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Justifiably, we prioritise the (weakly-)initial-algebra approach to Data.* datatypes via... data definitions, but looking at haskell's Foldable for #2300 I'm conscious that we don't have any implementation of build for List, or more systematically, Church-style analogues for the basic datatypes of the library, together with their build-based equivalence to the standard ones.
This issue might be moot, but it has already come up for me in #1962 as to whether or not to take an initial algebra/free extension approach to Free constructions, or to proceed via a Church-style encoding. I think it's worth further discussion as an issue in its own right, as well as waking up the thread about API vs. implementation in the library design...