Nix

SHA256 Map Generation Method

With this snippet from project-files.mk, we can switch between two methods of generating ghc-x.y.z.sha256map.nix.

# How to generate project-nix/ghc-$(GHC_VERSION)/sha256map.nix?
# This is copied from ghc-$(GHC_VERSION).sha256map.nix.
#  - false to generate from *.dhall inputs via sha256map.hs.
#  - true to generate from stack.yaml via sha256map.py.
SHA256MAP_VIA_PYTHON ?= false

# If true, generate the sha256map from the stack.yaml with python,
# overriding the recipe for this target.
ifeq ($(SHA256MAP_VIA_PYTHON), true)
ghc-$(GHC_VERSION).sha256map.nix: stack.yaml
⇥ updo/project-nix/sha256map.py <$^ >$@
endif

The default is to use the sha256map.hs script to generate it¹:

$ make -f project-files.mk ghc-x.y.z.sha256map.nix
echo \
  '[./project-dhall/ghc-x.y.z/deps-external.dhall
  , ./project-dhall/ghc-x.y.z/deps-internal.dhall
  , ./project-dhall/ghc-x.y.z/forks-external.dhall
  , ./project-dhall/ghc-x.y.z/forks-internal.dhall
  , ([] : List {loc : Text, tag : Text, sub : List Text})
  ]' \
  | ./updo/project-nix/sha256map.hs > ghc-x.y.z.sha256map.nix

To use the sha256map.py script instead that is much slower:

$ SHA256MAP_VIA_PYTHON=true \
  make -f project-files.mk project-nix/ghc-x.y.z/sha256map.nix
updo/project-nix/sha256map.py <stack.yaml >ghc-x.y.z.sha256map.nix
mkdir -p project-nix/ghc-x.y.z && cp ghc-x.y.z.sha256map.nix project-nix/ghc-x.y.z/sha256map.nix
rm ghc-x.y.z.sha256map.nix

You can read more about Updo Nix and its use with haskell.nix.

Footnotes

1. Using ghc-x.y.z and lts-m.n in the example output, not the actual GHC version or resolver.