Abstract
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. Programs written in Backpack 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 in the GHC compiler and the Cabal package system.