParTy ty))    tys = go ty tys
    go _                     _   = ([], orig_ty)  -- Failure to match
splitHsFunType orig_ty@(L _ (HsAppTy t1 t2))
  = go t1 [t2]
  where  -- Look for (->) t1 t2, possibly with parenthesisation
    go (L _ (HsTyVar fn))    tys | fn == funTyConName
                                 , [t1,t2] <- tys
                                 , (args, res) <- splitHsFunType t2
                                 = (t1:args, res)
    go (L _ (HsAppTy t1 t2)) tys = go t1 (t2:tys)
    go (L _ (HsParTy ty))    tys = go ty tys
    go _                     _   = ([], orig_ty)  -- Failure to match