This is a small tutorial on the very basics of the Backpack module system. (Well... at least it started that way. It has grown a bit over time.)

It requires cabal-install >= 3.2.0.0 and GHC >= 9.0.1. You can install both using ghcup.

It is a multi-package project. You can build all the packages from this folder using:

cabal build all

Any corrections welcome!

lessons

• 00 - convenience libraries
• 01 - renaming modules
• 02 - signatures
• 03 - signature merging
• 04 - signature thinning
• 05 - abstract typeclasses
• 06 - abstracting monad stacks
• 07 - module identity
• 08 - transitively indefinite packages
• 09 - template haskell
• 10 - coercing proofs
• 11 - controlling encapsulation
• 12 - abstracting type families

Further resources for learning Backpack

User guides

• A detailed description in the GHC User Guide of what can be put into a module signature. This is Backpack seen from the language side.

• The Backpack section of the Cabal User Guide and also the sections about module signatures and mixins. This is Backpack seen from the package manager side.

The theory behind Backpack

• Edward Z. Yang's thesis is quite readable and gives a good account of the motivations for Backpack.

• Edward Z. Yang's blog.

• Scott Kilpatrick's thesis. Not as directly applicable to the current implementation of Backpack as Edward Z. Yang's thesis, but useful insights can be gleaned here.

The implementation

• Backpack design ticket #3038.

• relationship between ComponentId, UnitId, MungedPackageId, PackageId etc..

• about units.

• GHC Issue #20890: Multiple home units and Backpack together.

Other Backpack-related stuff

• The Backpack and Module Signature entries in the Haskellwiki.

• GHC proposal. Interesting bits about the motivation and limits of Backpack.

• I wrote a few Backpack tips & tricks here and here.

• Example of an abstract package on Hackage which uses signatures. Discussed here.

• A signature for streaming libraries

• picnic: put containers into a backpack reddit Features an interesting example of abstracting over classes. (See also lesson 5 of this tutorial.)

• a blog post about common stanzas in .cabal files. Not directly related to Backpack, but both build-depends and mixins fields can be put into common stanzas, which can then be imported by multiple libraries/executables in the package. This can help avoid duplication in the .cabal file. An example.

• using Backpack to get around problems with overlapping instances

• Monad Transformers and Effects with Backpack. reddit.

• version 0.3.0 of cryptographic library "raaz" uses Bapckpack internally. ergonomic issues.

• moo-nad A tiny library that combines module signatures and the "ReaderT pattern".

• a few extant bugs, and a possible way forward

Stuff about module systems in general

• Understanding and Evolving the ML Module System by Derek Dreyer.

• Modular Type Classes (2007) reconstruction of Haskell typeclasses in terms of ML modules "classes are signatures, instances are modules"

• ML Modules and Haskell Type Classes: A Constructive Comparison" (2008)

• Logical relations as types by Sterling and Harper. video. This went way over my head, but it seems to propose a method to verify that a module implementation has the same external behaviour as some (presumably simpler) reference implementation.

• Modules as dependent records tweet.

• Do-it-Yourself Module Systems PhD thesis.

• Abstract Data Types without the Types.

• Repo with a bibliography about ML module systems. web page. Mastodon post.