Backpack to Work: Towards Practical Mixin Linking for Haskell

In this paper, we describe an evolution of the Backpack mixin package system which respects the division between package manager and compiler in the Haskell ecosystem: Backpack’16. Programs written in Backpack’16 are processed in two phases: first, a mixin linking phase which computes a “wiring diagram” of components indifferent to the actual Haskell source code, and then a typechecking phase on the output of mixin linking which processes Haskell source. This is not merely a paper design: our architecture was principally motivated by our experiences implementing Backpack’16 in the GHC compiler and the Cabal package system.