quoting @prophet
mli files are mostly used to constrain the visibility of definitions whereas hs-boot files are about allowing mutual recursion between modules (which OCaml doesn't support, even with mli files!)
But the mechanism by which they achieve their goals is nearly identical even though the perception of it is so vastly different.
I guess the conclusion to draw from this is that both sides are wrong: IMO, mli files are not nearly as good as OCamlers think they are, but hs-boot files aren't as ugly as Haskellers think either.
-- prettySrcLoc and prettyCallStack are defined here to avoid hs-boot
-- files. See Note [Definition of CallStack]
Backpack's design is primarily driven by compatibility considerations (“how do we build upon GHC's existing foundation?”), rather than elegance. In particular, Backpack doesn't eliminate those ugly .hs-boot files, it just automates and hides their generation and processing.
