Way back when, Shachaf Ben-Kiki suggested an efficient implementation of `at` for Data.Map in https://mail.haskell.org/pipermail/libraries/2013-May/019761.html but that was never approved for inclusion. I believe I've come up with a somewhat nasty but efficient implementation of `ix` for Data.Sequence (hand-specialized and hand-unboxed), based on the code for `adjust`. See https://gist.github.com/treeowl/b80c2c490563765fc7881be30f3dba24 lens has continued to grow in popularity, so I think the time has probably come to talk again about adding `at` for Data.Map, and to talk about adding `ix` for Data.Sequence. Note: for consistency with the rest of the Data.Sequence API, `ix` would be named `adjustA`, and would have its first two arguments flipped. David Feuer