Hello friends, I'm hoping to land Backpack (the patch is two years in the making now!) for GHC 8.2, even though 8.2 is intended to be a stabilization and performance release. In this email, I will make the case that the Backpack patch is relatively self-contained and will not affect non-Backpack users, and explain how you might be affected by the patch. The most recent patchset can be found at https://github.com/ezyang/ghc/tree/ghc-backpack ; look for "The Backpack patch" in the commit log, since I plan on squashing these commits before committing. OK, here is what is in the patchset that affects non-Backpack users: - The primary, overarching change is that there is now a distinction between "semantic" and "identity" modules. This solves a problem with hsig files: suppose you have A.hsig which defines data T in package p. The resulting A.hi is most accurately identified as p[A=<A>]:A; however, the Name for T should be <A>.T, not p[A=<A>]:A.T. The former is an identity module (it *identifies* interfaces), whereas the latter is a semantic module (it is what is used in names). Without Backpack, these two identities are always the same. There are a number of places in the existing codepaths where we have to distinguish between the two concepts (sometimes you want semantic, sometimes you want identity); we DEFAULT to the identity module but I fix a few cases where semantic modules were desired instead. - UnitId is generalized to also record how a component is instantiated (a mapping from module names to modules.) Without Backpack this is blank and equivalent to the existing UnitId. - Packages has been refactored to also handle Backpack `-unit-id` flags, since we have to do some extra work to determine what inherited signatures merge where. - The patch REVERTS all of the old support we had for hsig files; -sig-of is no more. spinda had mentioned that Liquid Haskell used this in some cases, and I'm looking at supporting *just* their particular use-case. - A new driver mode --backpack, which process test.bkp files. This mode is used PURELY for testing, and end users are not supposed to use it. These all live in their own modules (BkpSyn, DriverBkp) except for some modest, convenient extensions to the lexer and parser to parse multiple modules at once. The patch also comes with rewritten typechecking support for hsig files, and instantiating indefinite packages on the fly, but these are all standalone pieces of functionality which are not tied to any existing compiler infrastructure. Thanks, Edward