gistfile1.txt

        instantiateExtIxFun :: (MonadFreshNames m, Show d, Show u, Show ret) =>
                              Ident -> ExtIxFun ->
                              m ([PatElemT (MemInfo d u ret)], IxFun)
        instantiateExtIxFun idd ext_ixfn = do
          let isAndPtps = S.toList $
                          foldMap onlyExts $
                          foldMap leafExpTypes ext_ixfn

          -- Find the existentials that reuse the sizeidents, and
          -- those that need new pattern elements.  Assumes that the
          -- Exts form a contiguous interval of integers.
          let (size_exts, new_exts) =
                span ((<length sizeidents) . fst) $ sort isAndPtps
          (new_substs, patels) <-
            fmap unzip $ forM new_exts $ \(i, t) -> do
            v <- newVName $ baseString (identName idd) <> "_ixfn"
            return ((Ext i, LeafExp (Free v) t),
                    PatElem v $ MemPrim t)
          let size_substs = map (\((i, t), ident) ->
                                    (Ext i, LeafExp (Free (identName ident)) t))
                            (zip size_exts sizeidents)
              substs = M.fromList $ new_substs <> size_substs
          ixfn <- instantiateIxFun $ IxFun.substituteInIxFun substs ext_ixfn

          return (patels, ixfn)