This is the launch page for Backpack, actively maintained by Edward (as of Apr 2017).

Backpack is a proposal for retrofitting Haskell with an applicative, mix-in module system. It has been implemented in GHC 8.2.

The documentation for how to use Backpack is a bit scattered about at this point, but here are useful, up-to-date (as of 2017-04-02, prior to GHC 8.2's release) references:

  • This pair of blog posts: Try Backpack, ghc --backpack and Cabal packages have up-to-date tutorials for using the main features of Backpack, with and without Cabal.
  • There is not yet a manual entry in Cabal for how Cabal works. This section is under development.
  • Edward Z. Yang's thesis contains detailed information about the specification and implementation of Backpack. We also have an older paper draft which was submitted to ICFP'16.
  • Hackage does not yet support uploads of Backpack-using packages. next.hackage is a Hackage instances running a development branch of Hackage that can handle Backpack; for now, Backpack-enabled packages should be uploaded here.

Some more out-of-date documents:

  • Backpack specification. This was subsumed by my thesis but once Backpack stabilizes it will be worth distilling the thesis PDF back into a more web-friendly format.

Known gotchas

Make sure cabal-version is recent enough. (#4448) If you set the cabal-version of your package too low, you may get this error:

    Mix-in refers to non-existent package 'pkg'
    (did you forget to add the package to build-depends?)
    In the stanza 'executable myexe'
    In the inplace package 'pkg'

This is because internal libraries are feature-gated by the cabal-version of your package. Setting it to cabal-version: >= 2.0 is enough to resolve the problem.

You can't instantiate a dependency with a locally defined module. Consider the following package:

  other-modules: StrImpl
  build-depends: foo-indef
  mixins: foo-indef requires (Str as StrImpl)

This looks like it should work, but actually it will fail:

    Non-library component has unfilled requirements: StrImpl
    In the stanza 'library'
    In the inplace package 'mypkg-1.2'

The reason for this is Backpack does not (currently) support instantiating a package with a locally defined module: since the module can turn around and *import* the mixed in foo-indef, which would result in mutual recursion (not presently supported.)

To solve this problem, just create a new library to define the implementing module. This library can be in the same package using the convenience library mechanism:

library str-impl
  exposed-modules: StrImpl

  build-depends: str-impl, foo-indef
  mixins: foo-indef requires (Str as StrImpl)

Backpack-related tickets

Backpack-related tickets are marked with keyword 'backpack'. If the ticket is assigned to ezyang, it means he's planning on working on it.

Ticket Type Summary Priority Owner
#1409 feature request Allow recursively dependent modules transparently (without .hs-boot or anything) normal
#9351 feature request add ability to version symbols .c for packages with C code normal
#10749 bug Boot file instances should imply superclasses normal ezyang
#10827 feature request GHCi should support interpeting multiple packages/units with separate DynFlags normal
#12703 feature request Expand Backpack's signature matching relation beyond definitional equality normal
#13151 task Make all never-exported IfaceDecls implicit normal ezyang
#13266 bug Source locations from signature merging/matching are bad normal
#13469 feature request -fdefer-type-errors for Backpack normal
#10266 task Split base for Backpack low ezyang
#10681 feature request Teach GHC to interpret all hs files as two levels of hs-boot files (abstract types only/full types + values) low ezyang
#12680 feature request Permit type equality instances in signatures low
#13149 task Giving Backpack a Promotion low
#13262 feature request Allow type synonym family application in instance head if it has no free variables low
#13361 bug Better type synonym merging/subtyping for Backpack low
#10871 feature request Implement "fat" interface files which can be directly compiled without source lowest ezyang
#12717 feature request Permit data types in signatures to be implemented with equivalent pattern synonyms (and vice versa) lowest

Last modified 5 days ago Last modified on Apr 26, 2017 2:37:28 PM