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haskell: comment about configuration-nix.nix's intersectAttrs

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Benno Fünfstück 2017-03-12 22:45:19 +01:00
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pkgs/development/haskell-modules/configuration-nix.nix
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@ -29,6 +29,17 @@
with import ./lib.nix { inherit pkgs; }; with import ./lib.nix { inherit pkgs; };
# All of the overrides in this set should look like:
#
# foo = ... something involving super.foo ...
#
# but that means that we add `foo` attribute even is there is no `super.foo`! So if
# you want to use this configuration for a package set that only contains a subset of
# the packages that have overrides defined here, you'll end up with a set that contains
# a bunch of attributes that trigger an evaluation error.
#
# To avoid this, we use `intersectAttrs` here so we never add packages that are not present
# in the parent package set (`super`).
self: super: builtins.intersectAttrs super { self: super: builtins.intersectAttrs super {
# Apply NixOS-specific patches. # Apply NixOS-specific patches.