I'm supportive of doing this, and am working with Edward on it. Yell if you
object.
Simon
| -Original Message-
| From: ghc-devs [mailto:ghc-devs-boun...@haskell.org] On Behalf Of
| Edward Z. Yang
| Sent: 11 August 2016 20:07
| To: ghc-devs <ghc-devs@haskell.org>
| Subject: Landing Backpack in HEAD
|
| 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; however, the Name for T should be .T,
|not p[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
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